Enterprise API adoption has gone beyond predictions. APIs have become the 'coolest' way of exposing
business functionalities to the outside world. Both your public and private APIs need to be protected,
monitored, and managed. This white paper focuses on API security. There are many options available that
could be very confusing. When should you select one over another is a question that frequently comes up –
and you need to cautiously identify and isolate the tradeoffs.
Security is not an afterthought. It has to be an integral part of any development project. The same applies
to APIs as well. API security has evolved significantly in the past five years. The growth of standards to
date has been exponential. OAuth is the most widely adopted standard, and is possibly now the de-facto
standard for API security
2. What is OAuth?
The web community (e.g. Google, Yahoo, and Flicker) banded together to solve a perplexing security
challenge: How to authorize user access to resources and delegate authorization decisions across trusted
parties. OAuth is a security standard based on many pre-OAuth vendor specific protocols such as Google
AuthSub, Yahoo BBAuth, and Flicker Auth.
3. OAuth 1.0 vs. OAuth 2.0 — What's the Difference Between the Two?
OAuth 1.0 is a standard, built for identity delegation. OAuth 2.0 is a highly extensible authorization
framework. The best selling point in OAuth 2.0 is its extensibility by being an authorization framework.
OAuth 1.0 is coupled with signature-based security. Although it has provisions to use different signature
algorithms, it's still signature based. One of the key negative comments on OAuth 1.0 is the burden
enforced on OAuth clients for signature calculation and validation. This is not a completely valid argument.
This is where we need proper tools to the rescue. Why does an application developer need to worry about
signature handling? Delegate that to a third-party library and stay calm. If you think OAuth 2.0 is better
than OAuth 1.0 because of the simplicity added through the OAuth 2.0 Bearer Token profile (against the
signature based tokens in 1.0) – you've been misled.
To re-iterate this point, the biggest advantage of OAuth 2.0 is its extensibility. The core OAuth 2.0
specification is not tightly coupled with a token type. There are several OAuth profiles discussed under the
IETF OAuth working group at the moment. The Bearer token profile is already a proposed IETF standard -
4. Bearer Token Profile
The Bearer token profile is currently the most widely used for API Security. The access token used under
the Bearer token profile is a randomly generated string. Anyone who is in possession of this token can use it
to access a secured API. In fact, that is what its name implies too. The protection of this token is facilitated
through the underlying transport channel via TLS. TLS only provides the security while in transit. It"s the
responsibility of the OAuth Token Issuer (or the Authorization Server) and the OAuth client to protect
the access token while being stored. In most cases, the access token needs to be encrypted. Moreover,
the token issuer needs to guarantee the randomness of the generated access token, and it has to be long
enough to exhaust any brute-force attacks.
5. OAuth 2.0
OAuth 2.0 has three major phases
(to be precise, Phase 1 and Phase 2 could overlap based on the grant type).
- Requesting an Authorization Grant
- Exchanging the Authorization Grant for an Access Token
- Access the resource with the Access Token
OAuth 2.0 core specification does not mandate any access token type. In addition, the requester or the
client cannot decide which token type it needs. It"s purely up to the Authorization Server to decide which
token type is to be returned in the Access Token response, which is Phase 2.
The access token type provides the client the information required to successfully utilize the access token
to make a request to the protected resource (along with type-specific attributes). The client must not use
an access token if it does not understand the token type.
Each access token type definition specifies the additional attributes (if any) sent to the client together with
the "access_token" response parameter. It also defines the HTTP authentication method used to include
the access token when making a request to the protected resource.
For example, the following is what you get for the Access Token response irrespective of which grant
type you use (to be precise, if the grant type is client credentials, there won"t be any refresh_token in the
HTTP/1.1 200 OK
The above is for the Bearer and the following is for the MAC.
HTTP/1.1 200 OK
6. MAC Token Profile
The MAC token profile is very much closer to what we have in OAuth 1.0.
The OAuth authorization server will issue a MAC key along with the signature algorithm to be used and an
access token that can be used as an identifier for the MAC key. Once the client has access to the MAC key,
it can use it to sign a normalized string derived from the request to the resource server. Unlike in Bearer
token, the MAC key will never be shared between the client and the resource server. It"s only known to the
authorization server and the client. Once the resource server gets the signed message with MAC headers, it
has to validate the signature by talking to the authorization server. Under the MAC token profile, TLS is only
needed for the first step, the initial handshake where the client gets the MAC key from the authorization
server. Calls to the resource server need not be on TLS, as we never expose the MAC key over the wire.
