Secure Engineering Guidelines

1. Introduction

This document summarizes the "Secure Coding Guidelines" that should be followed by WSO2 engineers while engineering WSO2 products, as well as applications used within the organization.

The purpose of this document is to increase the security awareness and make sure the products and the applications developed by WSO2 are inherently secure, by making sure security best practices are followed throughout the Software Development Life Cycle.

1.1 Organization of the Document

"OWASP Top 10 - 2017 Prevention" section of the document categorizes different attacks or security threats that engineers must focus on while engineering a product or an application. Prevention techniques are discussed in generic form, and there are sections that discuss programming language specific prevention techniques.

“OWASP Top 10 - 2013 Prevention” section of the document categorizes OWASP Top 10 2013 list of the most critical application security risks. But OWASP has updated its top 10 list in 2017. Please refer to OWASP Top 10 2017 for updated list.

“OWASP Mobile Top 10 Prevention” section of the document categorizes different attacks or security threats that engineers must focus on while engineering mobile applications. Prevention techniques are discussed in generic form, and there are sections that discuss mobile platform specific prevention techniques.

"General Recommendations for Secure Coding" section of the document discusses any security threats that are not captured within OWASP Top 10 and several generic recommendations that summarize prevention mechanisms which address multiple security threats. For example, the "Cryptographic Algorithms" section discusses general recommendations on selecting cryptographic algorithms, and sections such as "Security Related HTTP Headers" and "Securing Cookies", summarize prevention techniques used across preventing multiple attacks.

"Tooling Recommendations for Secure Coding" section brings together all the documentations relevant to security related tooling used within WSO2, and recommendations relevant to usage of such tools in the engineering process.

1.2 Formatting of the Document

Formatting for "example incorrect usage":

Formatting for "example correct usage":

Formatting for documenting items that require Platform Security Team's approval:

Formatting for documenting sample code blocks or sample code outputs used in explanations (not incorrect or correct usage samples, but general code or output used for explanations):

Formatting for documenting example scenarios used in explanations:

Formatting for referencing to other documents owned by WSO2.

1.3 Revision History

2. OWASP Top 10 - 2017 Prevention

2.1 A1 - Injection

2.1.1 SQL Injection

A SQL injection attack consists of insertion or "injection" of a SQL query via the input data from the client to the application. A successful SQL injection exploit can read sensitive data from the database, modify database data (Insert/Update/Delete), execute administration operations on the database (such as shutdown the DBMS), recover the content of a given file present on the DBMS file system and in some cases issue commands to the operating system. SQL injection attacks are a type of injection attack, in which SQL commands are injected into data-plane input in order to effect the execution of predefined SQL commands ¹.

2.1.1.1 Prevention Techniques

Use secure language constructs (Example: PreparedStatement in Java) when executing SQL statements. Use language specific best practices when using secure language constructs to avoid any possible misuses.

Restructure the methods so that the application does not accept table names, column names, ordering information, offset details or any other value that cannot be parameterized using language specific best practices.

If user input must be used in sections that cannot be parameterized using secure language constructs, perform allowlisting against the user input.

2.1.1.2 Java Specific Recommendations

Use PreparedStatements to prevent SQL injections. The statement will be compiled and the user variables will be assigned to the query parameters in the runtime. Since user variables are being set to a precompiled SQL statement, this approach avoids possibility of SQL injections.

When processing dynamic query segments that cannot be set as PreparedStatement parameters (table names, column names, ordering information, offset details), validate user input against a allowlist. This approach avoids the risk of providing end user the ability to append anything uncontrolled to the SQL query.

Example 1:

Example 2:

2.1.1.3 PHP Specific Recommendations

Use PHP Data Objects (PDO) Extension or MySQL Improved Extension when executing queries that include user inputs. Make sure to use parameterizing capabilities of the extensions.

Using PHP Data Objects (PDO) Extension^2,3

Using MySQL Improved Extension ^4,3

Example with query segments that cannot be parameterized with PDO or MySQL Improved Extension⁵:

2.1.2 LDAP Injection

LDAP Injection is an attack used to exploit web based applications that construct LDAP statements based on user input. When an application fails to properly sanitize user input, it's possible to modify LDAP statements using a local proxy. This could result in the execution of arbitrary commands such as granting permissions to unauthorized queries, and content modification inside the LDAP tree. The same advanced exploitation techniques available in SQL Injection can be similarly applied in LDAP Injection ^6,7.

All user inputs that are getting directly appended to any LDAP queries should be filtered through an encoding function that does proper encoding for LDAP⁸.

2.1.3 OS Command Injection

Command injection is an attack in which the goal is execution of arbitrary commands on the host operating system via a vulnerable application. Command injection attacks are possible when an application passes unsafe user supplied data (forms, cookies, HTTP headers etc.) to a system shell. In this attack, the attacker-supplied operating system commands are usually executed with the privileges of the vulnerable application. Command injection attacks are possible largely due to insufficient input validation⁹.

2.1.3.1 Prevention Techniques

Products or applications should not use user inputs in constructing OS commands.

2.1.3.2 Java Specific Recommendations

WSO2 Products should not use java.lang.Runtime.getRuntime(), java.lang.ProcessBuilder or any other method to execute OS commands, constructed based on user input.

2.1.3.3 PHP Specific Recommendations

PHP applications should not use "exec" function¹⁰, "WScript.Shell"¹¹ or any other method to execute OS commands, constructed based on user input.

2.1.4 HTTP Response Splitting (CRLF Injection)

Including unvalidated data in an HTTP header allows an attacker to specify the entirety of the HTTP response rendered by the browser. When an HTTP request contains unexpected CR (carriage return, also given by %0d or \r) and LF (line feed, also given by %0a or \n) characters, the server may respond with an output stream that is interpreted as two different HTTP responses (instead of one). An attacker can control the second response and mount attacks such as cross-site scripting and cache poisoning attacks¹⁶.

For example, if below snippet was used to set cookie value, malicious value "Wiley Hacker\r\nContent-Length:45\r\n\r\nHTTP/1.1 200 OK\r\n...." can result in generating two HTTP responses¹⁷.

2.1.4.1 Prevention Techniques

With regards to WSO2 Carbon 4 based products, Apache Tomcat will handle this internally, by disallowing "carriage return" and "line feed" characters from the header names or values^18,19.

If HTTP response generation is done in any other component, or any other HTTP transport implementation, it should be reviewed and confirmed to have a mechanism similar to Tomcat 6¹⁸ and Tomcat 7¹⁹ that performs necessary filtering. If such mechanism is not present in transport implementation, a central filter should be used to read all the headers and do the necessary sanitization before passing the response to transport.

Sample filter implementation is available in WSO2 Carbon 4.4.x branch²⁰. However, this filter is not in use, since Tomcat provides the necessary protection.

2.1.5 Log Injection / Log Forging (CRLF Injection)

Including unvalidated data in log files allows an attacker to forge log entries or inject malicious content into logs. When a log entry contains unexpected CR (carriage return, also given by %0d or \r) and LF (line feed, also given by %0a or \n) characters, the server may record them in the log files/monitoring systems as different events. This can be used to forge log entries and might result in business level implications, if logs are used for further actions, such as reconciliation²¹.

For example, if below snippet was used to print a log entry, malicious input "example1\r\n$100 payment made to author: example2" can result in generating two log lines.

2.1.5.1 Prevention Techniques

In WSO2 Carbon 4 (4.4.3 onwards), log appenders can be configured to append a UUID to each log entry. UUID will be randomly generated during server startup and the UUID regeneration time can be configured if such behavior is required. Therefore, even if a log line was forged, it would be possible to isolate the forged entry, since it will not contain the relevant valid UUID.

UUID in logs can be enabled by adding %K in log4j pattern and relevant configuration is further documented in Administration Guide²².

2.2 A2 - Broken Authentication

2.2.1 Session Hijacking

The Session Hijacking attack consists of the exploitation of the web session control mechanism, which is normally managed with a session token.

