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, specifically within the mod_ssl module. The vulnerability arises in configurations where mod_ssl is set up with multiple virtual hosts, 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 trusted client authenticated for one virtual host can bypass access controls and gain access to other virtual hosts if the SSLStrictSNIVHostCheck directive is not enabled. SSLStrictSNIVHostCheck enforces strict Server Name Indication (SNI) checks during the TLS handshake, ensuring that resumed sessions are bound to the correct virtual host. Without this check, the TLS session resumption mechanism can be exploited to reuse a session established for one virtual host to access another, effectively bypassing client certificate restrictions. This flaw compromises confidentiality and integrity by allowing unauthorized clients to access sensitive resources or data on virtual hosts they should not have access to. The vulnerability can be exploited remotely without requiring any privileges or user interaction, increasing its risk profile. Although no known exploits are currently reported in the wild, the high CVSS score of 9.1 reflects the severity and ease of exploitation. The issue is particularly relevant for organizations hosting multiple virtual hosts with client certificate-based access controls, a common setup in enterprise and government environments. The vulnerability was publicly disclosed on July 10, 2025, and no official patches were linked at the time, emphasizing the importance of configuration-based mitigations.

For European organizations, this vulnerability poses a significant risk to the confidentiality and integrity of web services hosted on Apache HTTP Server with mod_ssl configured for multiple virtual hosts using client certificate restrictions. Unauthorized access to virtual hosts can lead to data breaches, exposure of sensitive information, and potential lateral movement within the network. Critical sectors such as finance, healthcare, government, and telecommunications that rely on Apache HTTP Server for secure client authentication are particularly vulnerable. The ability to bypass access controls without authentication or user interaction increases the likelihood of exploitation by threat actors. This could result in regulatory non-compliance under GDPR due to unauthorized data access and potential reputational damage. Additionally, organizations using TLS 1.3 session resumption to optimize performance are directly impacted, as this feature is integral to the exploit. The vulnerability undermines trust in client certificate-based authentication schemes, which are often used to enforce strict access policies in multi-tenant environments. Overall, the impact is severe, with potential for widespread unauthorized access and data compromise across affected European enterprises.

To mitigate CVE-2025-23048, European organizations should immediately verify their Apache HTTP Server mod_ssl configurations for multiple virtual hosts using distinct trusted client certificate sets. The primary mitigation is to enable the SSLStrictSNIVHostCheck directive on all virtual hosts, which enforces strict SNI checks during TLS session resumption, preventing session reuse across virtual hosts. Administrators should audit their configurations to ensure SSLStrictSNIVHostCheck is set to 'on' explicitly. If enabling SSLStrictSNIVHostCheck is not feasible due to compatibility or operational constraints, organizations should consider disabling TLS 1.3 session resumption temporarily until patches are available. Monitoring and logging TLS session resumptions and client certificate authentications can help detect anomalous access patterns indicative of exploitation attempts. Organizations should also plan to upgrade Apache HTTP Server to patched versions once released by the Apache Software Foundation. Network segmentation and strict access controls around web servers can limit the blast radius of potential exploitation. Finally, security teams should educate administrators about this vulnerability and incorporate checks into vulnerability management and configuration auditing processes to ensure ongoing compliance.