MAC access token response has two additional attributes. mac_key and the mac_algorithm. To rephrase
"each access token type definition specifies the additional attributes (if any) sent to the client together with
the "access_token" response parameter".
The MAC token profile defines the HTTP MAC access authentication scheme, providing a method for
making authenticated HTTP requests with partial cryptographic verification of the request, covering the
HTTP method, request URI, and host. In the above response, access_token is the MAC key identifier. Unlike
in Bearer, the MAC token profile never passes its top secret over the wire.
The access_token, or the MAC key identifier, is a string identifying the MAC key used to calculate the
request MAC. The string is usually opaque to the client. The server typically assigns a specific scope
and lifetime to each set of MAC credentials. The identifier may denote a unique value used to retrieve
the authorization information (e.g. from a database), or self-contain the authorization information in a
verifiable manner (i.e. a string consisting of some data and a signature).
The mac_key is a shared symmetric secret used as the MAC algorithm key. The server will not re-issue a
previously issued MAC key and MAC key identifier combination.
Phase 3 will utilize the access token obtained in Phase 2 to access the protected resource.
Shown below is the Authorization HTTP header when the Bearer token is used.
Authorization: Bearer mF_9.B5f-4.1JqM
This adds very low overhead on the client side. It simply needs to pass the exact access_token it got from
the Authorization Server in Phase 2.
Under the MAC token profile, this is how it would look like.
Authorization: MAC id="h480djs93hd8"
id is the MAC key identifier or the access_token from Phase 2.
ts is the request timestamp. The value is a positive integer set by the client when making each request to
the number of seconds elapsed from a fixed point in time (e.g. January 1, 1970 00:00:00 GMT). This value is
unique across all requests with the same timestamp and MAC key identifier combination.
nonce is a unique string generated by the client. The value is unique across all requests with the same
timestamp and MAC key identifier combination.
The client uses the MAC algorithm and the MAC key to calculate the request mac.
Either we use Bearer or MAC - the end user or the resource owner is identified using the access_token.
Authorization, throttling, monitoring or any other quality of service operations can be carried out against
the access_token irrespective of which token profile you use.
APIs are not just for internal employees. Customers and partners can access public APIs, where we do not
maintain credentials internally. In that case, we cannot directly authenticate them. So, we would need to
have a federated authentication setup for APIs, where we would trust a given partner domain, but not
individuals. The SAML 2.0 Bearer Assertion Profile for OAuth 2.0 addresses this concern.
7. SAML 2.0 Bearer Assertion Profile
The SAML 2.0 Bearer Assertion Profile, which is built on top of OAuth 2.0 Assertion Profile, defines the
use of a SAML 2.0 Bearer Assertion as a way of requesting an OAuth 2.0 access token as well as a way of
authenticating the client. Under OAuth 2.0, the way of requesting an access token is known as a grant
type. Apart from making the token type decoupled from the core specification, it also makes the grant type
decoupled too. The grant type defines a protocol to get the authorized access token from the resource
owner. The OAuth 2.0 core specification defines four grant types - authorization code, implicit, client
credentials, and resource owner password. However, it is not limited to four. A grant type is another way of
extending the OAuth 2.0 framework. OAuth 1.0 was coupled to a single grant type, which is almost similar
to the authorization code grant type in 2.0.
SAML2 Bearer Assertion Profile defines its own grant type (urn:ietf:params:oauth:grant-type:saml2-
bearer). Using this grant type a client can get either a MAC token or a Bearer token from the OAuth
A good use case for SAML2 grant type is a SAML2 Single Sign On (SSO) scenario. A partner employee can
login to a web application using SAML2 SSO (you have to trust the partner's SAML2 IdP) and later the web
application needs to access a secured API on behalf of the logged in user. To do that, the web application
can use the SAML2 assertion already provided and exchange that to an OAuth access token via SAML2
grant type. There you would need to have an OAuth Authorization Server running inside our domain -
which trusts the external SAML2 IdP.
