CVE-2025-27773: SimpleSAMLphp Auth Bypass Vulnerability

CVE-2025-27773 Overview

CVE-2025-27773 is a signature confusion vulnerability in the SimpleSAMLphp SAML2 library, a widely-used PHP library that provides SAML2 (Security Assertion Markup Language 2.0) functionality for authentication and single sign-on implementations. The vulnerability exists in the HTTP-Redirect binding mechanism and allows an attacker with any signed SAMLResponse via the HTTP-Redirect binding to cause the application to accept an unsigned message, effectively bypassing signature validation controls.

This authentication bypass vulnerability poses significant risk to organizations using SimpleSAMLphp for federated identity management, as it could allow attackers to forge SAML assertions and gain unauthorized access to protected resources.

Critical Impact

An attacker can exploit this signature confusion attack to bypass SAML signature validation, potentially allowing unauthorized authentication and access to protected applications using SimpleSAMLphp for single sign-on.

Affected Products

• SimpleSAMLphp SAML2 library versions prior to 4.17.0
• SimpleSAMLphp SAML2 library versions prior to 5.0.0-alpha.20
• Applications and identity providers utilizing vulnerable SimpleSAMLphp SAML2 library versions

Discovery Timeline

• March 11, 2025 - CVE-2025-27773 published to NVD
• May 9, 2025 - Last updated in NVD database

Technical Details for CVE-2025-27773

Vulnerability Analysis

This vulnerability is classified under CWE-347 (Improper Verification of Cryptographic Signature), which occurs when software does not properly verify the cryptographic signature attached to a message or data. In the context of SAML2, signature verification is critical to ensuring that authentication assertions originate from a trusted identity provider and have not been tampered with.

The signature confusion attack exploits a flaw in how the HTTPRedirect.php binding handler processes incoming SAML messages. When processing SAMLResponse messages received via the HTTP-Redirect binding, the vulnerable code fails to properly correlate the signature validation status with the actual message being processed. This allows an attacker who possesses any legitimately signed SAMLResponse to manipulate the application into accepting an entirely different, unsigned message.

The vulnerability has a network attack vector, requires no privileges or user interaction, and can impact the confidentiality and integrity of systems beyond the vulnerable component (changed scope). The primary impact is on integrity, allowing attackers to bypass authentication controls.

Root Cause

The root cause lies in the improper handling of signature verification state within the HTTP-Redirect binding implementation. The vulnerable code in HTTPRedirect.php (specifically around lines 104-113 and lines 178-217) does not maintain proper state isolation between signature verification and message processing, creating a window where an attacker can substitute messages.

This is a classic signature confusion pattern where the verification step and the processing step can be decoupled by an attacker, allowing signed content from one context to "vouch for" unsigned content in another.

Attack Vector

The attack leverages the HTTP-Redirect binding, which is one of the standard SAML2 bindings used for transmitting SAML messages via URL parameters. An attacker would need to:

1. Obtain any legitimately signed SAMLResponse that uses the HTTP-Redirect binding (this could be from a previous session or intercepted traffic)
2. Craft a malicious unsigned SAML assertion containing forged authentication claims
3. Exploit the signature confusion vulnerability to make the application accept the unsigned assertion

The vulnerability is exploitable over the network with low attack complexity and requires no authentication or user interaction, making it particularly dangerous for internet-facing SSO implementations.

For detailed technical information about the vulnerability mechanism, refer to the GitHub Security Advisory and the fix commit.

Detection Methods for CVE-2025-27773

Indicators of Compromise

• Anomalous SAML authentication events with mismatched signature validation states
• Authentication logs showing successful SSO authentications without corresponding valid signature verifications
• Unusual patterns in HTTP-Redirect binding traffic, particularly requests with manipulated SAMLResponse parameters
• User sessions established from unexpected identity providers or with unexpected attribute values

Detection Strategies

• Monitor SAML processing logs for signature validation failures followed by successful authentication
• Implement deep packet inspection for SAML traffic to detect malformed or suspicious assertions
• Deploy web application firewall rules to detect anomalous SAML message patterns in HTTP-Redirect binding requests
• Enable verbose logging on SimpleSAMLphp instances to capture detailed signature verification events

Monitoring Recommendations

• Configure alerting on authentication events where signature validation status is ambiguous or inconsistent
• Establish baselines for normal SAML authentication patterns and alert on deviations
• Monitor for multiple failed signature validations from the same source followed by successful authentications
• Implement centralized logging for all SAML-based authentication to enable correlation analysis

How to Mitigate CVE-2025-27773

Immediate Actions Required

• Upgrade SimpleSAMLphp SAML2 library to version 4.17.0 or later for the 4.x branch
• Upgrade SimpleSAMLphp SAML2 library to version 5.0.0-alpha.20 or later for the 5.x branch
• Audit authentication logs for any signs of exploitation prior to patching
• Consider temporarily disabling HTTP-Redirect binding if immediate patching is not possible

Patch Information

The vulnerability has been fixed in SimpleSAMLphp SAML2 library versions 4.17.0 and 5.0.0-alpha.20. The fix ensures proper correlation between signature validation and message processing in the HTTP-Redirect binding handler. The patch is available via the official commit.

For Debian-based systems, refer to the Debian LTS Announcement for package updates.

Workarounds

• If upgrading is not immediately possible, consider using HTTP-POST binding instead of HTTP-Redirect binding where feasible
• Implement additional network-level controls to restrict access to SAML endpoints
• Enable strict signature validation settings in SimpleSAMLphp configuration
• Deploy a web application firewall with rules specifically designed to validate SAML message integrity

# Verify current SimpleSAMLphp SAML2 library version via Composer
composer show simplesamlphp/saml2

# Update to patched version
composer require simplesamlphp/saml2:^4.17.0

# Or for 5.x alpha branch
composer require simplesamlphp/saml2:^5.0.0-alpha.20