REDHAT-BUG-2516141: Medium severity Red Hat Quay vulnerability

A flaw was found in Red Hat Quay's external LDAP authentication handling. When an LDAP referral is returned during authentication, the referral handler at data/users/externalldap.py:700 constructs a search filter using the raw usernameoremail input without applying escapefilterchars(), unlike the normal authentication path which correctly escapes the input. This allows LDAP filter metacharacters (, (, )) in the username to be injected into the referral path's search filter. While SCOPEBASE limits the search to a single DN (preventing directory enumeration) and a separate simplebinds password check prevents direct authentication bypass, an attacker could use this to perform user-existence oracle attacks at the referral DN and potentially influence which DN enters the password bind in multi-domain Active Directory environments.