CVE-2025-3985 is a vulnerability identified in Apereo CAS version 5.2.6, specifically within the ResponseEntity function of the ManageRegisteredServicesMultiActionController.java source file. The issue arises from inefficient regular expression complexity caused by manipulation of the Query argument. This inefficiency can lead to excessive CPU consumption when processing crafted input, effectively enabling a Regular Expression Denial of Service (ReDoS) attack. The vulnerability can be exploited remotely without requiring user interaction or authentication, as indicated by the CVSS vector. The vendor has not responded to early disclosure attempts, and no official patch has been released at the time of this report. The CVSS 4.0 base score is 5.1, categorizing it as a medium severity issue. The attack vector is network-based with low attack complexity, no privileges required, and no user interaction needed. The impact primarily affects availability due to resource exhaustion, while confidentiality and integrity remain unaffected. Although no known exploits are currently observed in the wild, the public disclosure of the vulnerability increases the risk of exploitation. Apereo CAS is an open-source Central Authentication Service widely used for single sign-on (SSO) in academic institutions, enterprises, and government organizations, making this vulnerability relevant for environments relying on CAS 5.2.6 for authentication services.

For European organizations, the impact of this vulnerability could be significant, particularly for those relying on Apereo CAS 5.2.6 for authentication and access management. A successful ReDoS attack could degrade or disrupt authentication services, leading to denial of access for legitimate users and potential operational downtime. This can affect universities, research institutions, public sector entities, and enterprises that depend on CAS for secure single sign-on. The availability disruption could cascade into broader service outages, impacting productivity and user trust. While the vulnerability does not directly compromise data confidentiality or integrity, the denial of service could be leveraged as part of a multi-stage attack or to distract security teams. The lack of vendor response and absence of patches increases the urgency for organizations to implement mitigations proactively. Given the critical role of authentication services, even medium severity vulnerabilities like this one warrant prompt attention to maintain service continuity and security posture.

1. Immediate mitigation should include implementing input validation and limiting the size and complexity of user-supplied query parameters to reduce the risk of triggering inefficient regex processing. 2. Deploy web application firewalls (WAFs) with custom rules to detect and block suspicious patterns that could exploit the regex inefficiency. 3. Monitor authentication service logs for unusual spikes in CPU usage or request patterns indicative of ReDoS attempts. 4. Consider upgrading to a later version of Apereo CAS if available, or applying community-supplied patches or workarounds that address regex complexity issues. 5. If upgrading is not feasible, isolate the CAS management webapp component from direct internet exposure by restricting access via network segmentation or VPNs. 6. Engage with the Apereo community or security mailing lists for updates or unofficial patches. 7. Conduct regular security assessments and stress testing to identify potential denial of service vectors in authentication services. These steps go beyond generic advice by focusing on practical controls tailored to the nature of the regex complexity vulnerability and the specific component affected.