REST Security Cheat Sheet
Last revision (mm/dd/yy): 05/16/2018
REST (or REpresentational State Transfer) is an architectural style first described in Roy Fielding's Ph.D. dissertation on Architectural Styles and the Design of Network-based Software Architectures. It evolved as Fielding wrote the HTTP/1.1 and URI specs and has been proven to be well-suited for developing distributed hypermedia applications. While REST is more widely applicable, it is most commonly used within the context of communicating with services via HTTP.
The key abstraction of information in REST is a resource. A REST API resource is identified by a URI, usually a HTTP URL. REST components use connectors to perform actions on a resource by using a representation to capture the current or intended state of the resource and transferring that representation. The primary connector types are client and server, secondary connectors include cache, resolver and tunnel. In order to implement flows with REST APIs, resources are typically created, read, updated and deleted. For example, an ecommerce site may offer methods to create an empty shopping cart, to add items to the cart and to check out the cart.
Another key feature of REST applications is the use of standard HTTP verbs and error codes in the pursuit or removing unnecessary variation among different services.
Another key feature of REST applications is the use of HATEOS or Hypermedia as the Engine of Application State. This provides REST applications a self-documenting nature making it easier for developers to interact with a REST service without a priori knowledge.
Secure REST services must only provide HTTPS endpoints. This protects authentication credentials in transit, for example passwords, API keys or JSON Web Tokens. It also allows clients to authenticate the service and guarantees integrity of the transmitted data.
See the Transport Layer Protection Cheat Sheet for additional information.
Consider the use of mutually authenticated client-side certificates to provide additional protection for highly privileged web services.
Non-public REST services must perform access control at each API endpoint. Web services in monolithic applications implement this by means of user authentication, authorisation logic and session management. This has several drawbacks for modern architectures which compose multiple micro services following the RESTful style.
There seems to be a convergence towards using JSON Web Tokens (JWT) as the format for security tokens. JWTs are JSON data structures containing a set of claims that can be used for access control decisions. A cryptographic signature or message authentication code (MAC) can be used to protect the integrity of the JWT.
If MACs are used for integrity protection, every service that is able to validate JWTs can also create new JWTs using the same key. This means that all services using the same key have to mutually trust each other. Another consequence of this is that a compromise of any service also compromises all other services sharing the same key. See https://tools.ietf.org/html/rfc7515#section-10.5 for additional information.
The relying party or token consumer validates a JWT by verifying its integrity and claims contained.
Some claims have been standardised and should be present in JWT used for access controls. At least the following of the standard claims should be verified:
Public REST services without access control run the risk of being farmed leading to excessive bills for bandwidth or compute cycles. API keys can be used to mitigate this risk. They are also often used by organisation to monetize APIs; instead of blocking high-frequency calls, clients are given access in accordance to a purchased access plan.
API keys can reduce the impact of denial-of-service attacks. However, when they are issued to third-party clients, they are relatively easy to compromise.
Restrict HTTP methods
In Java EE in particular, this can be difficult to implement properly. See Bypassing Web Authentication and Authorization with HTTP Verb Tampering for an explanation of this common misconfiguration.
Validate content types
A REST request or response body should match the intended content type in the header. Otherwise this could cause misinterpretation at the consumer/producer side and lead to code injection/execution.
Validate request content types
Send safe response content types
It is common for REST services to allow multiple response types (e.g. "application/xml" or "application/json", and the client specifies the preferred order of response types by the Accept header in the request.
To make sure the content of a given resources is interpreted correctly by the browser, the server should always send the Content-Type header with the correct Content-Type, and preferably the Content-Type header should include a charset. The server should also send an X-Content-Type-Options: nosniff to make sure the browser does not try to detect a different Content-Type than what is actually sent (can lead to XSS).
Additionally the client should send an X-Frame-Options: deny to protect against drag'n drop clickjacking attacks in older browsers.
Sensitive information in HTTP requests
RESTful web services should be careful to prevent leaking credentials. Passwords, security tokens, and API keys should not appear in the URL, as this can be captured in web server logs, which makes them intrinsically valuable.
HTTP Return Code
HTTP defines status codes . When designing REST API, don't just use 200 for success or 404 for error. Always use the semantically appropriate status code for the response.
Here is a non-exhaustive selection of security related REST API status codes. Use it to ensure you return the correct code.
Authors and primary editors
Erlend Oftedal - firstname.lastname@example.org