Vulnerability in Apereo CAS Client software

CERT Polska has received a report about vulnerability in Apereo CAS Client software and participated in coordination of its disclosure.

The vulnerabilityCVE-2026-15243: Apereo CAS Client accepts any CA-trusted certificate for any hostname, provided the URL the client is calling matches the configured allowlist or regex. An attacker with a MITM position (DNS poisoning, rogue Wi-Fi, malicious proxy, etc.) can provide any CA-signed certificate for a hostname that matches the configured allowlist or regex. This can lead to intercepting the CAS exchange, capturing the Ticket-Granting Ticket (TGT), and subsequently obtaining Service Tickets on behalf of the victim.

Because maintainers contact attempts were unsuccessful, vulnerabilities have only been confirmed in version 4.1.0 (Java Apereo CAS Client) and 3.6.4 (Jasig CAS Client) but may also affect other versions.

We thank Rafał Łykowski, Łukasz Kollbek and Krzysztof Surówka from Qualtrics for the responsible vulnerability report.