REST Security Cheat Sheet

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REST or REpresentational State Transfer is a means of expressing specific entities in a system by URL path elements, REST is not an architecture but it is an architectural style to build services on top of the Web. REST allows interaction with a web-based system via simplified URL's rather than complex request body or POST parameters to request specific items from the system. This document serves as a guide (although not exhaustive) of best practices to help REST-based services.

Check Authorization for User-Specific Entities

While REST is useful for targeting specific entities in the system, the URL itself should not be the only authorizing token for sensitive entities (such as account transactions, personally-identifying information, etc.). Proper authentication and authorization should take place. The authorizing credentials (token etc.) should be sent as an Authorization header, and cookies can be used in cases where Authorization headers are infeasible.

Authentication for a REST-based service should not take place by passing the user ID and password in path elements of the URL. URL's are commonly cached by browsers, proxies, etc. so sensitive information should not be included directly in the URL. Furthermore, no Personally-Identifiable Information (such as bank-account number, credit card number, etc.) should be used as a parameter to request an entity. See the [DOR (Direct Object Reference) Prevention Cheat Sheet] for strategies for preventing Direct Object Reference weaknesses.

Whitelist-Only Content-Types

When POSTing og PUTing new data, the client will specify the a Content-Type (e.g. application/xml or application/json) of the incoming data. The client should never assume the Content-Type, but always check that the Content-Type header and the content is of the same type. A lack of Content-Type header or an unexpected Content-Type header, should result in the server rejecting the Content with a 406 Not Acceptable response.

Whitelist-Only Response 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. Do NOT simply copy the Accept header to the Content-type header of the response. Reject the request (ideally with a 406 Not Acceptable response) if the Accept header does not specifically contain one of the allowable types.

Because there are many MIME types for the typical response types, it's important to document for clients specifically which MIME types should be used.

Whitelist Allowable Methods

It is common with RESTful services to allow multiple methods for a given URL for different operations on that entity. For example, a GET request might read the entity while POST would update an existing entity, PUT would create a new entity, and DELETE would delete an existing entity. It is important for the service to properly restrict the allowable verbs such that only the allowed verbs will work, all others return a proper response code (for example, a 403 Forbidden).

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.

Security headers for RESTful resources available to browsers

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. 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.

CSRF protection

For JSON resources available to browsers, it's important to make sure any PUT, POST and DELETE request is protected from Cross Site Request Forgery. Typically one would use a token based approach. See [Cross-Site_Request_Forgery_(CSRF)_Prevention_Cheat_Sheet] for more information on how to implement CSRF-protection.

XML based services

XML-based services must ensure that they are protected against common XML based attacks by using secure XML-parsing. This typically means XML External Entity attacks, XML-signature wrapping etc. See the [1] for examples of such attacks.

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