Draft REST cheat sheet

Last revision (mm/dd/yy): 09/11/2017 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 Externalising the Security Token Service and Identity Provider Modern access control protocols such as OAuth 2.0 and OpenID Connect (OIDC) 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. OAuth 2.0 and OIDC's authorization server is an example of an STS. 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. A single, centralised Security Token Service (STS) issuing access tokens for all APIs in an organisation facilitates enforcing access control policies. The STS is a choke point through which all access requests must pass. 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, not necessarily. Arguments for externalising the STS also apply to the IdP. Moreover, upgrading to stronger authentication protocols becomes easier with authentication-mechanism agnostic API endpoints. IdPs can be used without STS, e.g. for validating Basic Authentication credentials, but this exposes credentials to the relying party leads to higher coupling between service and authentication mechanism. Whitelist HTTP methods RESTful APIs expose resources that can be acted upon by HTTP verbs such as 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. 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. API Keys 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. API rate limits The objectives of API Rate 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. Content types A REST request or response body should match the advertised content-type in the header. Document all supported content types in your API Reject requests containing unexpected content type headers with HTTP response status 406 Unacceptable or 415 Unsupported Media Type Ensure content type headers in your response match the body content For XML content types ensure appropriate XML parser hardening, see the cheat sheet Avoid accidentally exposing unintended content types by explicitly defining content types e.g. @consumes("application/json"); @produces("application/json"). This avoid XXE-attack vectors for example. Input validation Do not trust input parameters/objects Validate input: length / range / format and type Achieve an implicit input validation by using strong types like numbers, dates, times in API parameters Sanitize string inputs with regexps Reject unexpected/illegal content Make use of validation/sanitation libraries or frameworks in your specific language Define an appropriate request size limit and reject requests exceeding the limit with HTTP response status 413 Request Entity Too Large Consider logging input validation failures. Assume that someone who is performing hundreds of failed input validations per second is up to no good. 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. Output encoding 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. CORS HTTPS 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. 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: https://example.com/resourceCollection/<id>/action https://twitter.com/vanderaj/lists NOT OK: https://example.com/controller/<id>/action?apiKey=a53f435643de32 (API Key in URL) http://example.com/controller/<id>/action?apiKey=a53f435643de32 (transaction not protected by TLS; API Key in URL) 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 OWASP Cheat Sheet Series V - T - E Cheat Sheets Developer / Builder 3rd Party Javascript Management Access Control AJAX Security Cheat Sheet Authentication (ES) Bean Validation Cheat Sheet Choosing and Using Security Questions Clickjacking Defense Credential Stuffing Prevention Cheat Sheet Cross-Site Request Forgery (CSRF) Prevention Cryptographic Storage C-Based Toolchain Hardening CSS Security Deserialization DOM based XSS Prevention Forgot Password HTML5 Security HTTP Strict Transport Security Injection Prevention Cheat Sheet Injection Prevention Cheat Sheet in Java JSON Web Token (JWT) Cheat Sheet for Java Input Validation Insecure Direct Object Reference Prevention JAAS Key Management LDAP Injection Prevention Logging Mass Assignment Cheat Sheet .NET Security OS Command Injection Defense Cheat Sheet OWASP Top Ten Password Storage Pinning Query Parameterization REST Security Ruby on Rails Session Management SAML Security SQL Injection Prevention Transaction Authorization Transport Layer Protection TLS Cipher String Configuration Unvalidated Redirects and Forwards User Privacy Protection Web Service Security XSS (Cross Site Scripting) Prevention XML External Entity (XXE) Prevention Cheat Sheet Assessment / Breaker Attack Surface Analysis REST Assessment Web Application Security Testing XML Security Cheat Sheet XSS Filter Evasion Mobile Android Testing IOS Developer Mobile Jailbreaking OpSec / Defender Virtual Patching Vulnerability Disclosure Draft and Beta Application Security Architecture Business Logic Security Content Security Policy Denial of Service Cheat Sheet Grails Secure Code Review IOS Application Security Testing PHP Security Regular Expression Security Cheatsheet Secure Coding Secure SDLC Threat Modeling All Pages In This Category Authors and primary editors Erlend Oftedal - [email protected] Andrew van der Stock - [email protected] Tony Hsu Hsiang Chih- [email protected] Johan Peeters - [email protected] Other cheatsheets V - T - E Cheat Sheets Developer / Builder 3rd Party Javascript Management Access Control AJAX Security Cheat Sheet Authentication (ES) Bean Validation Cheat Sheet Choosing and Using Security Questions Clickjacking Defense Credential Stuffing Prevention Cheat Sheet Cross-Site Request Forgery (CSRF) Prevention Cryptographic Storage C-Based Toolchain Hardening CSS Security Deserialization DOM based XSS Prevention Forgot Password HTML5 Security HTTP Strict Transport Security Injection Prevention Cheat Sheet Injection Prevention Cheat Sheet in Java JSON Web Token (JWT) Cheat Sheet for Java Input Validation Insecure Direct Object Reference Prevention JAAS Key Management LDAP Injection Prevention Logging Mass Assignment Cheat Sheet .NET Security OS Command Injection Defense Cheat Sheet OWASP Top Ten Password Storage Pinning Query Parameterization REST Security Ruby on Rails Session Management SAML Security SQL Injection Prevention Transaction Authorization Transport Layer Protection TLS Cipher String Configuration Unvalidated Redirects and Forwards User Privacy Protection Web Service Security XSS (Cross Site Scripting) Prevention XML External Entity (XXE) Prevention Cheat Sheet Assessment / Breaker Attack Surface Analysis REST Assessment Web Application Security Testing XML Security Cheat Sheet XSS Filter Evasion Mobile Android Testing IOS Developer Mobile Jailbreaking OpSec / Defender Virtual Patching Vulnerability Disclosure Draft and Beta Application Security Architecture Business Logic Security Content Security Policy Denial of Service Cheat Sheet Grails Secure Code Review IOS Application Security Testing PHP Security Regular Expression Security Cheatsheet Secure Coding Secure SDLC Threat Modeling All Pages In This Category