CVE-2021-41267: Inconsistent Interpretation of HTTP Requests ('HTTP Request Smuggling')

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, 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.