XML External Entities (XXE)¶
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:
- 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.
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:
Example #1: The attacker attempts to extract data from the server¶
<?xml version="1.0" encoding="ISO-8859-1"?> <!DOCTYPE foo [ <!ELEMENT foo ANY > <!ENTITY xxe SYSTEM "file:///etc/passwd" >]> <foo>&xxe;</foo>
Example #2: An attacker probes the server's private network by changing the above ENTITY line to¶
<!ENTITY xxe SYSTEM "https://192.168.1.1/private" >]>
Example #3: An attacker attempts a denial-of-service attack by including a potentially endless file¶
<!ENTITY xxe SYSTEM "file:///dev/random" >]>
How to prevent XML External Entities (XXE)¶
Developer training is essential to identify and mitigate XXE. Besides that, preventing XXE requires:
- Whenever possible, use less complex data formats such as JSON, and avoiding serialization of sensitive data.
- Patch or upgrade all XML processors and libraries in use by the application or on the underlying operating system. Use dependency checkers. Update SOAP to SOAP 1.2 or higher.
- Disable XML external entity and DTD processing in all XML parsers in the application, as per the OWASP Cheat Sheet 'XXE Prevention'.
- Implement positive ("whitelisting") server-side input validation, filtering, or sanitization to prevent hostile data within XML documents, headers, or nodes.
- Verify that XML or XSL file upload functionality validates incoming XML using XSD validation or similar.
- SAST tools can help detect XXE in source code, although manual code review is the best alternative in large, complex applications with many integrations.
If these controls are not possible, consider using virtual patching, API security gateways, Web Application Firewalls (WAF), or Interactive Application Security Testing (IAST) tools to detect, monitor, and block XXE attacks.
Hdiv actively protects applications against XML External Entities (XXE) attacks.