REST Security Cheat Sheet

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DRAFT MODE - WORK IN PROGRESS

Introduction

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.

REST APIs are stateless. Stateful APIs do not adhere to the REST architectural style. State in the REST acronym refers to the state of the resource which the API accesses, not the state of a session within which the API is called. While there may be good reasons for building a stateful API, it is important to realize that managing sessions is complex and difficult to do securely. Stateful services are out of scope of this Cheat Sheet. Passing state from client to backend, while making the service technically stateless, is an anti-pattern that should also be avoided as it is prone to replay and impersonation attacks.

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. Each of these REST calls is stateless and the endpoint should check whether the caller is authorized to perform the requested operation.

Access Control

Protocols for Authentication and Authorization

Basic and Digest Authentication

Authentication has been in the HTTP spec since 1995 when the first draft of RFC 1945 was published. Initially, only Basic Authentication was standardized, later to be supplemented with Digest Authentication, e.g. in RFC 2069, in an attempt to mitigate the obvious insecurity of sending username and password in cleartext. Given that it relies on MD5 hashes, it is unclear that it affords much better security. Digest Authentication never gained a lot of traction, but Basic Authentication became very popular. HTTPS mitigates the most obvious security problems with Basic Authentication.

While using Basic Authentication to control access to REST APIs seems straightforward, designers need to be aware of following limitations:

  • In order to make a REST call on behalf of a user, the user must hand over credentials to the client. The client could, for example, be a mobile application. In effect, the client has been given unlimited scope to impersonate the user. In other words, the user has granted all access rights to all resources in the authentication realm forever, or at least until the password is changed.
  • Each API endpoint must have access to the password database. Effectively, this means that all APIs are part of the attack surface. Since a password database is a high-value target, its attack surface should be carefully controlled. This becomes almost impossible as the number of endpoints grows and the API need to evolve quickly.
  • Authentication and authorization logic runs on every endpoint. Ideally, this would be shared code, subjected to rigorous QA, but this may be difficult to realise as distinct API endpoints may have different release cycles. Any security patch would impact all endpoints.
  • Basic Authentication is inherently tied to passwords. While passwords are convenient and cheap, they are unsuitable as the sole means of authenticating to gain access to high-value resources. Introducing a second factor to complement Basic Authentication is probably technically doable, but is unlikely to lead to an elegant design and would further exacerbate the difficulties of releasing and maintaining the purpose-build authentication library.
  • Before users can make use of an API, they must be registered with the service. This is at odds with casual use.

Digest Authentication suffers from the same drawbacks.

Basic and Digest Authentication are therefore not suitable authentication mechanisms except for the simplest, standalone, low-value REST API that provides services to a well-defined, stable community. For anything else, we look for authN/Z technologies that

  • remove the responsibility for implementing the authentication protocol from the endpoint,
  • limits the scope of the access rights granted to the client,
  • limits access permissions in time,
  • avoids exposing user credentials to the client,
  • interoperates with a variety of identity providers,
  • supports multi-factor authentication,

to name but of a few.

In the remainder of the section, modern protocols that address these concerns are reviewed.

OAuth 2.0

The prime goal of OAuth 2.0 is to let users, in their capacity of resource owners, grant clients limited access to resources they control. Resources reside on resource servers which require an access token for access. Access tokens are issued by an authorization server. This is a departure from previous protocols where a client would receive user credentials with which they could impersonate the resource owner. Instead users having to completely trust the client, they can, and should, limit the privileges they grant the client to only cover the job at hand. This not only mitigates the risk of a malicious client somewhat, it also limits the impact of a client compromise.

OAuth 2.0 defines 3 ways in which the client can prove its entitlement to access a resource:

  • Authorization Code Grant
  • Implicit Grant
  • Resource Owner Password Grant

It also defines a 4th grant type where the client is also the resource owner, the Client Credentials Grant.

The protocol in the first 3 grant types involves 5 parties, the resource owner or user, the user agent, e.g. the browser, the client, e.g. a Single Page Application implemented in JavaScript, the authorization server and the resource server, or, in other words, the REST API. In the 4th grant type, the resource owner and client coincide.

