CVE-2025-49812 is a vulnerability classified under CWE-287 (Improper Authentication) affecting the Apache HTTP Server, specifically versions up to 2.4.63 when configured with mod_ssl using the 'SSLEngine optional' directive. This configuration allows HTTP to HTTPS upgrades via TLS, but due to an HTTP desynchronization flaw, an attacker positioned as a man-in-the-middle can intercept and hijack HTTP sessions by manipulating the TLS upgrade handshake. The attack exploits the server's failure to properly authenticate the TLS upgrade request, enabling session hijacking without requiring any credentials or user interaction. The vulnerability is network exploitable remotely (AV:N), requires high attack complexity (AC:H), and does not require privileges or user interaction. The impact is high on confidentiality and integrity, allowing attackers to impersonate legitimate users and potentially access sensitive data or perform unauthorized actions. The Apache Software Foundation has mitigated this issue by removing support for TLS upgrades in version 2.4.64, effectively eliminating the vulnerable code path. No known exploits have been reported in the wild yet, but the vulnerability’s characteristics make it a significant threat to deployments relying on optional TLS upgrades. Organizations using affected versions should upgrade promptly and review their SSL/TLS configurations to avoid optional TLS upgrades. Additional network security controls such as strict TLS enforcement and monitoring for anomalous session behavior can help reduce risk.

For European organizations, this vulnerability poses a substantial risk to the confidentiality and integrity of web sessions served by Apache HTTP Server instances configured with 'SSLEngine optional'. Successful exploitation allows attackers to hijack user sessions, potentially leading to unauthorized access to sensitive information, data leakage, and impersonation attacks. This is particularly critical for sectors handling personal data under GDPR, financial services, healthcare, and government services where session integrity is paramount. The vulnerability does not affect availability, but the loss of confidentiality and integrity can result in regulatory penalties, reputational damage, and operational disruption. Since Apache HTTP Server is widely deployed across Europe in both public and private sectors, especially in countries with large internet infrastructure like Germany, France, the UK, and the Netherlands, the threat surface is significant. The requirement for a man-in-the-middle position means that attackers need network access, which could be achieved via compromised networks or malicious insiders. The high attack complexity somewhat limits mass exploitation but targeted attacks against high-value targets remain a concern.

The primary mitigation is to upgrade Apache HTTP Server to version 2.4.64 or later, which removes support for TLS upgrades and eliminates the vulnerable code path. Organizations should audit their Apache configurations to identify and disable any use of 'SSLEngine optional' that enables TLS upgrades. Where upgrading immediately is not feasible, disabling TLS upgrades and enforcing strict HTTPS connections without fallback to HTTP is recommended. Network-level mitigations include deploying TLS interception detection, enforcing HTTP Strict Transport Security (HSTS), and monitoring for unusual session behaviors indicative of desynchronization attacks. Additionally, organizations should ensure that their network infrastructure prevents man-in-the-middle positioning by using secure network segments, VPNs, and encrypted internal communications. Regular security assessments and penetration testing should include checks for HTTP desynchronization vulnerabilities. Finally, educating system administrators about the risks of optional TLS upgrades and proper SSL/TLS configuration best practices will help prevent similar issues.