Because HTTP communication uses different TCP connections, the web server needs a method to recognize every user's connections. The most useful method depends on a token that the Web Server sends to the client browser after a successful client authentication. A session token is normally composed of a string of variable width and it could be used in different ways, like in the URL, in the header of the HTTP requisition as a cookie, in other parts of the header of the HTTP request, or yet in the body of the HTTP request. (In WSO2 products, session ID is maintained as a cookie and will be communicated between browser and the server through HTTP headers).

The Session Hijacking attack compromises the session token by stealing or predicting a valid session token to gain unauthorized access to the Web Server²³.

The session token could be compromised in different ways; the most common methods controllable by application developer are:

• Session Sniffing
• Eavesdropping attack
• Predictable session token
• Client-side attacks (XSS, malicious JavaScript Codes, Trojans, etc)

2.2.1.1 Prevention Techniques

Preventing session sniffing and eavesdropping attacks

HTTP Strict-Transport-Security Header (HSTS)

HTTP Strict Transport Security (HSTS) is an opt-in security enhancement that is specified by a web application through the use of a special response header. Once a supported browser receives this header, that browser will prevent any communications from being sent over HTTP, to the specified domain and will instead send all communications over HTTPS²⁴.

HSTS header prevents sensitive information exposure over unencrypted channels by avoiding SSLStrip attacks²⁵ and by preventing a victim from using HTTP links that correspond to HTTPS-protected site, which could lead to session cookie exposure, if not configured properly.

HTTP Public-Key-Pins Header (HPKP)

To ensure the authenticity of a server's public key used in TLS sessions, this public key is wrapped into an X.509 certificate which is usually signed by a certificate authority (CA). Web clients such as browsers trust a lot of these CAs, which can all create certificates for arbitrary domain names. If an attacker is able to compromise a single CA, they can perform MITM attacks on various TLS connections. HPKP can circumvent this threat for the HTTPS protocol by telling the client which public key belongs to a certain web server²⁶.

HPKP is a Trust on First Use (TOFU) technique. The first time a web server tells a client via a special HTTP header which public keys belong to it, the client stores this information for a given period of time. When the client visits the server again, it expects at least one certificate in the certificate chain to contain a public key whose fingerprint is already known via HPKP. If the server delivers an unknown public key, the client should present a warning to the user²⁶. In addition, HPKP header prevents attack patterns such as SSLSplit²⁷ attack.

Avoiding predictable session token

Please refer to documentation section "Session Prediction".

Preventing client-side attacks

Please refer to documentation section "Cross-Site Scripting (XSS)".

Cookie security

Cookie attributes should be set properly, in order to prevent session related cookies from getting exposed over unencrypted channels. Please refer to documentation section "Securing Cookie".

2.2.2 Session Fixation

Session Fixation is an attack that permits an attacker to hijack a valid user session (a type of a Session Hijacking attack). The attack explores a limitation in the way the web application manages the session ID, more specifically the vulnerable web application. When authenticating a user, it doesn't assign a new session ID, making it possible to use an existing session ID. The attack consists of obtaining a valid session ID (e.g. by connecting to the application), inducing a user to authenticate himself with that session ID, and then hijacking the user-validated session by the knowledge of the used session ID. The attacker has to provide a legitimate Web application session ID and try to make the victim's browser use it²⁸.

2.2.2.1 Prevention Techniques

User Login Flow

When a user is successfully authenticated, a new session should be initiated and the guest session that was maintained to access the login sequence should be invalidated. This approach makes sure that an attacker is unable to use a compromised guest session token to gain access to an active user session.

User Logout Flow

When a user logs out of the system, any existing session should be invalidated so that an attacker is unable to login back into the system by re-submitting the previous session token.

2.2.2.2 Java Specific Recommendations

User Login Flow

Invalidate the HttpSession and create a new session after authentication is complete, and before setting any values to the user session.

User Logout Flow

Invalidate the HttpSession, rather than just removing user specific attributes.

Other Best Practices

Do not use HttpServletResponse#encodeRedirectURL() or HttpServletResponse#encodeURL, in any operation, since it will append the session ID to the resulting URL. Since WSO2 products are dependent on cookies, there is no functional impact of not running URLs through these methods.

2.2.2.3 PHP Specific Recommendations

User Login Flow

Use session_regenerate_id function to renew the session after successful authentication.

User Logout Flow

Destroy current session using session_destroy function, rather than just removing user specific attributes.

2.2.3 Session Prediction

Session prediction attack focuses on predicting session ID values that permit an attacker to bypass the authentication schema of an application. By analyzing and understanding the session ID generation process, an attacker can predict a valid session ID value and get access to the application.

In the first step, the attacker needs to collect some valid session ID values that are used to identify authenticated users. Then, they must understand the structure of session ID, the information that is used to create it, and the encryption or hash algorithm used by application to protect it. Some bad implementations use sessions IDs composed by username or other predictable information, like timestamp or client IP address. In the worst case, this information is used in the clear text or coded using some weak algorithm like base64 encoding.

In addition, the attacker can implement a brute force technique to generate and test different values of session ID until they successfully gets access to the application³⁵.

2.2.3.1 Prevention Techniques

Using a longer random number or string as the session key will reduce the risk that an attacker could simply guess a valid session key through trial and error or brute force attacks. Session identifiers should be at least 128 bits long to prevent brute-force session guessing attacks, according to OWASP recommendations³⁰.

2.2.3.2 Java Specific Recommendations

WSO2 Carbon 4 based products can use the Apache Tomcat context.xml file to configure the session ID generation. Default session ID length is 16 bytes, which is exactly 128 bit. Therefore, WSO2 Carbon 4 based products match with current recommendation. However, if the recommendations change and if any product needs to increase the session ID length, it is possible to use "SessionIdGenerator" element of context.xml file to make required changes as documented in Tomcat 7 "SessionIdGenerator Component" documentation³¹.

2.2.3.3 PHP Specific Recommendations

Below minimum session configuration should be used with PHP applications³² :

Full set of recommended session configuration is:

2.3 A3 - Sensitive Data Exposure

2.3.1 Heap Inspection Attacks

When sensitive data such as a password or an encryption key is not removed from memory, it could be exposed to an attacker using a "heap inspection" attack that reads the sensitive data using memory dumps or other methods⁴⁵.

2.3.1.1 Prevention Techniques

It is recommended to store sensitive information such as user passwords in mutable data types or data structures. Once usage is complete, it is essential to clear the memory space and make sure none of the store data is present in the memory.

2.3.1.2 Java Specific Recommendations

It would seem logical to collect and store the password in an object of type java.lang.String. However, here's the caveat: Objects of type String are immutable, i.e., there are no methods defined that allow you to change (overwrite) or zero out the contents of a String after usage.

Even if String object is no longer referenced and available for garbage collection, the object can stay in the memory for multiple garbage collection cycles, allowing a heap inspection attack to capture relevant values.

This feature makes String objects unsuitable for storing security-sensitive information such as user passwords. It is essential to always collect and store security-sensitive information in a char array or a mutable data type instead⁴⁶.

Once operations that require password are executed, it is required to clear the values from the array elements, before moving the variable out of scope.

2.3.2 Privacy Violation - Password Auto Complete

Browsers will sometimes ask a user if they wish to remember the password that they just entered. The browser will then store the password, and automatically enter it whenever the same authentication form is visited. This is a convenience for the user⁴⁷.

Having the browser store passwords is not only a convenience for end-users, but also for an attacker. If an attacker can gain access to the victim's browser (e.g. through a Cross Site Scripting attack, or through a shared computer), then they can retrieve the stored passwords. It is not uncommon for browsers to store these passwords in an easily retrievable manner, but even if the browsers were to store the passwords encrypted and only retrievable through the use of a master password, an attacker could retrieve the password by visiting the target web application's authentication form, entering the victim's username, and letting the browser to enter the password⁴⁷.

