Symfony/Http-Kernel is the HTTP kernel component for Symfony, a PHP framework for web and console applications and a set of reusable PHP components. Headers that are not part of the "trusted_headers" allowed list are ignored and protect users from "Cache poisoning" attacks. In Symfony 5.2, maintainers added support for the X-Forwarded-Prefix headers, but this header was accessible in SubRequest, even if it was not part of the "trusted_headers" allowed list.

An attacker could leverage this opportunity to forge requests containing a X-Forwarded-Prefix header, leading to a web cache poisoning issue. Versions 5.3.12 and later have a patch to ensure that the X-Forwarded-Prefix header is not forwarded to subrequests when it is not trusted.