Listen to this Post
This authentication bypass vulnerability stems from an improper handling of canonicalization failures within the libxml2 library, as utilized by the xmlseclibs PHP library for XML Digital Signature processing. The flaw occurs during the document transformation phase. When libxml2’s canonicalization function is invoked on a specifically crafted, invalid XML input, it can fail silently and return an empty string instead of a properly canonicalized node set or throwing an error. The xmlseclibs library does not treat this empty result as a failure. It proceeds to compute the cryptographic digest (DigestValue) over this empty string. This allows an attacker to generate a signature for an empty data set. Since the validation logic only checks if the computed digest matches the provided DigestValue in the signature, and both are based on the same empty canonical form, the validation passes. Crucially, a valid signature created for an empty string in one context (e.g., a prior legitimate session or a misconfigured service) can be extracted and replayed in a different authentication context. The receiving system, following the same flawed validation path, will accept the replayed signature as valid for its own empty canonicalization output, thereby bypassing authentication checks entirely.
Platform: libxml2 / xmlseclibs
Version: Commit f4131320
Vulnerability: Authentication Bypass
Severity: Critical
date: Unspecified
Prediction: Patch Expected Q3 2024
What Undercode Say:
Analytics
git clone https://github.com/robrichards/xmlseclibs.git
cd xmlseclibs
git show f4131320c6dcd460f1b0c67f16f8bf24ce4b5c3e
grep -n “canonicalize” src/XMLSecurityDSig.php
How Exploit
Craft invalid XML.
Trigger canonicalization failure.
Capture resulting signature.
Replay signature.
Protection from this CVE
Treat empty canonicalization as fatal error.
Add explicit nil/empty checks.
Abort validation on failure.
Impact:
Authentication Bypass
Signature Replay Attacks
Trust Chain Compromise
🎯Let’s Practice Exploiting & Learn Patching For Free:
Sources:
Reported By: github.com
Extra Source Hub:
Undercode
