Have a look at input validation cheat sheet for comprehensive explanation. 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 XXEProcessing) and similar attacks. Validate content types. A REST request or response body should match the intended content type in the header.

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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.

1. Lastly, it is good practice to consult the OWASP Cheat Sheet ‘XXE Prevention’ for additional information when securing your applications against this type of vulnerability. Davos Networks protects your data. Davos Networks is Swiss based, value-added Managed Security Service Provider and Reseller.
2. In this section, we'll explain what XML external entity injection is, describe some common examples, explain how to find and exploit various kinds of XXE injection, and summarize how to prevent XXE injection attacks. XML external entity injection (also known as XXE) is a web security vulnerability.
3. XML External Entity Prevention Cheat Sheet¶ Introduction¶. XML eXternal Entity injection (XXE), which is now part of the OWASP Top 10 via the point A4, is a type of attack against an application that parses XML input. XXE issue is referenced under the ID 611 in the Common Weakness Enumeration referential. This attack occurs when untrusted XML input containing a reference to an external.
4. The prevention cheat sheet above would definitely help you set up the basic fence. However, security is strongest when you take a defense in depth approach. You’ll want to identify more ways an attack can happen and the weak points of your app, via point-in-time snapshots like pentests and finding ways to gain more visibility on an ongoing.

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.

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 HATEOAS 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.

• in order to minimise latency and reduce coupling between services, the access control decision should be taken locally by REST endpoints
• user authentication should be centralised in a Identity Provider (IdP), which issues access tokens

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.

• Ensure JWTs are integrity protected by either a signature or a MAC. Do not allow the unsecured JWTs: {'alg':'none'}.
  • See here
• In general, signatures should be preferred over MACs for integrity protection of JWTs.

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 here for additional information.

The relying party or token consumer validates a JWT by verifying its integrity and claims contained.

• A relying party must verify the integrity of the JWT based on its own configuration or hard-coded logic. It must not rely on the information of the JWT header to select the verification algorithm. See here and here

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:

• iss or issuer - is this a trusted issuer? Is it the expected owner of the signing key?
• aud or audience - is the relying party in the target audience for this JWT?
• exp or expiration time - is the current time before the end of the validity period of this token?
• nbf or not before time - is the current time after the start of the validity period of this token?

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.

• Require API keys for every request to the protected endpoint.
• Return 429 Too Many Requests HTTP response code if requests are coming in too quickly.
• Revoke the API key if the client violates the usage agreement.
• Do not rely exclusively on API keys to protect sensitive, critical or high-value resources.

• Apply a whitelist of permitted HTTP Methods e.g. GET, POST, PUT.
• Reject all requests not matching the whitelist with HTTP response code 405 Method not allowed.
• Make sure the caller is authorised to use the incoming HTTP method on the resource collection, action, and record

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.

• Do not trust input parameters/objects.
• Validate input: length / range / format and type.
• Achieve an implicit input validation by using strong types like numbers, booleans, dates, times or fixed data ranges in API parameters.
• Constrain 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.
• Have a look at input validation cheat sheet for comprehensive explanation.
• 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_Processing) and similar attacks.

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.

• Document all supported content types in your API.

Validate request content types

• Reject requests containing unexpected or missing content type headers with HTTP response status 406 Unacceptable or 415 Unsupported Media Type.
• For XML content types ensure appropriate XML parser hardening, see the XXE cheat sheet.
• Avoid accidentally exposing unintended content types by explicitly defining content types e.g. Jersey (Java) @consumes('application/json'); @produces('application/json'). This avoids XXE-attack_Processing) vectors for example.

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.

• 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.

Services including script code (e.g. JavaScript) in their responses must be especially careful to defend against header injection attack.

• Ensure sending intended content type headers in your response matching your body content e.g. application/json and not application/javascript.

• Avoid exposing management endpoints via Internet.
• If management endpoints must be accessible via the Internet, make sure that users must use a strong authentication mechanism, e.g. multi-factor.
• Expose management endpoints via different HTTP ports or hosts preferably on a different NIC and restricted subnet.
• Restrict access to these endpoints by firewall rules or use of access control lists.

• Respond with generic error messages - avoid revealing details of the failure unnecessarily.
• Do not pass technical details (e.g. call stacks or other internal hints) to the client.

• Write audit logs before and after security related events.
• Consider logging token validation errors in order to detect attacks.
• Take care of log injection attacks by sanitising log data beforehand.

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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 the X-Content-Type-Options: nosniffsecurity header 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 the X-Frame-Options: denysecurity header to protect against drag'n drop clickjacking attacks in older browsers.

Cross-Origin Resource Sharing (CORS) is a W3C standard to flexibly specify what cross-domain requests are permitted. By delivering appropriate CORS Headers your REST API signals to the browser which domains, AKA origins, are allowed to make JavaScript calls to the REST service.

• Disable CORS headers if cross-domain calls are not supported/expected.
• Be as specific as possible and as general as necessary when setting the origins of cross-domain calls.

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.

• In POST/PUT requests sensitive data should be transferred in the request body or request headers.
• In GET requests sensitive data should be transferred in an HTTP Header.

OK:

https://example.com/resourceCollection/[ID]/action

https://twitter.com/vanderaj/lists

NOT OK:

https://example.com/controller/123/action?apiKey=a53f435643de32 because API Key is into the URL.

HTTP defines status code. 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.

Additional information about HTTP return code usage in REST API can be found here and here.

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

Jan Wolff - jan.wolff@owasp.org

Rocco Gränitz - rocco.graenitz@owasp.org

Manh Pham - manhpt2811@gmail.com

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Ask questionsUpdate: XML External Entity (XXE) Prevention Cheat Sheet

The JAXB > Java 8 and Up sub-section on the XXE Cheat Sheet can be misleading. The advice in this section succinctly states

Since JDK-8010393, which is in OpenJDK 8 beta 86, javax.xml.bind.Unmarshaller instances are safe by default. The other classes mentioned here are still unsafe by default in Java 8.

This statement is too broad. It does not account for all the nuances that can lead the javax.xml.bind.Unmarshaller to be vulnerable or used in a way that is vulnerable on OpenJDK 1.8 based runtimes. An application parsing untrusted XML on an OpenJDK 1.8 runtime may be vulnerable to XXE if

• the JAXB runtime has been inadvertently replaced with a different JAXB runtime that is not safe-by-default (e.g. EclipseLink MOXy)
• the secure SAX parser that the OpenJDK 1.8 Unmarshaller uses has been inadvertently replaced with a different implementation that is not safe-by-default (e.g. xercesImpl:2.8.0)
• the application uses one of the unmarshal overloads that does not delegate XML parsing to the safe-by-default SAX parser

Elaborating on that last point, even if the Unmarshaller is considered safe by default, not all of its unmarshal methods use the safe by default XMLReader. Calling unmarshal with any of the following argument types still requires the user to explicitly configure the XML parsing to be safe

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• javax.xml.transform.stream.SAXSource
• org.w3c.dom.Node
• javax.xml.stream.XMLStreamReader
• javax.xml.stream.XMLEventReader

The following is a detailed analysis that explains why users should not rely on the safe by default nature of the Unmarshaller in OpenJDK 1.8 https://www.contrastsecurity.com/security-influencers/xml-xxe-pitfalls-with-jaxb

Recommend removing this sub-section from the Cheat Sheet

Xxe Cheat Sheet 2020