CVE-2025-23048 is a vulnerability in the Apache HTTP Server versions 2.4.35 through 2.4.63, specifically affecting configurations that use mod_ssl with multiple virtual hosts, each restricted to different sets of trusted client certificates. The vulnerability arises when TLS 1.3 session resumption is used, allowing an access control bypass by trusted clients. In such configurations, if the SSLStrictSNIVHostCheck directive is not enabled on the virtual hosts, a client authenticated and trusted for one virtual host can potentially gain unauthorized access to another virtual host that it should not have access to. This occurs because the server does not properly enforce strict Server Name Indication (SNI) checks during TLS session resumption, leading to improper access control (CWE-284). The issue is rooted in the interaction between TLS 1.3 session resumption and mod_ssl's handling of client certificate validation across multiple virtual hosts. This vulnerability can allow unauthorized access to sensitive resources or services hosted on other virtual hosts within the same Apache HTTP Server instance, undermining the confidentiality and integrity of those resources.

For European organizations, this vulnerability poses a significant risk, especially for those hosting multiple services or applications on a single Apache HTTP Server instance using mod_ssl with client certificate authentication. The access control bypass could lead to unauthorized data access, potentially exposing sensitive personal data protected under GDPR, intellectual property, or internal business information. The impact is heightened in sectors such as finance, healthcare, government, and critical infrastructure, where strict access controls are mandatory. Exploitation could facilitate lateral movement within an organization's network, enabling attackers to escalate privileges or exfiltrate data across virtual hosts. Although no known exploits are currently reported in the wild, the ease of exploitation via TLS 1.3 session resumption and the widespread use of Apache HTTP Server in Europe make this a credible threat. The vulnerability undermines trust in client certificate-based authentication mechanisms, which are often used for high-security environments.

To mitigate this vulnerability, European organizations should immediately verify their Apache HTTP Server configurations if they use mod_ssl with multiple virtual hosts and client certificate authentication. The primary mitigation is to enable the SSLStrictSNIVHostCheck directive on all virtual hosts to enforce strict SNI checks during TLS session resumption, preventing clients from accessing unauthorized virtual hosts. Organizations should also consider upgrading Apache HTTP Server to a version beyond 2.4.63 once patches become available, as this will likely include a fix for this vulnerability. In the interim, disabling TLS 1.3 session resumption or restricting session resumption to trusted clients may reduce risk. Regularly auditing virtual host configurations and client certificate policies is recommended to ensure no misconfigurations exist. Additionally, monitoring server logs for unusual access patterns or unexpected client certificate usage can help detect exploitation attempts early. Finally, organizations should maintain an incident response plan tailored to web server compromises involving access control bypasses.