Unlike the four other grant types defined in OAuth 2.0 core specification, the SAML2 grant type needs the
resource owner to define the allowed scope for a given client out-of-band.
8. JSON Web Token (JWT) Bearer Profile
JSON Web Token (JWT) Bearer Profile is almost the same as the SAML2 Assertion Profile. Instead
of SAML tokens, this uses JSON Web Tokens. JWT Bearer profile also introduces a new grant type
The provision for extensibility made OAuth 2.0 very much superior to OAuth 1.0. That does not mean it's
perfect by any means.
To be the de facto standard for API security, OAuth 2.0 needs to operate in a highly distributed manner and
still be interoperable. We need to have clear boundaries and well-defined interfaces between the client,
the authorization server, and the resource server. OAuth 2.0 specification breaks it into two major flows.
The first is the process of getting the access token from the authorization server - which is based on a grant
type. The second is the process of using it in a request to the resource server. The way the resource server
talks to the authorization server to validate the token is not addressed in the core specification. Hence, it
has resulted in vendor specific APIs to creep between the resource server and the authorization server. This
kills interoperability. The resource server is coupled with the authorization server and this results in vendor
10. Token Introspection
The Internet draft OAuth Token Introspection, which is currently being discussed under the IETF OAuth
working group, defines a method for a client or a protected resource (resource server) to query an OAuth
authorization server to determine metadata about an OAuth token. The resource server needs to send the
access token and the resource id (which is going to be accessed) to the authorization server's introspection
endpoint. The authorization server can check the validity of the token - evaluate any access control rules
around it - and send back the response to the resource server. In addition to the token validity information,
it will further return back the scopes, client_id and some other metadata associated with the token.
Apart from having a well-defined interface between the OAuth authorization server and the resource
server, a given authorization server should also have the capability to issue tokens of different types. To
do this, the client should bring the required token type it needs in the authorization request. But in the
OAuth authorization request, there is no token type defined. This limits the capability of the authorization
server to handle multiple token types simultaneously or it will require a form of out-of-band mechanism to
associate token types against clients.
11. Server Metadata
Both the authorization server and the resource server should have the ability to expose their capabilities
and requirements through a standard metadata endpoint.
The resource server should be able to expose its metadata by resource, which type of token a given request
expects, and the required scope likewise. Moreover, the requirements could change based on the token
type. If it is a MAC token, then the resource server needs to declare which signature algorithm it expects.
This could be possibly supported via an OAuth extension to the WADL (Web Application Description
Language). Similarly, the authorization server also needs to expose its metadata. These could be the
supported token types, and grant types likewise.
The User-Managed Access (UMA) Profile of OAuth 2.0 introduces a standard endpoint to share metadata
at the authorization server level. The authorization server can publish its token endpoint, supported token
types, and supported grant types via this UMA authorization server configuration data endpoint as a JSON
The UMA profile also mandates a set of UMA specific metadata to be published through this endpoint.
This couples the authorization server to UMA, which also addresses a bigger problem than the need to
discover authorization server metadata. It would be more ideal to introduce the need to publish/discover
authorization server metadata through an independent OAuth profile and extend that in UMA to address
more UMA specific requirements.
12. User-Managed Access
The problem addressed by UMA is far beyond than just exposing Authorization Server metadata. UMA is
undoubtedly going to be one of the key ingredients in any ecosystem for API security.
UMA defines how resource owners can control protected-resource access by clients operated by arbitrary
requesting parties, where the resources reside on any number of resource servers, and where a centralized
authorization server governs access based on resource owner policy. UMA defines two standard interfaces
for the Authorization Server. One interface is between the Authorization Server and the Resource Server
(protection API), while the other is between the Authorization Server and Client (authorization API).
To initiate the UMA flow, the resource owner has to introduce all his resource servers to the centralized
authorization server. With this, each resource server will get an access_token from the authorization
server - and that can be used by resource servers to access the protection API exposed by the authorization
server. The API consists of an OAuth resource set registration endpoint as defined by OAuth Resource
Registration draft specification, an endpoint for registering client-requested permissions, and an OAuth
token introspection endpoint.
The Client or the Requesting party can be unknown to the resource owner. When it tries to access a
resource, the resource server will provide the necessary details - so, the requesting party can talk to the
authorization server via Authorization API and get a Requesting Party Token (RPT). This API once again is
OAuth protected, so the requesting party should be known to the authorization server.
