CVE-2025-31335: OpenSAML C++ Auth Bypass Vulnerability

CVE-2025-31335 Overview

CVE-2025-31335 is a signature forgery vulnerability in the OpenSAML C++ library before version 3.3.1. The vulnerability allows attackers to forge signed SAML messages via parameter manipulation when using SAML bindings that rely on non-XML signatures. This affects authentication and authorization systems that depend on SAML for secure identity federation.

Critical Impact

Attackers can forge SAML authentication assertions, potentially bypassing authentication controls and impersonating legitimate users in federated identity systems.

Affected Products

• OpenSAML C++ library versions prior to 3.3.1
• Shibboleth Service Provider implementations using vulnerable OpenSAML versions
• Systems using SAML HTTP-Redirect or HTTP-POST bindings with non-XML signatures

Discovery Timeline

• 2025-03-13 - Shibboleth releases security advisory
• 2025-03-28 - CVE CVE-2025-31335 published to NVD
• 2025-03-28 - Last updated in NVD database

Technical Details for CVE-2025-31335

Vulnerability Analysis

This vulnerability relates to CWE-347 (Improper Verification of Cryptographic Signature). The OpenSAML C++ library contains a flaw in how it handles signature verification for SAML messages when using bindings that employ non-XML signatures, such as HTTP-Redirect binding. The issue enables attackers to manipulate parameters in signed SAML messages without invalidating the cryptographic signature, effectively allowing message forgery.

SAML bindings like HTTP-Redirect use signatures applied to URL-encoded parameters rather than XML digital signatures. The vulnerability exists in how OpenSAML processes and validates these parameters, creating an opportunity for manipulation that bypasses signature verification.

Root Cause

The root cause lies in improper verification of cryptographic signatures in the OpenSAML C++ library. When processing SAML messages with non-XML signatures, the library fails to properly bind all security-relevant parameters to the signature verification process. This allows an attacker to modify certain message parameters after signing while the signature remains valid, violating the integrity guarantees that digital signatures are meant to provide.

Attack Vector

The attack is network-based and requires the attacker to intercept or observe legitimate SAML messages. The attacker can then manipulate specific parameters in the SAML request or response while preserving the original signature. This forged message can be submitted to the service provider, which will incorrectly validate the signature and process the manipulated content.

The attack scenario typically involves:

1. Intercepting a legitimately signed SAML message
2. Modifying security-relevant parameters (such as subject identity or authorization attributes)
3. Submitting the manipulated message to the target service provider
4. Bypassing authentication or gaining unauthorized access due to improper signature verification

For detailed technical information about the vulnerability mechanism, see the Shibboleth Security Advisory and the Shibboleth Issue Tracker.

Detection Methods for CVE-2025-31335

Indicators of Compromise

• Unexpected SAML authentication events for users who did not initiate login
• SAML assertions with mismatched or suspicious attribute values
• Authentication logs showing access from unexpected locations or times for federated identities
• Anomalous patterns in SAML message parameters compared to baseline traffic

Detection Strategies

• Monitor SAML authentication logs for anomalous login patterns or unexpected identity claims
• Implement logging of all SAML assertion attributes and compare against expected values
• Deploy network monitoring to detect manipulation of SAML messages in transit
• Review audit logs for authentication events that correlate with suspicious activity

Monitoring Recommendations

• Enable verbose logging for SAML service provider components to capture full assertion details
• Implement alerting on authentication events involving high-privilege accounts via SAML
• Monitor for multiple authentication attempts using variations of the same base SAML message
• Correlate SAML authentication events with subsequent user activity for anomaly detection

How to Mitigate CVE-2025-31335

Immediate Actions Required

• Upgrade OpenSAML C++ library to version 3.3.1 or later immediately
• Review SAML service provider configurations to ensure proper signature validation settings
• Audit recent SAML authentication logs for signs of exploitation
• Consider temporarily disabling SAML bindings that use non-XML signatures if upgrade is not immediately possible

Patch Information

The vulnerability has been addressed in OpenSAML C++ version 3.3.1. The fix is available in the Shibboleth Git repository at commit 22a610b322e2178abd03e97cdbc8fb50b45efaee. Debian users should refer to the Debian Security Announcement for distribution-specific patch information.

Workarounds

• If immediate patching is not possible, consider restricting SAML authentication to XML-signature-based bindings only
• Implement additional application-level validation of SAML assertion attributes
• Deploy network-level controls to limit SAML traffic to trusted identity providers
• Enable strict signature verification modes in service provider configuration where available

# Example: Verify OpenSAML version on Linux systems
rpm -qa | grep opensaml
dpkg -l | grep opensaml

# Check Shibboleth SP version
shibd -v