Due to associated security risks relevant to password auto complete, it is recommended to turn autocomplete off, for sensitive text fields. This includes password inputs.

In order to support password managers and increase usability features of browsers, some modern browsers do not honor autocomplete attribute. However, regardless of browser's behavior, it is advised to turn autocomplete off, to address security compliance related issues.

2.4 A4 - XML External Entity (XXE)

An XML External Entity attack is a type of attack against an application that parses XML input. This attack occurs when XML input containing a reference to an external entity is processed by a weakly configured XML parser. This attack may lead to the disclosure of confidential data, denial of service, server-side request forgery, port scanning from the perspective of the machine where the parser is located, and other system impacts¹².

2.4.1.1 Java Specific Recommendations

In order to resolve this issue, it is required to configure the XML parser correctly performing following actions.

1. Enable the DocumentBuilderFactory namespace awareness
2. Disable the DocumentBuilderFactory entity reference expansion
3. Enable the DocumentBuilderFactory secure processing feature
4. Disable the DocumentBuilderFactory "http://xml.org/sax/features/external-general-entities" feature
5. Use a SecurityManager for the DocumentBuilderFactory
6. Use a custom EntityResolver for the DocumentBuilder created using above DocumentBuilderFactory

DocumentBuilderFactory (DOM Parser)

XMLInputFactory (Stax Parser)

2.4.1.2 PHP Specific Recommendations

If libxml is used in XML processing, libxml_disable_entity_loader function¹³ must be used to protection against XXE attacks^14,15.

2.5 A5 - 2017 – Broken Access Control

2.5.1 Insecure Direct Object References

2.5.1.1 Path Traversal

A path traversal attack (also known as directory traversal) aims to access files and directories that are stored outside the web root folder. By manipulating variables that reference files with "dot-dot-slash (../)" sequences and its variations or by using absolute file paths, it may be possible to access arbitrary files and directories stored on file system including application source code or configuration and critical system files⁴⁰.

Prevention Techniques

Absolute paths should not be accepted from the end-user during any operation, apart from administrative configurations.

The general recommendation is to avoid accepting paths or path fragments (used to build absolute or relative paths) from the end-user.

However, if any such requirement arises, it is necessary to follow language specific recommendations for validating the accepted path fragment.

Java Specific Recommendations

Normalize⁴¹ the final path constructed and check if the normalized path is within the expected boundary. The purpose of this validation is to check if any malicious user has used ".." or other traversal techniques to move out of the expected base directory the action should be performed on.

PHP Specific Recommendations

Normalize the final path constructed and check if the normalized path is within the expected boundary. Purpose of this validation is to check if any malicious user has used ".." or other traversal techniques to move out of the expected base directory the action should be performed on.

2.5.2 Missing Function Level Access Control

If the authentication check in sensitive request handlers is insufficient or non-existent, the vulnerability can be categorized as Missing Function Level Access Control⁴⁸.

2.5.2.1 Prevention Techniques

• Make sure authentication checks are present for any restricted URLs.
• Make sure user authorization (role-permission) is checked before allowing a user to execute any function. It is essential to make sure, authorization checks are done, before processing any state-changing operation.
• "login" permission should not be granted for any user that does not require access to Carbon Management Console (Carbon 4). (Example: API Manager - Store - Self-registered user).
• The enforcement mechanism(s) should deny all access by default, requiring explicit grants to specific roles for access to every function.
• If the function is involved in a workflow, check to make sure the conditions are in the proper state to allow access.
• Make sure audit logs are maintained properly and authentication and authorization related changes are logged in audit logs.

2.6 A6 - 2017 - Security Misconfiguration

This section is not within the scope of "Secure Coding Guidelines". However, necessary prevention techniques are summarized for information.

If a component is susceptible to attack due to an insecure configuration it would classify as security misconfiguration⁴⁴.

2.6.1.1 Prevention Techniques

Product Documentation

Product documentation should explain or reference to steps to be executed in order to perform required security hardening and production deployment guidelines should include security-related instructions.

All the default credentials, default certificates and security sensitive information such as ports being opened should be properly documented.

Deployments

All WSO2 deployments should follow security hardening guidelines and production deployment guidelines in security sensitive environments.

Production and staging environments should be configured identically. Configuration, as well as artifact deployment between these environments, should be automated and should follow a proper change management process to make sure no configuration changes are done in production environments without proper security evaluation.

External components of the deployment such as the operation system and runtime environment should be updated regularly and must be updated when relevant "security updates" are released.

Default credentials, default certificates, and any security sensitive default values should not be used in production environments (as recommended in security hardening guidelines and production deployment guidelines).

2.7 A7 - 2017 - Cross-Site Scripting (XSS)

Cross-Site Scripting allows an attacker to execute malicious code (scripts) against the web browser of the user. By leveraging this attack, an attacker could attempt to carry-out other forms of attacks such as stealing session cookies to perform Session Hijacking or stealing token values to conduct CSRF Attacks, launch a phishing attack etc³³.

2.7.1.1 Prevention Techniques

There are three recommended best practices that need to be followed in order to prevent XSS threats:

Input validation

User inputs should be validated whenever possible and only characters that are relevant to the particular field should be accepted. Input validation should be done on the server-side to prevent any bypass attempts, even though additional front-end validation can be done to increase user experience.

However, in WSO2 Carbon 4 based products, input validation is given a lower priority and is only done in essential screens. Therefore, it is a must to use proper output encoding and output sanitization techniques.

Output Encoding

Proper output encoding should be applied, where output consists of user input that is not expected to include HTML content.

Output Sanitization

Proper output sanitization should be applied, where output consists of user input that is expected to include HTML content.

Browser Level Protection

Modern browsers have built-in XSS prevention mechanisms. However, certain browsers require explicitly enabling these mechanisms using special response headers. "X-XSS-Protection: 1; mode=block" header should be set in HTTP responses to make sure browser level protection is enabled in all supported browsers.

For additional details on HTTP security related headers, refer to "Security Related HTTP Headers" section.

However, this is not a permanent or justifiable solution for XSS. Instead, browser level protection should only be considered as an additional protection mechanism.

2.7.1.2 Java Specific Recommendations

Output Encoding

OWASP Java Encoder³⁵ should be used to perform output encoding. OWASP Java Encoder is capable of performing contextual encoding. Following are the contexts and relevant examples that should be used:

Encode within HTML content

Encode Attributes used in JavaScript

Encode Variables in Javascript Blocks

Encode with URLs

Output Sanitization

OWASP Java HTML Sanitizer³⁶ should be used to perform output sanitization. OWASP Java HTML Sanitizer can be used to allow only certain set of HTML elements in a user input. In addition, it is possible to define allowed attributes and properties of attributes, relevant to an enabled element.

It is recommended to limit to pre-packaged FORMATTING sanitizer policy³⁷. However, if user should be allowed to use images, tables or links, other pre-packaged sanitizer policies can be used.

Browser Level Protection

"org.apache.catalina.filters.HttpHeaderSecurityFilter" Servlet Filter should be used to add X-XSS-Protection header to the HTTP response.

2.7.1.3 JavaScript Specific Recommendations

Output Encoding

When adding content into HTML document using client-side script (such as JavaScript), avoid inserting HTML content directly into the document. Avoid .html(), .innerHTML, and other related functions.

Use methods such as .text(), .textContent, for adding dynamic text content into DOM. When adding HTML content is required, programmatically create DOM nodes and append content to the newly created DOM and append the new DOM node into HTML document.

If dynamic input is used to construct a HTML attribute double quote and single quote should be additionally escaped.

2.7.1.4 PHP Specific Recommendations

Output Encoding

"htmlspecialchars" function³⁸ should be used whenever user input is printed back into a response. Mentioned function converts special characters in the input to HTML entities.

Use ENT_QUOTES flag to enable encoding both single and double quotes.