Once the client has the RPT, it can present it to the Resource Server and get access to the protected
resource. The Resource Server uses OAuth introspection endpoint of the Authorization Server to validate
This is a highly distributed, decoupled set up and can be further extended by incorporating SAML2 grant
13. Token Revocation
Token revocation is also an important aspect in API security.
Most of the OAuth authorization servers currently utilize vendor specific APIs. This couples the resource
owner to a proprietary API, leading to vendor lock-in. This aspect is not yet being addressed by the OAuth
working group. The Token Revocation RFC 7009 addresses a different concern. This proposes an endpoint
for OAuth authorization servers, which allows clients to notify the authorization server when a previously
obtained refresh or access token is no longer needed.
In most cases token revocation by the resource owner will be more prominent than token revocation by the
client as proposed in this draft. The challenge in developing a profile to revoke access tokens/refresh tokens
by the resource owner is the lack of token metadata at the resource owner end. The resource owner does
not have visibility to the access token. In that case, the resource owner needs to talk to a standard end
point at the authorization server to discover the clients it had authorized before. As per the OAuth 2.0 core
specification, a client is known to the authorization server via the client-id attribute. Passing this back to the
resource owner is less meaningful as in most cases it's an arbitrary string. This can be fixed by introducing a
new attribute called "friendly-name".
14. Resource Owner Initiated Delegation
The model proposed in both OAuth 1.0 as well as in OAuth 2.0 is client initiated. The client is the one who
starts the OAuth flow, by first requesting an access token. How about the other way around? Resource
owner initiated OAuth delegation. For example I am a user of an online photo-sharing site. There can be
multiple clients like Facebook applications and Twitter applications registered with it. Now, I want to pick
some client applications from the list and give them access to my photos under different scopes. Let's
take another example; I am an employee of Foo.com. I'll be going on vacation for two weeks, so I want
to delegate some of my access rights to Peter only for that period of time. Conceptually, OAuth fits nicely
here. But this is a use case that is initiated by the Resource Owner and is not being addressed in the
OAuth specification. This would require introducing a new resource owner initiated grant type. The Owner
Authorization Grant Type Profile Internet draft for OAuth 2.0 addresses a similar concern by allowing the
resource owner to directly authorize a relying party or a client to access a resource.
15. Token Chaining
Delegated access control talks about performing actions on behalf of another user. This is what OAuth
addresses. Delegated "chained" access control takes one step beyond this. The OASIS WS-Trust (a
speciation built on top of WS-Security for SOAP) specification addressed this concern from its 1.4 version
on wards, by introducing the "Act-As" attribute. The resource owner delegates access to the client and the
client uses the authorized access token to invoke a service that resides in the resource server. This is OAuth
so far. In a real enterprise use case it's a common requirement that the resource or the service needs to
access another service or a set of services to cater to a given request. In this scenario, the first service going
to act as the client to the second service, and also it needs to act on behalf of the original resource owner.
Using the access token passed to it as it is, is not the ideal solution. The Chain Grant Type Internet draft for
OAuth 2.0 is an effort to fix this. It defines a method by which an OAuth protected service or a resource can
use a received OAuth token from its client, in turn, to act as a client and access another OAuth protected
service. This specification still at its draft-1 would require maturing soon to address these concerns in real
enterprise API security scenarios.
16. OAuth 2.0 Vs. OpenID Connect
OpenID Connect is a profile built on top OAuth 2.0. OAuth talks about access delegation while OpenID
Connect talks about authentication. In other words, OpenID Connect builds an identity layer on top of
Authentication is the act of confirming the truth of an attribute of a datum or entity. If I say, I am Peter - I
need to prove that. I can prove that with something I know, something I have or with something I am. Once
proven who I claim to be, then the system can trust me. Sometimes systems do not want to identify end
users by just a name. A name could be your unique form of identification, but there are other attributes
too. In order to get past border control, you would need to identify yourself by name, and provide a
photo, as well as do fingerprint, and retina checks. Those are validated real-time against data from the visa
office, which would have already issued a visa to you. That is proving your identity. Proving your identity is
Authorization is about what you can do in the country. You could prove your identity at the border control
by name, photo, as well as fingerprint and retina scans, but it's your visa that determines what you can
do. To enter a country, you would need to have a valid visa. A valid visa is not a part of your identity, but
determines what you can do. Moreover, what you can do in the country depends on the type of visa
too, e.g. what you do with a B1 or B2 visa in the US differs from what you can do with an L1 or L2. That is
authorization. OAuth 2.0 is about authorization. Not about authentication.
