Impact

An attacker may be able to exploit this vulnerability to bypass authentication or authorization mechanisms in systems that rely on xml-crypto for verifying signed XML documents. The vulnerability allows an attacker to modify a valid signed XML message in a way that still passes signature verification checks. For example, it could be used to alter critical identity or access control attributes, enabling an attacker with a valid account to escalate privileges or impersonate another user.

Patches

All versions <= 6.0.0 are affected. Please upgrade to version 6.0.1.

If you are still using v2.x or v3.x please upgrade to the associated patch version.

Indicators of Compromise

When logging XML payloads, check for the following indicators. If the payload includes encrypted elements, ensure you analyze the decrypted version for a complete assessment. (If encryption is not used, analyze the original XML document directly). This applies to various XML-based authentication and authorization flows, such as SAML Response payloads.

Multiple SignedInfo Nodes

There should not be more than one SignedInfo node inside a Signature. If you find multiple SignedInfo nodes, it could indicate an attack.

<Signature>
    <SomeNode>
      <SignedInfo>
        <Reference URI="somefakereference">
          <DigestValue>forgeddigestvalue</DigestValue>
        </Reference>
      </SignedInfo>
    </SomeNode>
    <SignedInfo>
        <Reference URI="realsignedreference">
          <DigestValue>realdigestvalue</DigestValue>
        </Reference>
      </SignedInfo>
    </SignedInfo>
</Signature>

Code to test

Pass in the decrypted version of the document

decryptedDocument = ... // yours to implement

// This check is per-Signature node, not per-document
const signedInfoNodes = xpath.select(".//*[local-name(.)='SignedInfo']", signatureNode);

if (signedInfoNodes.length === 0) {
  // Not necessarily a compromise, but invalid. Should contain exactly one SignedInfo node
  // Yours to implement
}

if (signedInfoNodes.length > 1) {
  // Compromise detected, yours to implement
}