Xxe Cheat Sheet
What are XML External Entities (XXE)?
According to OWASP, “An XML External Entity attack is a type of attack against an application that parses XML input. This attack occurs when XML input containing a reference to an external entity is processed by a weakly configured XML parser. This attack may lead to the disclosure of confidential data, denial of service, server side request forgery, port scanning from the perspective of the machine where the parser is located, and other system impacts.”
Applications and in particular XML-based web services or downstream integrations might be vulnerable to attack if:
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 vectors for example. Send safe response content types.
Ask questions Update: 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. GitHub Gist: instantly share code, notes, and snippets. 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.
Xml External Entity
- The application accepts XML directly or XML uploads, especially from untrusted sources, or inserts untrusted data into XML documents, which is then parsed by an XML processor.
- Any of the XML processors in the application or SOAP based web services has document type definitions (DTDs) enabled. As the exact mechanism for disabling DTD processing varies by processor, it is good practice to consult a reference such as the OWASP Cheat Sheet 'XXE Prevention’.
- If your application uses SAML for identity processing within federated security or single sign on (SSO) purposes. SAML uses XML for identity assertions, and may be vulnerable.
- If the application uses SOAP prior to version 1.2, it is likely susceptible to XXE attacks if XML entities are being passed to the SOAP framework.
- Being vulnerable to XXE attacks likely means that the application is vulnerable to denial of service attacks including the Billion Laughs attack.
XXE Examples
Numerous public XXE issues have been discovered, including attacking embedded devices. XXE occurs in a lot of unexpected places, including deeply nested dependencies. The easiest way is to upload a malicious XML file, if accepted:
Xxe Payloads
Example #1: The attacker attempts to extract data from the server
Example #2: An attacker probes the server's private network by changing the above ENTITY line to
Xxe Attack Owasp
Example #3: An attacker attempts a denial-of-service attack by including a potentially endless file