CVE-2025-27773 is a high-severity vulnerability affecting the SimpleSAMLphp SAML2 PHP library, specifically versions prior to 4.17.0 and certain 5.0.0-alpha releases before 5.0.0-alpha.20. The vulnerability arises from improper verification of cryptographic signatures (CWE-347) in the HTTP-Redirect binding used for SAML2 authentication messages. In this context, an attacker who possesses any signed SAMLResponse transmitted via the HTTP-Redirect binding can exploit a signature confusion flaw to trick the application into accepting an unsigned message as valid. This effectively bypasses the intended cryptographic verification, undermining the integrity of the authentication process. The flaw is critical because it allows an attacker to forge or replay SAML assertions without a valid signature, potentially granting unauthorized access or elevating privileges within systems relying on SimpleSAMLphp for federated identity management. The vulnerability does not require authentication or user interaction and can be exploited remotely over the network. The issue was addressed in versions 4.17.0 and 5.0.0-alpha.20 by correcting the signature verification logic to properly distinguish between signed and unsigned messages in the HTTP-Redirect binding. No known exploits are currently reported in the wild, but the high CVSS score of 8.6 reflects the significant risk posed by this vulnerability if exploited.

For European organizations, especially those utilizing SimpleSAMLphp for Single Sign-On (SSO) and federated identity management, this vulnerability poses a substantial risk. Exploitation could allow attackers to bypass authentication controls, leading to unauthorized access to sensitive systems and data. This can result in data breaches, compromise of user accounts, and potential lateral movement within networks. Given the widespread adoption of SAML2 for identity federation in government, healthcare, finance, and large enterprises across Europe, the impact could be severe, including regulatory non-compliance with GDPR due to unauthorized data access. The integrity of authentication flows is critical in these sectors, and a failure here undermines trust in identity assertions, potentially disrupting business operations and damaging reputations.

European organizations should promptly upgrade SimpleSAMLphp to version 4.17.0 or later, or to 5.0.0-alpha.20 or later if using alpha releases. Beyond patching, organizations should audit their SAML2 implementations to ensure that signature verification is correctly enforced, particularly for HTTP-Redirect bindings. Implement additional monitoring on authentication logs to detect anomalous SAML assertions or unexpected authentication flows. Employ strict validation of SAML messages, including checking binding types and signature presence. Where possible, restrict the sources of SAML responses to trusted identity providers and implement network-level controls to limit exposure. Conduct penetration testing focused on SAML authentication to verify that signature verification cannot be bypassed. Finally, maintain an incident response plan tailored to identity federation compromises.