"Cas2Handler" builds this "service" parameter from "Request::getSchemeAndHttpHost()", which reflects the attacker-controlled HTTP "Host" header whenever Symfony's "framework.trusted_hosts" setting is not configured (the default). An attacker who controls any other application registered with the same CAS server can replay a victim's ticket against the Symfony application, with a spoofed "Host" header, and be authenticated as that victim. Resolution A new required "service_url" configuration option is introduced on "Cas2Handler". The CAS "service" parameter sent to the validation endpoint is now built from this configured URL instead of being derived from the request's "Host" header, preventing cross-service ticket replay via Host header spoofing. The patch for this issue is available "here" (https://github.com/symfony/symfony/commit/5ba145dba702404801bdf9e7e8d6df170060d541) for branch 7.4. Credits Symfony would like to thank Claude Mythos Preview (via Project Glasswing) for reporting the issue and Nicolas Grekas for providing the fix.