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 enables TLS upgrades on HTTP connections, allowing the server to switch from plaintext HTTP to encrypted HTTPS dynamically. The vulnerability arises from an HTTP desynchronisation attack vector, where a man-in-the-middle (MitM) attacker can exploit the way HTTP requests and responses are parsed and handled during the TLS upgrade process. By carefully crafting and injecting malicious HTTP payloads, the attacker can desynchronize the HTTP stream, causing the server and client to interpret the communication differently. This desynchronization enables the attacker to hijack an active HTTP session, effectively bypassing authentication mechanisms and gaining unauthorized access to sensitive session data or user privileges. The vulnerability is specifically tied to configurations using "SSLEngine optional" to allow TLS upgrades, which is not a default setting but may be used in environments requiring flexible TLS negotiation. The Apache Software Foundation has addressed this issue in version 2.4.64 by removing support for TLS upgrades, thereby eliminating the attack surface. No known exploits are currently reported in the wild, but the vulnerability represents a significant risk due to the potential for session hijacking and unauthorized access in affected deployments.

For European organizations, the impact of CVE-2025-49812 can be substantial, especially for those relying on Apache HTTP Server with mod_ssl configured for optional TLS upgrades. Successful exploitation can lead to session hijacking, allowing attackers to impersonate legitimate users, access confidential information, manipulate data, or perform unauthorized actions within web applications. This can compromise the confidentiality and integrity of sensitive data, including personal data protected under GDPR, potentially resulting in regulatory penalties and reputational damage. The availability of services may also be indirectly affected if attackers leverage hijacked sessions to disrupt operations or escalate privileges. Given the widespread use of Apache HTTP Server across European enterprises, government agencies, and critical infrastructure providers, the vulnerability poses a risk to sectors such as finance, healthcare, public administration, and e-commerce. The man-in-the-middle nature of the attack requires network-level access or interception capabilities, which could be facilitated in environments with insufficient network segmentation or insecure Wi-Fi networks. The absence of known exploits in the wild provides a window for proactive mitigation, but the potential impact on confidentiality and integrity remains high.

European organizations should take immediate steps to mitigate this vulnerability beyond generic patching advice. First, audit all Apache HTTP Server instances to identify those using mod_ssl with the "SSLEngine optional" directive and assess whether TLS upgrades are necessary for operational requirements. If TLS upgrades are not essential, disable the "SSLEngine optional" setting or configure it to "SSLEngine on" to enforce TLS from the start of the connection. Organizations should prioritize upgrading affected Apache HTTP Server instances to version 2.4.64 or later, which removes support for TLS upgrades and addresses the vulnerability. Network defenses should be enhanced to detect and prevent man-in-the-middle attacks, including deploying TLS inspection with trusted certificates, enforcing strict transport security (HSTS), and using network segmentation to limit exposure of critical servers. Additionally, monitoring HTTP traffic for anomalies indicative of desynchronization attacks can provide early detection. Web application session management should be reviewed to ensure robust session token handling, including short session lifetimes, secure cookie flags (HttpOnly, Secure, SameSite), and re-authentication for sensitive operations. Finally, organizations should conduct security awareness training focused on risks associated with insecure network environments to reduce the likelihood of successful MitM attacks.