CVE-2025-25293 is a high-severity vulnerability affecting the ruby-saml library, a widely used Security Assertion Markup Language (SAML) toolkit for Ruby applications implementing single sign-on (SSO) functionality. The vulnerability arises from an uncontrolled resource consumption issue (CWE-400) related to the handling of compressed SAML responses. Specifically, ruby-saml uses the zlib compression library to decompress incoming SAML assertions. However, the library performs a size check on the compressed message before decompression rather than on the inflated data. This allows an attacker to craft a malicious compressed SAML response that bypasses the size check, resulting in excessive resource consumption during decompression. The consequence is a remote Denial of Service (DoS) attack, where the targeted application becomes unresponsive or crashes due to resource exhaustion. The vulnerability affects ruby-saml versions prior to 1.12.4 and versions from 1.13.0 up to but not including 1.18.0. The issue was addressed in versions 1.12.4 and 1.18.0 by correcting the size validation logic to account for decompressed data size. The CVSS 4.0 score is 7.7 (high), reflecting the network attack vector, no required privileges or user interaction, and a high impact on availability. No known exploits are currently reported in the wild, but the vulnerability is publicly disclosed and patchable.

For European organizations, the impact of this vulnerability can be significant, especially for those relying on ruby-saml for SSO integration in web applications and enterprise identity management. Exploitation can lead to service outages, disrupting user authentication flows and potentially causing downtime for critical business applications. This can affect productivity, customer trust, and compliance with regulations such as GDPR if service availability is compromised. Additionally, repeated or large-scale DoS attacks could strain infrastructure resources, increasing operational costs. Since SAML is commonly used in federated identity scenarios across sectors like finance, healthcare, government, and education, the disruption could have cascading effects on inter-organizational workflows and access to essential services. The vulnerability does not directly compromise confidentiality or integrity but poses a high risk to availability, which is critical for maintaining continuous access to services.

European organizations should immediately assess their use of ruby-saml and identify affected versions. The primary mitigation is to upgrade ruby-saml to version 1.12.4 or 1.18.0 or later, where the vulnerability is fixed. If immediate upgrade is not feasible, organizations should implement network-level protections such as rate limiting and filtering to detect and block suspicious SAML responses with abnormal compression characteristics. Application-level input validation enhancements can be considered to verify decompressed message sizes before processing. Monitoring application logs for unusual decompression errors or resource spikes can provide early detection of attempted exploitation. Additionally, organizations should review their SSO deployment architecture to ensure redundancy and failover capabilities to minimize downtime in case of DoS attempts. Coordinating with identity providers to enforce strict message size and compression policies can further reduce risk. Finally, maintaining up-to-date dependency management and vulnerability scanning processes will help prevent similar issues in the future.