The vulnerability CVE-2025-12543 affects the Undertow HTTP server core, which fails to properly validate the Host header in HTTP requests. This improper input validation can be exploited to poison caches, perform server-side request forgery (SSRF), or hijack user sessions. The issue impacts Red Hat JBoss Enterprise Application Platform 8.1 versions prior to 8.1.3. Red Hat has issued security advisories and updates that upgrade Undertow to fixed versions, mitigating this vulnerability.

Successful exploitation of this vulnerability allows attackers to send HTTP requests with malformed Host headers that are not rejected by the server. This can enable cache poisoning, potentially causing users to receive malicious content, internal network scanning through SSRF, and hijacking of user sessions. The CVSS v3.1 base score is 9.6 (critical), reflecting the high impact on confidentiality, integrity, and availability with network attack vector and no privileges required.

Red Hat has released official security updates in Red Hat JBoss Enterprise Application Platform 8.1.3 for Red Hat Enterprise Linux 8 and 9 that fix this vulnerability by upgrading the Undertow component. Users should apply these updates promptly after ensuring all prior errata are applied and backing up their systems. Detailed update instructions are available from Red Hat. No other mitigation is indicated by the vendor advisory.