Output Sanitization

HTML Purifier³⁹ should be used to sanitize HTML content. Default HTML Purifier rules set is recommended.

Browser Level Protection

2.8 A8 - 2017 - Insecure Deserialization

Deserialization is the process of creating an object from binary data or text data. It is an opposite process of serialization. When serialized data is in control of an attacker, insecure deserialization flaws can enable an attacker to cause remote code execution upon deserialization or create a malicious deserialized objects that can cause remote code execution and date tampering upon usage¹¹⁹.

2.8.1.1 Prevention Techniques

Use language specific guidelines

Use language-specific guidelines to enumerate safe methodologies for deserializing data that can't be trusted.

Additional prevention techniques¹²⁰

• Implementing integrity checks such as digital signatures on any serialized objects to prevent hostile object creation or data tampering.
• Log deserialization exceptions and failures, such as where the incoming type is not the expected type, or the deserialization throws exceptions.

2.8.1.2 PHP Specific Recommendations

Check the use of 'unserialize()' and review how the external parameters are accepted.Use a safe, standard data interchange format such as JSON (via json_decode() and json_encode()) if you need to pass serialized data to the user. If you need to deserialize externally-stored data, consider using hash hmac() for data validation. Make sure data is not modified by anyone but you¹²¹.

2.8.1.3 JAVA Specific Recommendations

Secure java.io.ObjectInputStream

The technique is overriding the ObjectInputStream#resolveClass() method to prevent arbitrary classes from being deserialized. This safe behavior can be wrapped by using SerialKiller. SerialKiller inspects Java classes during naming resolution and allows a combination of denylisting or allowlisting to secure the application¹²⁰.

In order to detect malicious payloads or allow trusted classes only, SerialKiller should be used instead of the standard java.io.ObjectInputStream¹²².

The second argument is the location of SerialKiller's configuration file. InvalidClassException exceptions should be used to gracefully handle insecure object deserialization.

Prevent Data Leakage

If there are data members of an object that should never be controlled by end users during deserialization or exposed to users during serialization, they should be declared as "transient".

For a class that is defined as Serializable, the sensitive variables should be declared as 'private transient'. For example, the variable 'name' and 'age' of the class Foo were declared as transient to avoid being serialized¹²⁰.

Prevent Deserialization of Domain Objects

If a Class is must implement Serializable only due to their hierarchy, in order to guarantee that the objects can’t be deserialized, readObject method should be declare with the final modifier¹²⁰.

Harden own java.io.ObjectInputStream

Override ObjectInputStream.html#resolveClass() to restrict which classes are allowed to be deserialized¹²⁰.

A example code ensures the SampleObjectInputStream class is guaranteed not to deserialize any type other than the Foo class.

2.9 A9 - 2017 – Using Known Vulnerable Components

Outdated components may have known vulnerabilities that are exploitable. Public databases such as NVD⁵³ and ExploitDB⁵⁴ offer information on known vulnerabilities and exploits available.

2.9.1 Introducing New External Dependencies

When introducing new external dependency components, use the most up-to-date version of the component.

Selected most up-to-date version should be scanned by using OWASP Dependency Check for known vulnerabilities by following "Engineering Guidelines - External Dependency Analysis using OWASP Dependency Check".

External dependency, as well as all transitive dependencies getting added should be scanned using instructions from "OWASP Dependency Check CLI" section of the document.

Generated report should be attached with the usual "Approval Request" and the approval request should be copied to Security Leads Group email.

If most up-to-date version contains a known vulnerability, and if there is no active development happening, other alternatives should be considered, rather than using the vulnerable library.

2.9.2 Vulnerabilities in Current Dependencies

OWASP Dependency Check maven plugin should be integrated into the build process of latest product builds, by following "Engineering Guidelines - External Dependency Analysis using OWASP Dependency Check".

Engineers can execute Dependency Check by calling the " dependency-check:check" maven goal, in development environment. However, Jenkins will execute this task by default during the scheduled builds, to identify if any latest product build contains external dependencies with known security vulnerabilities.

When it is identified that a security vulnerability has been identified for a particular external dependency, it is recommended to do following:

• Initiate relevant discussion at Security Leads Group mailing list with subject "Dependency Vulnerability - [DependencyName] - [DependencyVersion] - [CVE]".
• Analyze the impact on the usage of WSO2, relevant to the particular dependency and identify if usage of WSO2 makes any product vulnerable.
  1. If yes, take necessary corrective actions to migrate to a higher version of the dependency with no known security vulnerabilities.
  2. If yes, and no new version of the dependency exists, take necessary actions to nullify the impact of the vulnerability on usage of WSO2, by introducing additional validations or security checks.

1. If no, update the mail thread with the reasoning and request approval of Platform Security Team to add relevant mitigation information into OWASP Dependency Check, suppression file relevant to the component.

2.10 A10 - 2017 - Insufficient logging and Monitoring

Insufficient logging and ineffective integration with security incident response systems allow attackers to pivot to other systems and maintain persistent threats for weeks or months before being detected. This leads attacker to tamper, extract or system data¹²³.

2.10.1.1 Prevention Techniques

As per the risk of the data stored or processed by the WSO2 products¹²⁴.

• Ensure all login, access control failures, and server-side input validation failures can be logged with sufficient user context to identify suspicious or malicious accounts, and held for sufficient time to allow delayed forensic analysis.
• Ensure that logs are generated in a format that can be easily consumed by a centralized log management solutions.
• Ensure high-value transactions have an audit trail with integrity controls to prevent tampering or deletion, such as append-only database tables or similar.

3. OWASP Top 10 - 2013 Prevention

This section discusses OWASP Top 10 2013 list of the most critical application security risks.

Note : OWASP has updated its top 10 list of the most critical application security risks in 2017. Please refer to OWASP top 10 2017 for updated list.

3.1 A1 – Injection

Please refer to OWASP Top 10 2017 A1 - Injection

Please refer to OWASP Top 10 2017 A4 - XML External Entity (XXE)

3.2 A2 – Broken Authentication and Session Management

Please refer to OWASP Top 10 2017 A2 - Broken Authentication

3.3 A3 – Cross-Site Scripting (XSS)

Please refer to OWASP Top 10 2017 A7 - Cross-Site Scripting (XSS)

3.4 A4 – Insecure Direct Object References

Please refer to OWASP Top 10 2017 A5 - Broken Access Control

3.5 A5 – Security Misconfiguration

Please refer to OWASP Top 10 2017 A6 - Security Misconfiguration

3.6 A6 – Sensitive Data Exposure

Please refer to OWASP Top 10 2017 A3 - Sensitive Data Exposure

3.7 A7 – Missing Function Level Access Control

Please refer to OWASP Top 10 2017 A5 - Broken Access Control

3.8 A8 - Cross-Site Request Forgery (CSRF)

Cross-Site Request Forgery (CSRF) is an attack that forces an end user to execute unwanted actions on a web application in which they're currently authenticated. CSRF attacks specifically target state-changing requests, not theft of data, since the attacker has no way to see the response to the forged request. With a little help of social engineering (such as sending a link via email or chat), an attacker may trick the users of a web application into executing actions of the attacker's choosing. If the victim is a normal user, a successful CSRF attack can force the user to perform state-changing requests like transferring funds, changing their email address, and so forth. If the victim is an administrative account, CSRF can compromise the entire web application⁴⁹.

3.8.1.1 Prevention Techniques

There are multiple techniques usable for preventing CSRF attacks which are further documented at Cross-Site Request Forgery (CSRF) Prevention Cheat Sheet⁵⁰.

Synchronizer Token Pattern

With Synchronizer Token Pattern, any state-changing operation requires a secure random token which is generally known as CSRF Token. CSRF Token is a unique large random string generated per user session, using a secure pseudorandom number generator.

Once user session is created, per-session CSRF Token should be added to the server session as well. Basically, the storage of CSRF Token happens in server session.

