CVE-2025-12543: Host Header Validation Bypass in Undertow

CVE-2025-12543 is a critical vulnerability in the Undertow HTTP server core that permits attackers to send malicious or unexpected Host headers, which are improperly accepted as valid, posing risks such as cache poisoning, unsafe redirects, and cross-tenant mix-ups. This issue affects enterprise Java environments using Undertow directly or indirectly, including platforms like WildFly and JBoss EAP. Organizations are advised to upgrade immediately as patches become available and enforce strict Host header validation at both the application and edge layers. The vulnerability stems from Undertow's failure to strictly validate Host header values, allowing attacker-controlled headers to be trusted by various application components. Potential impacts include the leaking of sensitive information, unauthorized access in multi-tenant environments, and exposure of internal functionalities. To mitigate the risk, Undertow has introduced a HostHeaderHandler to ensure early validation, rejecting invalid or ambiguous Host headers with a 400 Bad Request error. Additional measures include strict application-level validation, reverse proxy rules, and logging rejected headers to detect exploitation attempts, emphasizing the importance of addressing this vulnerability across enterprise systems due to its potential to cause systemic security failures.

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