CVE-2025-40934: XML-Sig for Perl XXE Vulnerability

CVE-2025-40934 Overview

CVE-2025-40934 is a signature validation bypass vulnerability in the XML::Sig Perl module affecting versions 0.27 through 0.67. The module incorrectly validates XML files when signatures are omitted, allowing attackers to bypass signature verification entirely by simply removing the signature element from an XML document.

XML::Sig is a Perl module designed to validate cryptographic signatures on XML files, commonly used in SAML (Security Assertion Markup Language) implementations and other XML-based authentication workflows. The vulnerability causes the module to return a successful validation result (true) when processing XML documents that contain no signatures, when it should instead return an error indicating the absence of required signatures.

Critical Impact

Attackers can bypass XML signature verification by removing signatures from documents, potentially enabling authentication bypass, data tampering, and unauthorized access to systems relying on XML::Sig for security validation.

Affected Products

• XML::Sig for Perl versions 0.27 through 0.67
• Applications using XML::Sig for SAML authentication
• Systems relying on XML::Sig for XML document signature validation

Discovery Timeline

• 2025-11-26 - CVE CVE-2025-40934 published to NVD
• 2025-12-30 - Last updated in NVD database

Technical Details for CVE-2025-40934

Vulnerability Analysis

This vulnerability represents a Improper Verification of Cryptographic Signature (CWE-347) flaw where the XML::Sig module fails to enforce the requirement that XML documents must contain valid signatures before returning a positive validation result.

When the verify() function is called on an XML document that lacks any signature element, the module does not detect this absence as an error condition. Instead, it proceeds through its validation logic and returns true, falsely indicating that the document passed signature verification. This behavior fundamentally undermines the security guarantee that XML::Sig is intended to provide.

The vulnerability is particularly dangerous in SAML authentication contexts, where XML signatures are the primary mechanism for ensuring the authenticity and integrity of authentication assertions. An attacker who can intercept or modify SAML responses could strip the signature entirely, and the vulnerable XML::Sig module would still report successful validation.

Root Cause

The root cause lies in insufficient validation logic within the XML::Sig module's signature verification routine. The module does not properly check for the presence of signature elements before attempting validation. When no signature is found, instead of raising an error or returning false, the code path leads to a successful return value. This is a logic flaw where the absence of something that should be validated is not treated as a validation failure.

Attack Vector

The attack vector for this vulnerability is network-based and requires no authentication or user interaction. An attacker positioned to intercept XML documents (such as SAML responses) can exploit this vulnerability through the following approach:

1. Intercept an XML document containing a valid signature
2. Remove the signature element entirely from the document
3. Optionally modify other elements in the document (such as user identity assertions)
4. Forward the modified, unsigned document to the target application
5. The vulnerable XML::Sig module validates the document as authentic

This attack is especially effective against SAML Single Sign-On (SSO) implementations where an attacker could forge authentication assertions, potentially gaining unauthorized access as any user.

The vulnerability mechanism involves the improper handling of missing signature elements. When XML::Sig receives an unsigned document, its verification function fails to detect the absence of the <Signature> element and incorrectly returns a successful validation status. For detailed technical analysis, see the GitHub Issue Discussion and the related Pull Request.

Detection Methods for CVE-2025-40934

Indicators of Compromise

• Successful authentication or authorization events for XML documents that lack signature elements
• SAML assertions being accepted without corresponding signature validation logs
• Unusual authentication patterns where users appear to authenticate without proper IdP signatures
• Log entries showing XML::Sig validation success for documents that should contain signatures

Detection Strategies

• Implement additional logging before XML::Sig validation to record whether incoming documents contain signature elements
• Monitor for SAML responses or XML documents that are processed successfully but lack <Signature> elements
• Review application logs for authentication success events that lack corresponding signature verification details
• Deploy network monitoring to detect XML documents in transit that have had signatures stripped

Monitoring Recommendations

• Audit all systems using XML::Sig for Perl and verify the installed version
• Implement pre-validation checks that verify signature presence before calling XML::Sig
• Configure alerting for any XML signature validation that completes without processing a signature element
• Review access logs for anomalous authentication patterns that could indicate exploitation

How to Mitigate CVE-2025-40934

Immediate Actions Required

• Identify all installations of XML::Sig for Perl and check version numbers against the vulnerable range (0.27 through 0.67)
• Upgrade XML::Sig to a patched version that addresses this signature validation bypass
• Implement additional validation logic to verify that XML documents contain signatures before passing them to XML::Sig
• Review recent authentication logs for signs of exploitation

Patch Information

The vulnerability has been addressed by the XML::Sig maintainers. Organizations should upgrade to a version newer than 0.67 that properly validates the presence of signatures before returning a successful verification result. Detailed information about the fix is available in the GitHub Pull Request. Organizations should also review the GitHub Issue Discussion for additional context and mitigation guidance.

Workarounds

• Implement a pre-validation check that verifies XML documents contain a <Signature> element before calling XML::Sig's verify function
• Use XPath queries to confirm signature presence: check for //ds:Signature or //Signature elements
• Consider temporarily using an alternative XML signature validation library until the upgrade can be performed
• Implement network-level controls to reject XML documents that lack expected signature elements

# Check installed XML::Sig version in Perl
perl -MXML::Sig -e 'print $XML::Sig::VERSION, "\n"'

# Verify if version is in vulnerable range (0.27-0.67)
# If vulnerable, update via CPAN:
cpanm XML::Sig