Generated CSRF Token should be added to all the forms, generating requests for state-changing operations, as a hidden input. The server should do additional validation and reject the request if the CSRF Token sent in the request does not match with the token available in the user session. In addition to forms, AJAX requests for state-changing operations should also include the CSRF Token.

Ultimately, the attacker will not be able to make a form submission to protected application, since attacker will need to know the per-session CSRF Token value in advance, to craft the attack vector.

This pattern is further discussed in "Core J2EE Patterns"⁵¹.

Double Submit Cookie

The difference between "Double Submit Cookie" and "Synchronize Token Pattern" is that, "Synchronize Token Pattern" uses server session for storing CSRF Token, whereas "Double Submit Cookie" approach uses cookies to store the CSRF Token.

Upon user login, per-session CSRF token value should be generated and it should be sent to the browser as a cookie. The server may decide not to store the CSRF token value.

When rendering HTML pages with forms, client side JavaScript should read the CSRF Token cookie value and inject the value all forms. In addition, AJAX requests should be modified to send the same value with AJAX requests.

When an application sends an HTTP request for a state changing operation (over HTTP method other than GET), the server should compare the CSRF Token received over the request cookies, with the CSRF Token value received in request payload (query parameter/post data). If values are not matching, request can be identified as a possible CSRF attack.

If an attacker creates a forged HTTP request with the intention of performing a CSRF attack, correct CSRF Token value will still be sent in the cookie, but they will not have required cross domain access to read the token value and inject same into the HTML forms.

3.8.1.2 Java Specific Recommendations

• WSO2 products based on WSO2 Carbon Kernel versions prior to 4.4.6 use "Referer Header" based CSRF prevention, which is no longer recommended.
• WSO2 products based on WSO2 Carbon Kernel 4 (4.4.6+) should use "Synchronizer Token Pattern" based CSRF prevention, using OWASP CSRFGuard⁵².
• WSO2 Carbon Kernel 5+ based products and any new applications should use "Double Submit Cookie" approach in CSRF prevention.

Synchronizer Token Pattern (WSO2 Carbon Kernel 4 (4.4.6+))

OWASP CSRFGuard is used to implement Synchronizer Token Pattern in WSO2 Carbon Kernel 4 (4.4.6+) products. OWASP CSRFGuard provides required classes to generate per-session token and to do necessary validation on state-changing operations. Furthermore, it provides a JavaScript which is capable of dynamically adding CSRF Token as a hidden input and override XMLHttpRequest to include CSRF Token in AJAX requests.

In summary, when integrating OWASP CSRFGuard with a product, it is required to do following changes:

• Make sure all the state-changing operations are performed using HTTP methods other than GET. GET requests must not be used for state-changing operations.
• In web.xml or jaggery.conf file, add CsrfGuardServletContextListener.
  • This class is responsible for loading CSRFGuard configuration and doing the initialization of component.
• In web.xml or jaggery.conf file, add CsrfGuardHttpSessionListener.
  • This class is responsible for generating and storing per-session CSRF Token.
• In web.xml or jaggery.conf file, add CsrfGuardFilter.
  • This class is responsible for comparing the submitted CSRF Token with the token in user session, and generating error response if an attack was detected.
• In web.xml or jaggery.conf file, add JavaScriptServlet.
  • This class is responsible for serving the JavaScript file that is dynamically adding the CSRF Token to forms and AJAX requests.
• Include "JavaScriptServlet" in the HTML template of the application, so that <head> element of all pages that need to be protected, should have JavaScriptServlet as the first JavaScript inclusion.
• Prepare and store per-application CSRF configuration file according to "repository/conf/security/Owasp.CsrfGuard.Carbon.properties" or reuse the Carbon CSRF configuration.
• Do thorough testing on CSRF protected application to verify that there is no functional impact.

Note : CSRF is dropped in OWASP top 10 list of the most critical application security risks in 2017.

3.9 A9 – Using Known Vulnerable Components

Please refer to OWASP Top 10 2017 A9 - Using Known Vulnerable Components

3.10 A10 - Unvalidated Redirects and Forwards

Unvalidated redirects and forwards are possible when a web application accepts untrusted input that could cause the web application to redirect the request to a URL contained within untrusted input. By modifying untrusted URL input to a malicious site, an attacker may successfully launch a phishing scam and steal user credentials. Because the server name in the modified link is identical to the original site, phishing attempts may have a more trustworthy appearance. Unvalidated redirect and forward attacks can also be used to maliciously craft a URL that would pass the application's access control check and then forward the attacker to privileged functions that they would normally not be able to access⁵⁵.

3.10.1.1 Prevention Techniques

Absolute forward URLs or fragments of forward URLs should not be accepted from the end-user during any operation. This is advised since an attacker can use a less restricted resource with a malicious forward, to circumvent URL based security control in place and send requests to a restricted resource.

Absolute redirect URLs should not be accepted from the end-user during any operation, apart from administrative configurations.

The general recommendation is to avoid accepting redirect or forward URL fragments (used to build absolute or relative redirect/forward URLs) from the end-user.

However, if there is such requirement, it is necessary to follow language specific recommendations for validating the accepted URL fragment.

3.10.1.2 Java Specific Recommendations

If an absolute URL is accepted from the end-user, it should be validated against a list of allowed redirect/forward URLs. However, since this the condition explained in Approval Required blocks above, Platform Security Team should be informed on the use case and approval is required.

When a portion of the redirect or forward URL is expected from the user, it is a must to validate if the appended fragment contains only the expected type of a value.

Note: Unvalidated Redirects and Forwards is dropped in OWASP top 10 list of the most critical application security risks in 2017.

4. OWASP Mobile Top 10 Prevention

4.1 Introduction

This section discusses OWASP Mobile Top 10 prevention techniques that should be followed by WSO2 engineers while engineering mobile applications.

4.2 M1 - Improper Platform Usage

This category covers misuse of a platform feature or failure to use platform security controls. It might include Android intents, platform permissions, misuse of TouchID, the Keychain, or some other security control that is part of the mobile operating system⁵⁷.

4.2.1 Prevention Techniques

The best method of preventing improper platform usage is to follow the guidelines and best practices published by the respective platforms, on secure development of the mobile applications. These guidelines include the proper way to implement the relevant features and how to maintain them. Also, make sure to check whether the application behaves as it was intended without misusing platform features.

4.2.1.1 Android Specific Recommendations

Using Intents

Intents are used to request an action such as starting an activity, starting a service or delivering a broadcast from another app component. Explicit intents specify the component by the fully qualified class name. Usually, these are used to start a component within the app. Implicit intents declare a general action to be handled by another application.

Do not use an implicit intent to start a service as implicit intents allow any other app component to respond to the intent and start the Service transparently to the user. Use an explicit intent where the component can be specified using the fully qualified class name.

Note: From Android 5.0 (API level 21) onwards, the system will throw an exception if an implicit intent is used to start a service.

Set the android:exported attribute to false for the specific activity, service or receiver to limit the exposure to other components.

Data received by public components should be treated as untrusted data and needs to be properly validated and sanitized before usage.

4.3 M2 - Insecure Data Storage

This category covers insecure data storage and unintended data leakage⁵⁷.

4.3.1 Prevention Techniques

4.3.1.1 Client-side data storage

Data stored in the mobile device have a risk of being exposed to an outside side attacker through a malware or a lost/stolen device. To reduce the impact of an attack, limit the amount of data stored in a mobile device and use a strong encryption algorithm to encrypt the stored data.

Android Specific Recommendations

Using internal storage

Do not create files with permissions MODE_WORLD_WRITEABLE⁵⁹ or MODE_WORLD_READABLE⁶⁰ as all applications will have permission to read or write to the file.Use a content provider⁶¹ to allow other applications to securely access and modify data.

Note: Constants MODE_WORLD_WRITEABLE and MODE_WORLD_READABLE were deprecated in API level 17

Using external storage

Files stored in the external storage has global read and write permissions. Therefore external storage should not be used to store any sensitive data. Also, input validations should be applied when using data from the external storage.

