Revision as of 11:26, 26 November 2012

DRAFT CHEAT SHEET - WORK IN PROGRESS

Introduction

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.

Authentication and Session Management

RESTful web services should use session based authentication, either by establishing a session token via a POST, or using an API key as a POST body argument. Usernames and passwords, session tokens and API keys should not appear in the URL, as this can be captured in web server logs and makes them intrinsically valuable.

OK:

• http://example.com/resourceCollection/<id>/action
• http://twitter.com/vanderaj/lists

NOT OK:

• http://example.com/controller/<id>/action?apiKey=a53f435643de32

Authorization

Protect HTTP methods

RESTful API often use GET (read), POST (create), PUT (replace/update) and DELETE (to delete a record). Not all of these are valid choices for every single resource collection, user, or action. Make sure the incoming HTTP method is valid for the session token/API key and associated resource collection, action, and record. For example, if you have an RESTful API for a library, it's not okay to allow anonymous users to DELETE book catalog entries, but it's fine for them to GET a book catalog entry, whereas for the librarian, both of these are valid uses.

Protect privileged actions and sensitive resource collections

Not every user has a right to every web service. This is vital, as you don't want administrative web services to be misused:

• http://example.com/admin/exportAllData

The session token or API key should be sent along as a cookie or body parameter to ensure that privileged collections or actions are properly protected from unauthorized use.

Protect against cross-site request forgery

Just because there's no web interface doesn't mean RESTful web services are secure against CSRF attacks. For high value transactions require a two step tango to obtain a CSRF token prior to submitting a high value transaction. There's no easy way to stop this issue if your application has cross-site scripting within the application, so also weed out all XSS issues.

Direct object references

It may seem obvious, but if you had a bank account REST web service, you have to make sure there is adequate checking of primary and foreign keys:

• http://example.com/account/325365436/transfer?amount=$100.00&toAccount=473846376

In this case, it would be possible to transfer money from any account to any other account, which is clearly insane. Not even a random token makes this safe.

• http://example.com/invoice/2362365

In this case, it would be possible to get a copy of all invoices.

Please make sure you understand how to protect against direct object references in the OWASP Top 10 2010.

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.

Related Articles

OWASP Cheat Sheets Project Homepage

• OWASP Cheat Sheet Series

Authors and Primary Editors

First Last - first.last [at] owasp.org
Erlend Oftedal - [email protected]