With OAuth 2.0, the client does not know about the end user (only exception is resource owner credentials
grant type). It simply gets an access token to access a resource on behalf of the user. With OpenID Connect,
the client will get an ID Token along with the access token. ID Token is a representation of the end user's
What does it mean by securing an API with OpenID Connect? Or is it totally meaningless?
OpenID Connect is at the Application level or at the Client level - not at the API level or at the Resource
Server level. OpenID Connect helps, client or the application to find out who the end user is, but for
the API that is meaningless. The only thing the API expects is the access token. If the resource owner,
or the API wants to find out who the end user is, it has to query the Authorization Server. The OAuth
Token Introspection specification currently does not support sending back the end user identity in the
introspection response, but it would be quite useful to have a user ID Token in the response (as in OpenID
Connect) and was proposed to the OAuth IETF working group.
17. Fine-Grained Access Control
OAuth is all about access delegation. The resource owner delegates a limited set of access rights to a third
party. In OAuth terminology, this is the "scope". A given access token has a scope associated with it and it
governs the access token's capabilities.
XACML (eXtensible Access Control Markup Language) is the de facto standard for fine-grained access
control. OAuth scope can be represented in XACML policies.
Say, for example, a user delegates access to his Facebook profile to a third party, under the scope "user_
activities". This provides access to the user's list of activities as the activities' connection. To achieve finegrained
access control, this can be represented in an XACML policy.
<Rule RuleId="permit_rule" Effect="Permit">
<Rule RuleId="deny_rule" Effect="Deny">
The above policy will be picked when the scope associated with the access token is equal to user_activities.
Authorization Server first needs to find all the scopes associated with the given access token and build the
XAML request accordingly.
Authorization Server first gets the following introspection request:
Authorization Server now needs to find the scope and the client id associated with the given token and build the XACML request.
18. API Security Ecosystem
18.1. Key Manager
Key Manager, or the Authorization Server, is an essential ingredient in an API security ecosystem. Following
highlights some of the prominent expectations from a key manager.
- Support for open standards ( OAuth, OpenID Connect, UMA, SAML 2.0, XACML)
- Ability to Integrate with heterogeneous user stores - LDAP / Active Directory / JDBC
- Tighten security - ability to protect token in transit as well as at rest
- Fine-grained access control
- Auditing and Reporting
- High availability and failover
- High Performance
18.2. API Gateway
API Gateway under the context of a security ecosystem, plays the role of a security gateway. This intercepts
all API calls to make sure only the legitimate calls pass through. In most cases for the public facing APIs, the
API Gateway will be deployed in the DMZ. The API Gateway should maintain a secured connection with the
18.3 WSO2 Identity Server (WSO2 IS)
WSO2 IS is an open source Identity and Access Management server that supports OAuth 1.0/2.0, SAML2,
OpenID, OpenID Connect, XACML, SCIM and many other identity management features. Building an
ecosystem for API security, WSO2 IS plays a key role as the Authorization Server. It has support for following
OAuth related standards/profiles.
- OAuth 1.0
- OAuth 2.0 Core Grant Types
- OAuth 2.0 Bearer Token Profile
- OAuth 2.0 SAML 2.0 Token Profile
- OAuth 2.0 Token Revocation Profile
- OpenID Connect
18.4. WSO2 API Manager
WSO2 API Manager, comes with three components; API Publisher, API Store, and API Gateway. This white
paper will only focus on the API Gateway part of it. WSO2 API Manager connects to the WSO2 IS over a
high performance and secured communication channel built on top of Thrift and TLS.
18.5. WSO2 Business Activity Monitor (WSO2 BAM)
WSO2 BAM can be used to monitor SOA metrics and key business indicators. It provides a highly scalable,
large scale monitoring and analytics solution, integrated with Apache Hadoop and Apache Cassandra.