It is not recommended to store executable files in the external storage. If the application requires loading executable files from external storage, make sure to sign and cryptographically verify the content before dynamic loading.

Database Related Recommendations

Using a third party encryption can withstand a threat to the native protection provided by the OS. The master key for the encryption should be randomly generated and encrypted using a passphrase from the user by the time the data is processed. Unencrypted master key or the passphrase should not be stored on the device.

4.3.1.2 HTTP response caching

HTTP responses can have sensitive information. Caching them can increase the risk of data leakage. Therefore response caching should be disabled for sensitive data.

Android Specific Recommendations

Do not use HttpResponseCache⁶³ to cache sensitive data.

4.3.1.3 Keyboard press caching

Mobile devices cache the keyboard input to be used in auto-suggesting words to the user. This feature can be a vulnerability when sensitive information of one user is cached and later suggests to a different user. Therefore auto suggest feature must be disabled to avoid keyboard press caching for sensitive information.

Android Specific Recommendations

The user dictionary in android saves the words entered by the user to be used for auto-correction. Since this dictionary is available to other applications without permission, sensitive information can get leaked to other apps. To prevent this, use android:inputType= “textNoSuggestions” for sensitive data fields or create custom keyboards with auto-suggest disabled.

4.3.1.4 Copy/Paste buffer caching

When using copy and paste, the data is initially copied into a clipboard. Malicious applications have the ability to access this clipboard cache to extract sensitive data. Disable the copy/paste functionality for sensitive data fields to prevent these type of copy/paste buffer caching.

4.3.1.5 Logging

Debug logs are used to identify flaws in the application. However, information provided by debug logs can be useful for an attacker to gather knowledge about the application. Therefore debug logs should be disabled in the production environment.

Android Specific Recommendations

Logs are a shared resource for Android. Any application with READ_LOGS permission can view them. Production applications should limit the logging by using debug flags and configure logging levels by defining custom log classes.

4.3.1.6 HTML5 local storage

HTML5 local storage can be used to store data within the browser between HTTP requests. Since this storage is accessible by JavaScript, cross-site scripting attacks can be used to steal the data. Therefore sensitive data should not be stored in HTML5 local storage.

4.3.1.7 Browser Cookie objects

Cookies are used by servers to store data in the browser. Often session related data are stored inside cookies. Use Secure flag to indicate that only HTTPS requests are allowed to transfer cookies and HTTP Only flag to make cookies inaccessible to JavaScript's Document.cookie API. Make sure that both Secure and HTTP Only flags are set to cookies containing sensitive data.

4.3.1.8 Analytics data sent to 3rd parties

Mobile applications may need to have access to user’s personal information for functionality purposes. However, the application should make sure that these sensitive pieces of information are not being sent to third parties violating the privacy of the user.

4.4 M3 - Insecure Communication

This section covers poor handshaking, incorrect SSL versions, weak negotiation, cleartext communication of sensitive assets, etc⁵⁷.

4.4.1 Prevention Techniques

4.4.1.1 SSL/TLS

Use TLS to serve all sensitive and nonsensitive traffic. This will prevent from having mixed SSL sessions where the user’s session ID might get exposed.

Android Specific Recommendations

The application can choose to avoid unencrypted HTTP traffic by using cleartextTrafficPermitted="false" in the network security config file⁶⁴.

4.4.1.2 Self Signed Certificates

Usually, self-signed certificates are used in the development stage for the easiness of the developer. However, do not accept self-signed certificates in the production application as they allow an attacker to easily intercept the communication using their own self-signed certificate.

4.4.1.3 Certificate Pinning

The application accepts all the certificates signed by trusted Certificate Authorities(CAs). If one of these trusted CA gets compromised and started issuing fraudulent certificates, they will be accepted by the application. Certificate pinning can be used store the certificate locally along with the domain name. This allows detecting fraudulent certificates as the application can compare previously stored certificate with the new one.

Android Specific Recommendations

Note: Always include a backup pin to use in an event of a certificate change.

Note: To protect from compromised CAs android has the ability to denylist certain certificates or even whole CAs. While this list was historically built into the operating system, starting in Android 4.2 this list can be remotely updated to deal with future compromises⁶⁵.

4.4.1.4 Hostname Verification

Hostname verification is verifying whether the hostname of the server that the application is trying to connect is specified in the certificate presented by the server. It is important to have this verification as this will make sure that the server has presented the right certificate.

Android Specific Recommendations

SSLSocket⁶⁶ does not perform hostname verification. The application has to verify the hostname of the certificate by calling the method getDefaultHostnameVerifier() with the expected hostname.

Using org.apache.http.conn.ssl.AllowAllHostnameVerifier or SSLSocketFactory.ALLOW_ALL_HOSTNAME_VERIFIER allows the application to accept all certificates. Make sure that these are not being used in the production code.

Note: org.apache.http.conn.ssl.AllowAllHostnameVerifier and SSLSocketFactory.ALLOW_ALL_HOSTNAME_VERIFIER were deprecated in API level 22.

4.4.1.5 SMS

SMS is a protocol that has been designed for user to user communication. It is unencrypted and not properly authenticated. Therefore, do not use SMS for sensitive data transfer.

Android Specific Recommendations

In Android, SMS messages are transmitted as broadcast intents. Anyone with the READ_SMS permission can read the SMS messages on the device. Therefore, do not use SMS for data transfer.

4.5 M4 - Insecure Authentication

This category captures notions of authenticating the end user or bad session management.This can include:

• Failing to identify the user at all when that should be required
• Failure to maintain the user's identity when it is required
• Weaknesses in session management⁵⁷

4.5.1 Prevention Techniques

4.5.1.1 Local Authentication

Local Authentication can be bypassed if the attacker has the ability to tamper the application. Therefore all the authentication checks should be performed by a backend server.

4.5.1.2 Password Policy

Ensure that a strong password policy is enforced by the authentication server. Use a similar strong password policy if the authentication is performed locally with the approval mentioned in the Local Authentication section.

4.5.1.3 Device Identifiers

Do not use device-specific identifiers for authentication. In an event of change of ownership of the phone, these IDs can expose previous owner’s data to the new owner.

4.5.1.4 Geo-Location

Geo-location should not be used as an authentication mechanism as attackers can easily spoof the geo-location. If an application requires the use of geo-location, ensure that the application has implemented proper geo-location spoof detection mechanism to detect location anomalies.

4.5.1.5 Token Revocation

In the event of a lost or stolen device, the application should have the ability to invalidate the authenticated session on the lost/stolen device by revoking a device-specific token.

4.6 M5 - Insufficient Cryptography

The code applies cryptography to a sensitive information asset. However, the cryptography is insufficient in some way. Note that anything and everything related to TLS or SSL goes in M3. Also, if the app fails to use cryptography at all when it should, that probably belongs in M2. This category is for issues where cryptography was attempted, but it wasn't done correctly⁵⁷.

4.6.1 Prevention Techniques

4.6.1.1 Key Generation

Use a secure random number generator to generate strong keys that can withstand brute force attacks.

Android Specific Recommendations

Use the secure random number generator, SecureRandom⁶⁷ to initialize any cryptographic keys generated by KeyGenerator⁶⁸. Use of a key that is not generated with a secure random number generator significantly weakens the strength of the algorithm and may allow offline attacks.

4.6.1.2 Key Storage

Do not store keys in plaintext format. Use a cryptographic vault for secure storage of keys and make sure that compromised or outdated keys are properly revoked.

Android Specific Recommendations

Use the KeyStore⁶⁹ for long-term storage and retrieval of cryptographic keys.

4.6.1.3 Custom Encryption Algorithms

Do not implement custom encryption algorithms. Use a proper algorithm that is widely used and accepted as secure.

