CVE-2025-3986: Apereo CAS DOS Vulnerability

CVE-2025-3986 Overview

CVE-2025-3986 is a denial-of-service vulnerability in Apereo Central Authentication Service (CAS) 5.2.6. The flaw resides in CasConfigurationMetadataServerController.java, part of the cas-server-core-configuration-metadata-repository module. Manipulation of the Name argument triggers inefficient regular expression complexity, classified under [CWE-400] Uncontrolled Resource Consumption. An authenticated remote attacker can submit crafted input that forces catastrophic backtracking, exhausting CPU resources on the server. The exploit details have been disclosed publicly. According to the disclosure record, the vendor was contacted prior to publication but did not respond.

Critical Impact

Remote attackers with low privileges can degrade availability of the Apereo CAS configuration metadata endpoint through a Regular Expression Denial of Service (ReDoS) attack against the Name parameter.

Affected Products

• Apereo Central Authentication Service 5.2.6
• cas-server-core-configuration-metadata-repository module
• Deployments exposing the configuration metadata REST controller

Discovery Timeline

• 2025-04-27 - CVE-2025-3986 published to NVD
• 2025-11-05 - Last updated in NVD database

Technical Details for CVE-2025-3986

Vulnerability Analysis

The vulnerability exists in the CasConfigurationMetadataServerController REST endpoint within Apereo CAS 5.2.6. This controller exposes configuration metadata via HTTP and accepts a Name parameter used to filter or search configuration properties. The endpoint applies a regular expression to the supplied Name value without bounding execution time or input length.

When an attacker submits a specially crafted string, the regex engine enters catastrophic backtracking. Each additional character in the malicious payload increases evaluation time exponentially. A small payload of a few dozen characters can occupy a worker thread for seconds or minutes.

Repeated requests scale the impact linearly with available threads. The result is sustained CPU exhaustion that prevents legitimate authentication traffic from being processed by the CAS server.

Root Cause

The root cause is an unsafe regular expression pattern combined with absent input validation on user-supplied search terms. The pattern likely contains nested quantifiers or overlapping alternations that exhibit exponential time complexity on adversarial input. The controller does not enforce a timeout, length limit, or rate limit on the search operation.

Attack Vector

The attack is remotely reachable over the network and requires low-privileged authentication to reach the configuration metadata controller. The attacker issues an HTTP request to the metadata search endpoint with a Name query parameter containing a ReDoS-triggering string. No user interaction is required, and the request body remains small, which makes the attack difficult to distinguish from normal traffic at the network layer.

The vulnerability mechanism is described in prose only; no verified proof-of-concept code is referenced in this advisory. See the VulDB entry #306322 for additional technical context.

Detection Methods for CVE-2025-3986

Indicators of Compromise

• HTTP requests targeting the CAS configuration metadata controller with unusually long or repetitive values in the Name parameter.
• Sustained CPU saturation on CAS Java processes correlated with inbound requests to /configurationMetadata or similar metadata routes.
• Thread pool exhaustion warnings in CAS application logs without corresponding legitimate traffic spikes.
• Repeated requests from a single source containing patterns such as long runs of repeating characters followed by a non-matching suffix.

Detection Strategies

• Inspect web access logs for requests to the configuration metadata endpoint where the Name parameter exceeds a reasonable length threshold, for example 64 characters.
• Deploy WAF rules that flag query parameters containing classic ReDoS patterns such as (a+)+, repeated character classes, or excessive nested groupings.
• Correlate JVM CPU metrics with HTTP request rates to detect asymmetric resource consumption attacks.

Monitoring Recommendations

• Enable per-request execution time logging on the CAS metadata controller and alert on outliers.
• Monitor authentication latency and queue depth to catch ReDoS-induced degradation before it affects users.
• Track unique source IPs hitting the configuration metadata endpoint and rate-limit those exceeding baseline volumes.

How to Mitigate CVE-2025-3986

Immediate Actions Required

• Restrict network access to the Apereo CAS configuration metadata endpoint so that it is reachable only from administrative networks.
• Enforce strict input validation on the Name parameter, rejecting values longer than a defined maximum or containing repetitive patterns.
• Apply rate limiting at the reverse proxy or WAF layer to cap requests per source against the metadata controller.
• Upgrade away from Apereo CAS 5.2.6 to a currently supported release line where the configuration metadata controller has been hardened or removed.

Patch Information

No vendor patch has been linked in the NVD record for CVE-2025-3986. The disclosure notes that the vendor did not respond to coordinated disclosure attempts. Apereo CAS 5.2.x is an end-of-life branch. Operators should plan migration to an actively maintained CAS major version and confirm in release notes whether the affected metadata controller has been removed or refactored.

Workarounds

• Disable the configuration metadata REST controller in production if it is not required by operational tooling.
• Place the CAS deployment behind a reverse proxy that enforces request timeouts, terminating long-running requests before they exhaust application threads.
• Configure a WAF rule to drop requests where the Name query parameter matches known ReDoS payload patterns or exceeds length limits.
• Isolate the CAS service in a dedicated process group with CPU quotas so a ReDoS attack cannot degrade co-located services.