CVE-2025-23419 is a vulnerability classified under CWE-287 (Improper Authentication) affecting F5 NGINX Open Source version 1.11.4. The vulnerability occurs when multiple server blocks are configured to share the same IP address and port, and the default server is configured to perform client certificate authentication. The root cause lies in the improper handling of TLS session resumption mechanisms, specifically TLS Session Tickets and SSL session cache. When these session resumption features are enabled, an attacker can exploit the reuse of TLS sessions to bypass client certificate authentication requirements on other server blocks sharing the same IP and port. This means that a malicious client can resume a TLS session initially authenticated with a client certificate on the default server and then access other servers that do not require or enforce client certificate authentication, effectively circumventing the intended security controls. The vulnerability requires network-level access and low privileges (PR:L), does not require user interaction (UI:N), and has a CVSS v3.1 base score of 4.3, indicating medium severity. The impact is primarily on confidentiality, as unauthorized access to services protected by client certificates can lead to information disclosure. The vulnerability does not affect software versions that have reached End of Technical Support (EoTS) and no known exploits have been reported in the wild. The issue is specific to configurations using TLS session tickets or SSL session cache in the default server with client certificate authentication enabled, which is a relatively specialized setup but common in environments requiring strong mutual TLS authentication.

For European organizations, this vulnerability poses a risk of unauthorized access to internal or external services protected by client certificate authentication, potentially leading to information disclosure or unauthorized data access. Sectors such as finance, healthcare, government, and critical infrastructure that rely on mutual TLS for secure communications are particularly vulnerable. The bypass of client certificate authentication undermines trust boundaries and could facilitate lateral movement within networks or unauthorized access to sensitive applications. Although the CVSS score is medium, the impact on confidentiality can be significant in regulated environments subject to GDPR and other data protection laws. The lack of known exploits reduces immediate risk, but the presence of this vulnerability in widely deployed open-source NGINX versions means that attackers could develop exploits, especially targeting organizations with complex multi-server block configurations. The vulnerability also complicates compliance with strict authentication requirements, potentially leading to regulatory and reputational consequences if exploited.

European organizations should take the following specific actions: 1) Audit NGINX configurations to identify multi-server block setups sharing the same IP and port, especially those using client certificate authentication on the default server. 2) Disable TLS Session Tickets and SSL session cache in the default server block to prevent session resumption attacks, by setting 'ssl_session_tickets off;' and 'ssl_session_cache off;' in the NGINX configuration. 3) Where possible, segregate services requiring client certificate authentication onto separate IP addresses or ports to avoid shared session contexts. 4) Monitor network traffic for anomalous TLS session resumptions that do not align with expected client certificate usage. 5) Plan and apply patches or upgrade to a fixed version of F5 NGINX Open Source once released by the vendor. 6) Implement additional layers of authentication or authorization controls at the application level to mitigate potential bypasses. 7) Conduct regular security assessments and penetration tests focusing on TLS configurations and session management. These measures go beyond generic advice by focusing on configuration hardening and architectural segregation specific to this vulnerability.