OpenID Connect

The OpenID Connect standard is based on OAuth 2.0 and is widely used in authentication.  It allows Clients to verify the identity of the End-User based on the authentication performed by an Authorization Server, as well as obtain basic profile information in JSON Web Token format.

https://openid.net/connect/

Externalising the Security Token Service

Modern access control protocols discussed in the previous section all rely on a security token being passed from client to server with a REST call. Clients obtain a security token from a Security Token Service, or STS for short.

A single, centralised Security Token Service (STS) issuing access tokens for all APIs published by an organisation facilitates enforcing access control policies. The STS is a choke point through which all access requests must pass.

In principle, access control protocols could be implemented at each individual API endpoint. Some frameworks encourage this, but the downside is that functionality is duplicated, making it more difficult to ensure security policies and patches are kept up to date and in sync across all endpoints. Key management also becomes more difficult.

Many STS implementations come bundled with an Identity Provider (IdP), but they are logically distinct:

  • the IdP authenticates the user, or client,
  • the STS issues tokens

Tokens may contain claims about the identity of the user, or client, as is the case for the Identity token in OIDC. Access tokens, on the other hand, do not necessarily contain identity claims.

Anti-farming

Many RESTful web services are put up, and then farmed, such as a price matching website or aggregation service. There's no technical method of preventing this use, so strongly consider means to encourage it as a business model by making high velocity farming is possible for a fee, or contractually limiting service using terms and conditions. CAPTCHAs and similar methods can help reduce simpler adversaries, but not well funded or technically competent adversaries. Using mutually assured client side TLS certificates may be a method of limiting access to trusted organisations, but this is by no means certain, particularly if certificates are posted deliberately or by accident to the Internet.

API keys can be used for every API request. If there is any suspicious behavior in the API requests, the caller can be identified by the API Key and its key revoked. Furthermore, rate limiting is often also implemented based on API keys. Note, however, that API keys are susceptible to theft and should not be the sole defence mechanism on high-value targets.

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 caller is authorised to use the incoming HTTP method on the 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. On the other hand, for the librarian, both of these are valid uses.

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 PUT would update an existing entity, POST 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 would work, while all others would 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.

Protect privileged actions and sensitive resource collections

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

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

For resources exposed by RESTful web services, 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.

CSRF is easily achieved even using random tokens if any XSS exists within your application, so please make sure you understand how to prevent XSS.

Insecure direct object references

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

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

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

This is essentially a data-contextual access control enforcement need. A URL or even a POSTed form should NEVER contain an access control "key" or similar that provides automatic verification. A data contextual check needs to be done, server side, with each request.

API Rate limits

The objective of the API Rate limits is to reduce massive API requests that cause denial of services, and also to mitigate potential brute-force attack, or misuses of the services. The API rate limits can be controlled at API gateway or WAF. The following API rate limits mechanism can be considered.

  • API rate limits per application or per API: Every API or application can only access the services for defined the number of requests per rate limit window.
  • API rate limits per GET or POST request: The allowed access requests may vary based on GET or POST requests per period.
  • HTTP error return code: If there are too many error return (i.e. 401, 404, 501...), the identifier of the API (API Key) will be blocked temporarily for further access.

The results of exceeding API rate limits can be temporarily blacklisted the application/API access or notification alert to relevant users/admin. The service should return HTTP return code. "429 Too Many Requests" - The error is used when there may be DOS attack detected or the request is rejected due to rate limiting.

Input validation

Input validation 101

Everything you know about input validation applies to RESTful web services, but add 10% because automated tools can easily fuzz your interfaces for hours on end at high velocity. So:

  • Assist the user > Reject input > Sanitize (filtering) > No input validation

Assisting the user makes the most sense, as the most common scenario is "problem exists between keyboard and chair" (PEBKAC). Help the user input high quality data into your web services, such as ensuring a Zip code makes sense for the supplied address, or the date makes sense. If not, reject that input. If they continue on, or it's a text field or some other difficult to validate field, input sanitization is a losing proposition but still better than XSS or SQL injection. If you're already reduced to sanitization or no input validation, make sure output encoding is very strong for your application.

Log input validation failures, particularly if you assume that client-side code you wrote is going to call your web services. The reality is that anyone can call your web services, so assume that someone who is performing hundreds of failed input validations per second is up to no good. Also consider rate limiting the API to a certain number of requests per hour or day to prevent abuse.

