CVE-2026-15243 in CAS Client
Summary
by MITRE • 07/24/2026
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.
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Analysis
by VulDB Data Team • 07/24/2026
This vulnerability resides within the Apereo CAS Client implementation where the certificate validation process fails to properly verify hostname matching against the expected target server. The flaw exists in the SSL/TLS certificate validation logic that accepts any CA-trusted certificate for any hostname when the URL matches configured allowlists or regex patterns. This represents a critical security weakness that directly violates the fundamental principles of secure communication and authentication protocols. The vulnerability stems from improper certificate hostname verification, which allows attackers to bypass the expected certificate validation mechanisms through man-in-the-middle attacks.
The technical implementation flaw occurs at the certificate validation layer where the system relies solely on CA trust without enforcing proper hostname binding checks. When an attacker positions themselves in a MITM scenario through DNS poisoning, rogue Wi-Fi networks, or malicious proxy servers, they can present any valid CA-signed certificate for a domain that matches the client's allowlist patterns. This creates a dangerous condition where the authentication system accepts certificates that should not be trusted based on hostname verification alone. The vulnerability specifically affects the SSL/TLS handshake process and certificate validation routines, creating an attack surface that enables session hijacking.
The operational impact of this vulnerability is severe and directly enables credential theft and session manipulation. An attacker who successfully exploits this weakness can intercept the CAS authentication exchange and capture the Ticket-Granting Ticket which serves as the primary authentication token for subsequent service requests. Once obtained, the TGT allows the attacker to generate Service Tickets on behalf of legitimate users without requiring their credentials. This provides unauthorized access to protected services and applications that rely on CAS authentication. The vulnerability affects the core authentication integrity and can lead to complete system compromise if exploited effectively.
The attack vectors available to exploit this vulnerability include DNS poisoning attacks, rogue wireless access points, malicious proxy servers, and other network position attacks that allow interception of TLS traffic. This aligns with the ATT&CK technique of credential access through network sniffing and man-in-the-middle attacks. The weakness also maps to CWE-295 which describes improper certificate validation and hostname verification failures. Organizations using affected versions of Apereo CAS Client 4.1.0 or Jasig CAS Client 3.6.4 face significant risk exposure, as the vulnerability allows attackers to impersonate legitimate users and gain unauthorized access to protected resources.
Mitigation strategies should include implementing proper hostname verification during SSL/TLS certificate validation, updating to patched versions of the affected software components, configuring strict certificate validation policies that enforce hostname matching, and deploying network monitoring to detect potential MITM activities. Organizations should also consider implementing additional security controls such as certificate pinning, enhanced network segmentation, and regular security assessments to identify and remediate similar vulnerabilities in other authentication systems. The fix requires ensuring that certificate validation routines properly check the Subject Alternative Name (SAN) fields and Common Name (CN) values against the expected hostname, rather than relying solely on CA trust relationships without hostname verification.