Android Specific Recommendations

Do not write custom protocols for implementing secure tunnels. Instead use HttpsURLConnection⁷⁰ or SSLSocket⁶⁶. If a custom protocol is needed, do not implement new algorithms. Instead, use existing cryptographic algorithms such as the implementations of AES and RSA provided in the Cipher⁷¹ class.

4.6.1.4 Deprecated Algorithms

OWASP defines following algorithms as deprecated and not to be used for encryption purposes⁷².

• RC2
• MD4
• MD5
• SHA1

4.7 M6 - Insecure Authorization

This is a category to capture any failures in the authorization. It is distinct from authentication issues. If the app does not authenticate users at all in a situation where it should, then that is an authentication failure, not an authorization failure⁵⁷.

4.7.1 Prevention Techniques

4.7.1.1 Permissions

Do not use roles and permission information coming from the mobile device for authorization. Backend code should independently verify that any incoming identifiers associated with a request (operands of a requested operation) that come along with the identify match up and belong to the incoming identity.

It is a must to request only the minimum number of permissions that your app requests, to reduce the risk of misusing permissions. In application documentation, it is required to mention the actual usage of each permission, so that users and security reviewers have access to this information.

Android Specific Recommendations

In Android, permissions are categorized into four protection levels⁷³.

normal

The default value. A lower-risk permission that gives requesting applications access to isolated application-level features, with minimal risk to other applications, the system, or the user. The system automatically grants this type of permission to a requesting application at installation, without asking for the user's explicit approval (though the user always has the option to review these permissions before installing).

dangerous

A higher-risk permission that would give a requesting application access to private user data or control over the device that can negatively impact the user. Because this type of permission introduces potential risk, the system may not automatically grant it to the requesting application. For example, any dangerous permissions requested by an application may be displayed to the user and require confirmation before proceeding, or some other approach may be taken to avoid the user automatically allowing the use of such facilities.

signature

A permission that the system grants only if the requesting application is signed with the same certificate as the application that declared the permission. If the certificates match, the system automatically grants the permission without notifying the user or asking for the user's explicit approval.

signatureOrSystem

A permission that the system grants only to applications that are in the Android system image or that are signed with the same certificate as the application that declared the permission.The "signatureOrSystem" permission is used for certain special situations where multiple vendors have applications built into a system image and need to share specific features explicitly because they are being built together.

When defining new permissions, use the dangerous protection level if the permission has the ability to access the stored data or affect the operation of other applications. Use the signature protection level if the permission has the ability to share data between applications signed with the same certificate.

When exposing data over Interprocess Communication(IPC), check the permissions of the data that are being exposed. Other applications might not have the same level of permission for the exposed data.

4.7.1.2 Proof Key for Code Exchange (PKCE)

Using OAuth 2.0 authorization code grant type is susceptible to interception attacks. An attacker has the ability to intercept the authorization code received from the authorization endpoint via an unprotected communication such as inter-application communication within the client’s operating system. Proof Key for Code Exchange (PKCE)⁷⁴ is used to mitigate this vulnerability. In PKCE,

1. A unique cryptographic random key (code verifier) is created by the application with every authorization request.
2. The code verifier is transformed into a code challenge and sent to the authorization server along with the transform method.
3. The authorization server stores the code challenge and the transform method.
4. During the request for an access token, the application has to send it’s generated code verifier to the authorization server.
5. The server transforms the received code verifier with the stored transform method and compares with the previously stored code challenge.

4.8 M7 - Poor Code Quality

This would be the catch-all for code-level implementation problems in the mobile client. That's distinct from server-side coding mistakes. This would capture things like buffer overflows, format string vulnerabilities, and various other code-level mistakes where the solution is to rewrite some code that's running on the mobile device⁵⁷.

4.8.1 Prevention Techniques

4.8.1.1 Input Validation

All the input from the app and user should be treated as untrusted data. Therefore, input validation must be used when handling such data.

Android Specific Recommendations

When accessing a content provider, use parameterized query methods such as query(), update(), and delete() to prevent from potential SQL injection from untrusted sources.

If the content provider is serving files based on filename, make sure that path traversals are filtered out.

4.8.1.2 Third Party Libraries

Applications rely heavily on third-party libraries. Making the application secure won’t be enough if the third party libraries contain vulnerabilities. Therefore, security auditing must thoroughly test third-party libraries for vulnerabilities.

4.8.1.3 Buffer Overflows

Buffer overflows are not possible in Java. However, the application is susceptible to buffer overflows if it contains native code such as C or C++. To avoid buffer overflows, make sure that length of the incoming buffer data will not exceed the length of the target buffer.

Android Specific Recommendations

Run Android Lint⁷⁵ on the application code using Android SDK and correct any identified issues.

4.9 M8 - Code Tampering

This category covers binary patching, local resource modification, method hooking, method swizzling, and dynamic memory modification.

Once the application is delivered to the mobile device, the code and data resources are resident there. An attacker can either directly modify the code, change the contents of memory dynamically, change or replace the system APIs that the application uses, or modify the application's data and resources. This can provide the attacker a direct method of subverting the intended use of the software for personal or monetary gain⁵⁷.

4.9.1 Prevention Techniques

4.9.1.1 Tamper Detection

Use tamper detection techniques such as checksums or digital signatures to identify code tampering.

In an event of code tampering use an appropriate mechanism such as wipe the user data or send a notification to the server to protect the sensitive data.

4.9.1.2 Restricting Debuggers

Prevent the operating system from permitting to attach a debugger to the application. This will increase the complexity of an attack⁷⁶.

Android Specific Recommendations

Set the android:debuggable=”false” in the application manifest to restrict the debuggers.

4.9.1.3 Stripping Binaries

Strip the native binaries to increase the difficulty for an attacker to debug or reverse engineer.

4.10 M9 - Reverse Engineering

This category includes analysis of the final core binary to determine its source code, libraries, algorithms, and other assets. Binary inspection tools give the attacker insight into the inner workings of the application. This may be used to exploit other nascent vulnerabilities in the application, as well as revealing information about back-end servers, cryptographic constants and ciphers, and intellectual property⁵⁷.

4.10.1 Prevention Techniques

4.10.1.1 Obfuscation

Use an obfuscation tool to modify the code to become difficult to understand if the application gets decompiled.To measure the effectiveness of the obfuscator, use a deobfuscator.

Android Specific Recommendations

ProGuard⁷⁷ is a Java class file shrinker, optimizer, obfuscator, and preverifier. The obfuscation step in ProGuard renames the remaining classes, fields, and methods using short meaningless names making it difficult for attackers to reverse engineer.

4.10.1.2 Hide Application Logic

In an event of reverse engineering, the attacker might get hold of the source code containing all the application logic. To reduce the impact critical parts of the application logic can be moved to a web service.

4.10.1.3 Use C/C++

Java is easier to decompile when compared to C/C++. Use C/C++ to write security sensitive sections of the code to make reverse engineering difficult for an attacker.

4.11 M10 - Extraneous Functionality

Often, developers include hidden backdoor functionality or other internal development security controls that are not intended to be released into a production environment. For example, a developer may accidentally include a password as a comment in a hybrid app. Another example includes disabling of 2-factor authentication during testing⁵⁷.

4.11.1 Prevention Techniques

• Examine the app's configuration settings to discover any hidden switches.
• Verify that all test code is not included in the final production build of the app.
• Examine all API endpoints accessed by the mobile app to verify that these endpoints are well documented and publicly available.
• Examine all log statements to ensure nothing overly descriptive about the backend is being written to the logs⁷⁸.

5. General Recommendations for Secure Coding

5.1 Server Side Request Forgery (SSRF) Prevention

By providing URLs to unexpected hosts or ports, attackers can make it appear that the server is sending the request, possibly bypassing access controls such as firewalls that prevent the attackers from accessing the URLs directly. The server can be used as a proxy to conduct port scanning of hosts in internal networks, use other URLs such as that can access documents on the system (using file://), or use other protocols such as gopher:// or tftp://, which may provide greater control over the contents of requests⁷⁹.