WSO2 BAM can seamlessly integrated with WSO2 API Gateway to collect statistics, and analyze and build
dashboards on identified business indicators.
19.1 Accessing an API Secured with OAuth, on Behalf of a User Logged into the System, with SAML2 Web SSO.
- User from domain Foo tries to access a web app deployed in the domain Bar. The web app is secured
with SAML2 Web SSO.
- Web App finds that the user does not have an authenticated session. It finds out from which domain
the request was initiated and redirects the user to the SAML2 IdP in his own domain.
- User authenticates to the SAML2 IdP in his own domain.v
- SAML2 IdP from domain Foo sends a SAML response back to the web app in domain Bar.
- Web app validates the SAML response. It has to trust the domain Foo SAML2 IdP. To access backend
APIs, on behalf of the logged in user, web app needs an OAuth access token. Web app talks to the
OAuth Authorization Server in its own domain, passing the SAML token.
- OAuth Authorization Server trusts the SAML2 IdP in domain Foo. Validates the SAML token and sends
back an access token.
- Web app invokes the API with the access token.
- API Gateway intercepts the request, finds the access token and calls OAuth Authorization Server to
- OAuth Authorization Server validates the token and sends back a JWT (JSON Web Token), which
includes end-user details, to the API Manager.
- API Gateway adds the JWT as an HTTP header and invokes the backend business API.
19.2. Accessing an API Secured with OAuth on Behalf of a User/System Authenticated to a SOAP Service with WS-Trust.
- User/System from domain Foo authenticates to the WS-Trust STS in his own domain.
- STS returns back a SAML token to access the SOAP service in domain Bar.
- User/System authenticates to the SOAP service in domain Bar with the SAML token.
- SOAP service validates the SAML token. It has to trust the domain Foo STS. To access backend APIs,
on behalf of the logged in user, SOAP service needs an OAuth access token. SOAP service talks to the
OAuth Authorization Server in its own domain, passing the SAML token.
- OAuth Authorization Server trusts the STS in domain Foo. Validates the SAML token and sends back an
- SOAP service invokes the API with the access token.
- API Gateway intercepts the request, finds the access token and calls the OAuth Authorization Server to
- The OAuth Authorization Server validates the token and sends back a JWT (JSON Web Token), which
includes end-user details to the API Manager.
- API Gateway adds the JWT as an HTTP header and invokes the backend business API.
19.3 Fine-Grained Access Control with XACML
- User/System accesses the API passing an access token.
- API Gateway intercepts the request, finds the access token, and calls the OAuth Authorization Server to
- Authorization Server, finds the scopes and the client id associated with access token, builds a XACML
request can call XACML PDP.
- XACML PDP evaluates the XACML requests against its policy set and returns back a XACML response.
- OAuth Authorization Server sends back a JWT (JSON Web Token), which includes end-user details to the
- API Gateway adds the JWT as an HTTP header and invokes the backend business API.
- OAuth 1.0 http://tools.ietf.org/html/rfc5849
- The OAuth 2.0 Authorization Framework http://tools.ietf.org/html/rfc6749
- The OAuth 2.0 Authorization Framework: Bearer Token Usage https://ietf.org/doc/rfc6750/
- OAuth 2.0 Token Revocation https://ietf.org/doc/rfc7009/
- OAuth 2.0 Message Authentication Code (MAC) Tokens https://ietf.org/doc/draft-ietf-oauth-v2-http-mac/
- SAML 2.0 Profile for OAuth 2.0 Client Authentication and Authorization Grants
- JSON Web Token (JWT) Profile for OAuth 2.0 Client Authentication and Authorization Grants
- User-Managehttps://ietf.org/doc/draft-ietf-oauth-jwt-bearer/d Access (UMA) Profile of OAuth 2.0 https://ietf.org/doc/draft-hardjono-oauth-umacore/
- OAuth Token Introspection http://tools.ietf.org/html/draft-richer-oauth-introspection-04
- WSO2 API Manager https://wso2.com/products/api-manager/
- WSO2 Identity Server https://wso2.com/products/identity-server/
- WSO2 Business Activity Monitor https://wso2.com/products/business-activity-monitor/
- XACML 3.0 http://docs.oasis-open.org/xacml/3.0/xacml-3.0-core-spec-os-en.html