CVE-2025-23048 is a critical vulnerability classified under CWE-284 (Improper Access Control) affecting Apache HTTP Server versions 2.4.35 through 2.4.63. The issue arises in mod_ssl configurations where multiple virtual hosts are set up, each restricted to different sets of trusted client certificates, typically enforced via SSLCACertificateFile or SSLCACertificatePath directives. When TLS 1.3 session resumption is used, a client authenticated and trusted on one virtual host can bypass access controls and gain unauthorized access to another virtual host if SSLStrictSNIVHostCheck is not enabled on either host. This occurs because the server does not properly enforce strict Server Name Indication (SNI) checks during session resumption, allowing the client to reuse a session associated with one virtual host to access another. The vulnerability does not require any privileges or user interaction and can be exploited remotely over the network. The impact includes unauthorized disclosure and modification of data on virtual hosts that should be restricted, compromising confidentiality and integrity. Although no known exploits are currently in the wild, the high CVSS score of 9.1 indicates a critical risk. The vulnerability affects configurations common in environments where client certificate authentication is used to segregate access across multiple virtual hosts on the same Apache server instance.

For European organizations, this vulnerability poses a significant risk to multi-tenant web services, government portals, financial institutions, and enterprises that rely on Apache HTTP Server with mod_ssl configured for client certificate-based access control across multiple virtual hosts. Unauthorized access could lead to data breaches, exposure of sensitive information, and potential lateral movement within networks. The confidentiality and integrity of data hosted on affected virtual hosts are at risk, potentially violating GDPR and other data protection regulations. Organizations with complex hosting environments or those providing services to multiple clients on shared infrastructure are particularly vulnerable. The ease of remote exploitation without authentication increases the threat level, potentially enabling attackers to bypass intended access restrictions and compromise sensitive resources.

To mitigate this vulnerability, administrators should immediately enable the SSLStrictSNIVHostCheck directive on all virtual hosts configured with mod_ssl and client certificate restrictions. This setting enforces strict SNI checks during TLS session resumption, preventing session reuse across virtual hosts. Additionally, organizations should plan to upgrade Apache HTTP Server to versions beyond 2.4.63 once official patches are released. In the interim, reviewing and auditing mod_ssl configurations for multiple virtual hosts with distinct client certificate requirements is critical. Network segmentation and monitoring TLS session resumption behaviors can provide additional detection capabilities. Implementing strict logging and alerting for anomalous access patterns related to TLS sessions may help identify exploitation attempts. Finally, educating system administrators about this vulnerability and ensuring timely patch management processes are essential to reduce exposure.