Secure parsing

Use a secure parser for parsing the incoming messages. If you are using XML, make sure to use a parser that is not vulnerable to XXE and similar attacks.

Strong typing

It's difficult to perform most attacks if the only allowed values are true or false, or a number, or one of a small number of acceptable values. Strongly type incoming data as quickly as possible.

Validate incoming content-types

When POSTing or PUTting new data, the client will specify the Content-Type (e.g. application/xml or application/json) of the incoming data. The server should never assume the Content-Type; it should always check that the Content-Type header and the content are 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.

Validate 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.

XML input validation

XML-based services must ensure that they are protected against common XML based attacks by using secure XML-parsing. This typically means protecting against XML External Entity attacks, XML-signature wrapping etc. See http://ws-attacks.org for examples of such attacks.

Framework-Provided Validation

Many frameworks, such as Jersey, allow for validation constraints to be enforced automatically by the framework at request or response time. (See Bean Validation Support for more information). While this does not validate the structure of JSON or XML data before being unmarshaled, it does provide automatic validation after unmarshaling, but before the data is presented to the application.

Output encoding

Send security headers

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.

JSON encoding

A key concern with JSON encoders is preventing arbitrary JavaScript remote code execution within the browser... or, if you're using node.js, on the server. It's vital that you use a proper JSON serializer to encode user-supplied data properly to prevent the execution of user-supplied input on the browser.

When inserting values into the browser DOM, strongly consider using .value/.innerText/.textContent rather than .innerHTML updates, as this protects against simple DOM XSS attacks.

XML encoding

XML should never be built by string concatenation. It should always be constructed using an XML serializer. This ensures that the XML content sent to the browser is parseable and does not contain XML injection. For more information, please see the Web Service Security Cheat Sheet.

Cryptography

Data in transit

Unless the public information is completely read-only, the use of TLS should be mandated, particularly where credentials, updates, deletions, and any value transactions are performed. The overhead of TLS is negligible on modern hardware, with a minor latency increase that is more than compensated by safety for the end user.

Consider the use of mutually authenticated client-side certificates to provide additional protection for highly privileged web services.

Data in storage

Leading practices are recommended as per any web application when it comes to correctly handling stored sensitive or regulated data. For more information, please see OWASP Top 10 2010 - A7 Insecure Cryptographic Storage.

Message Integrity

In addition to HTTPS/TLS, JSON Web Token (JWT) is an open standard (RFC 7519) that defines a compact and self-contained way for securely transmitting information between parties as a JSON object. JWT can not only be used to ensure the message integrity but also authentication of both message sender/receiver. The JWT includes the digital signature hash value of the message body to ensure the message integrity during the transmition.

https://jwt.io/introduction/

Confidentiality

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.

OK:

NOT OK:

HTTP Return Code

HTTP defines status code [1]. When designing REST API, don't just use 200 for success or 404 for error.

Here are some guideline to consider for each REST API status return code. Proper error handle may help to validate the incoming requests and better identify the potential security risks.

  • 200 OK - Response to a successful REST API action. The HTTP method can be GET, POST, PUT, PATCH or DELETE.
  • 201 Created - The request has been fulfilled and resource created. A URI for the created resource is returned in the Location header.
  • 202 Accepted - The request has been accepted for processing, but processing is not yet complete.
  • 400 Bad Request - The request is malformed, such as message body format error.
  • 401 Unauthorized - Wrong or no authencation ID/password provided.
  • 403 Forbidden - It's used when the authentication succeeded but authenticated user doesn't have permission to the request resource
  • 404 Not Found - When a non-existent resource is requested
  • 405 Method Not Allowed - The error for an unexpected HTTP method. For example, the REST API is expecting HTTP GET, but HTTP PUT is used.
  • 429 Too Many Requests - The error is used when there may be DOS attack detected or the request is rejected due to rate limiting

Related articles

OWASP Cheat Sheets Project Homepage


Authors and primary editors

Erlend Oftedal - erlend.oftedal@owasp.org
Andrew van der Stock - vanderaj@owasp.org
Tony Hsu Hsiang Chih- Hsiang_chihi@yahoo.com
Johan Peeters - yo@johanpeeters.com

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