This is identified as a high severity attack because it allows an attacker to perform tasks which are usually prevented by the perimeter security devices such as firewalls. For example, an attacker would be able to appear to internal and external nodes as the vulnerable host and perform following:

• Scan a network segment which is behind a firewall by analyzing responses received from the vulnerable host.
• Perform a service enumerate attack by enumerating through services that are running on a particular host.
• Bypass host based restrictions, policies or authentication mechanisms in place.
• Query and attack internal hosts that are not normally accessible.

In detail explanations and analysis on SSRF can be found at SSRF Bible Cheatsheet⁸⁰.

5.1.1.1 Prevention Techniques

Avoid Using User Inputs in Backend Requests

Avoid accepting information used in internal requests such as following from the user:

• URLs used in back channel operations (backend calls made, where WSO2 product acts as the client).
• IP addresses used in internal or back channel operations.
• Unvalidated and unrestricted system paths used in internal file access.
• Unvalidated XML payloads that are passed to XML parsers without proper security attributes set.

Perform Strict Error Handling

Display minimum information on the client side at event of an error or something unexpected occurs. For example, if content type validation failed, provide a generic error message such as "Invalid Data Retrieved". Also, ensure that the error message is same when the request fails in the backend and if invalid data is retrieved. This will make it hard to distinguish between open and closed ports or services.

Perform Strict Response Handling

Validate responses received from the remote resource. If a certain content type is expected by the application, validate the received response at the server-side before displaying it on client's side or processing it for the client.

5.1.1.2 Java Specific Recommendations

Proper Usage of Java Security Manager

Java Security Manager should be enabled in any production deployment and any other deployment that requires additional security (such as environments used for security related testing).

Steps relevant to enabling Java Security Manager is explained in "Enabling Java Security Manager" section of the "Administration Guide"⁸¹. Security policy file should be used to only allow network level connections to trusted and pre-identified hosts or subnets, using SocketPermission⁸².

5.2 ClickJacking and Cross Frame Scripting Prevention

Clickjacking, also known as a "UI redress attack", is when an attacker uses multiple transparent or opaque layers to trick a user into clicking on a button or a link on another page when they were intending to click on the top level page. Thus, the attacker is "hijacking" clicks meant for their page and routing them to another page, most likely owned by another application, domain, or both⁸³.

Cross-Frame Scripting (XFS) is an attack that combines malicious JavaScript with an iframe that loads a legitimate page in an effort to steal data from an unsuspecting user. This attack is usually only successful when combined with social engineering. An example would consist of an attacker convincing the user to navigate to a web page the attacker controls. The attacker's page then loads malicious JavaScript and an HTML iframe pointing to a legitimate site. Once the user enters credentials into the legitimate site within the iframe, the malicious JavaScript steals the keystrokes⁸⁴.

5.2.1.1 Prevention Techniques

X-Frame-Options

X-Frame-Options HTTP response header should be used to indicate that the browser should not allow rendering relevant response in a <frame> or <iframe>. "X-Frame-Options: DENY" should be used in all the cases, except in situations where the product itself needs to frame a page exposed elsewhere in the same product. In a situation where product itself needs to frame a page exposed elsewhere in the same product, "X-Frame-Options: SAMEORIGIN" can be used.

Content-Security-Policy

In addition to non-standard X-Frame-Options header, the standard frame-ancestors directive can be used in a Content-Security-Policy HTTP response header to indicate whether or not a browser should be allowed to render a page in a <frame> or <iframe>. "Content-Security-Policy: frame-ancestors 'none'" should be used in all cases, except in situations where the product itself needs to frame a page exposed elsewhere in the same product. In a situation where product itself needs to frame a page exposed elsewhere in the same product, "Content-Security-Policy: frame-ancestors 'self'" can be used.

5.2.1.2 Java Specific Recommendations

X-Frame-Options

"org.apache.catalina.filters.HttpHeaderSecurityFilter" Servlet Filter should be used to add X-Frame-Options header to the HTTP response.

5.3 Random Number Generation

When an undesirably low amount of entropy is available, Pseudo Random Number Generators are susceptible to suffer from insufficient entropy when they are initialized, because entropy data may not be available to them yet⁸⁵. This will leave patterns or clusters of values that are more likely to occur than others⁸⁶.

5.3.1.1 Java Specific Recommendations

Avoid usage of java.util.Random class for security sensitive operations and use java.security.SecureRandom.

• Periodically throw away the existing java.security.SecureRandom instance and create a new one. This will generate a new instance with a new seed.
• Periodically add new random material to the PRNG seed by making a call to java.security.SecureRandom.setSeed(java.security.SecureRandom.generateSeed(int)).⁸⁷
• SecureRandom.getInstanceStrong() introduced with Java 8 should not be used in web applications, since application gets blocked until PRNG could collect required amount of entropy. This could affect Docker containers badly, since when instance spawning happens, entropy gathering will take significant amount of time^88,89.

5.4 Cross-Origin Resource Sharing

A wildcard same-origin policy is appropriate when a page or API response is considered completely public content and it is intended to be accessible to everyone, including any code on any site.

"Access-Control-Allow-Origin: *" should not be used in WSO2 products. If such option should be available, user/admin should have a way to configure between using wildcard and using domain restriction.

5.5 Security Related HTTP Headers

There are HTTP response headers that can be used to configure the security controls enforced by browsers.

5.6 Securing Cookies

Sensitive cookies such as JSESSIONID should be secured to avoid stealing the cookies from an insecure network or with the use of an XSS vulnerability of the application.

5.6.1 Avoiding sending of cookies over insecure (unencrypted) networks

By setting "secure" attribute of the cookie, it is possible to instruct the browser not to send the cookie over insecure networks (not to send the cookie unless HTTPS is used).

5.6.2 Avoiding cookies being read by JavaScript and other client side scripts

If an XSS vulnerability is present in the web application a malicious JavaScript would be able to read sensitive information stored in cookies and communicate those to an external party.

An attacker could leverage this pattern to use an XSS vulnerability to read the session cookie from the victim's browser and conduct session hijacking attack.

To prevent JavaScripts and other client-side scripts from access cookie values, "HttpOnly" attribute should be set on the cookie.

5.6.3 Summary of Recommendations

All cookies containing sensitive information should:

• Include "secure" attribute
• Include "HttpOnly" attribute
• Include "path" attribute and the "path" attribute should contain accurate context information
• Not include "expires" attribute (cookie should be a session cookie)

5.7 ZIP Slip Vulnerability Prevention

Zip Slip is a critical directory traversal security vulnerability that can be exploited when extracting files from an archive¹²⁵. To exploit this vulnerability an attacker can create a malicious archive that holds directory traversal file names (e.g. ../../../../evil.sh). Exploitation of the said vulnerability could lead to overwriting executable files outside the target folder where the archived files are being extracted, thus causing the possibility of remote command execution.

This vulnerability can also be used to overwrite configuration files and other sensitive resources. The Snyk Security team is maintaining a GitHub repository¹²⁶ with all projects that have been found vulnerable to Zip Slip and have been responsibly disclosed to, including fix dates and versions.

5.7.1 Java Specific Recommendations

When using the java.util.zip library, the name of the file that is being extracted needs to be validated before storing.

When using a library other than java.util.zip, refer the Snyk Security team’s GitHub repository¹²⁶ for a fixed higher version.

6. Tooling Recommendations for Secure Coding

6.1 Security Related Dynamic Analysis

OWASP Zed Attack Proxy⁹⁰ (ZAP) is the recommended tool for performing dynamic security analysis.

6.2 Security Related Static Code Analysis

Find Security Bugs ⁹¹, FindBugs plugin is the recommended tool for performing static security analysis.

6.3 Dependency Vulnerability Analysis

OWASP Dependency Check⁹² is the recommended tool for performing dependency vulnerability analysis.