CVE-2025-49812 is a high-severity vulnerability affecting the Apache HTTP Server, specifically versions up to 2.4.63, in configurations that use mod_ssl with the "SSLEngine optional" directive to enable TLS upgrades. This vulnerability arises from improper authentication (CWE-287) that allows an attacker to perform an HTTP desynchronisation attack. In this scenario, a man-in-the-middle (MITM) attacker can exploit the TLS upgrade mechanism to hijack an HTTP session. The attack leverages the fact that when TLS is optionally enabled, the server may accept both encrypted and unencrypted requests on the same connection, allowing the attacker to desynchronize the HTTP request stream. This desynchronization can be exploited to inject malicious requests or intercept legitimate session data, effectively bypassing the intended security provided by TLS. The vulnerability does not require authentication or user interaction but does require network-level access to perform the MITM attack. The Apache Software Foundation has addressed this issue in version 2.4.64 by removing support for TLS upgrade, thereby eliminating the attack vector. The CVSS v3.1 base score is 7.4, reflecting a high severity due to the network attack vector, no privileges required, no user interaction, and high impact on confidentiality and integrity, though availability is not affected. No known exploits are currently reported in the wild, but the potential for session hijacking makes this a critical concern for affected deployments.

For European organizations, the impact of CVE-2025-49812 can be significant, especially for those relying on Apache HTTP Server with mod_ssl configured to allow optional TLS upgrades. Successful exploitation could lead to session hijacking, exposing sensitive user data, credentials, and potentially allowing attackers to impersonate legitimate users or manipulate web application behavior. This can result in data breaches, loss of customer trust, regulatory non-compliance (e.g., GDPR violations), and financial losses. Since many European enterprises, government agencies, and service providers use Apache HTTP Server as a core web infrastructure component, the vulnerability poses a widespread risk. The attack vector requires MITM capability, which could be feasible in scenarios involving unsecured or compromised networks, such as public Wi-Fi, or through advanced persistent threat actors targeting critical infrastructure. The confidentiality and integrity of communications are at risk, which is particularly critical for sectors like finance, healthcare, and public administration in Europe. The removal of TLS upgrade support in the patched version also implies that organizations relying on this feature must adapt their configurations, potentially impacting legacy systems or workflows.

European organizations should prioritize upgrading Apache HTTP Server to version 2.4.64 or later, which removes support for TLS upgrade and mitigates the vulnerability. For environments where immediate upgrade is not feasible, organizations should audit their mod_ssl configurations to identify and disable the "SSLEngine optional" setting, enforcing mandatory TLS usage instead. Network-level mitigations include deploying strict TLS enforcement via web application firewalls (WAFs) and intrusion detection/prevention systems (IDS/IPS) to detect and block HTTP desynchronisation attempts. Organizations should also ensure robust network segmentation and the use of secure, trusted networks to reduce the risk of MITM attacks. Monitoring and logging of HTTP and TLS traffic anomalies can help detect exploitation attempts early. Additionally, educating administrators about the risks of optional TLS configurations and encouraging the use of modern TLS versions and cipher suites will strengthen overall security posture. Finally, organizations should review their incident response plans to include scenarios involving session hijacking and HTTP desynchronisation attacks.