Privilege escalation via content-supplied principal trust

Incorrect authorization in Mozilla Firefox and Thunderbird before 134 and ESR before 128.6 allows attackers to escalate privileges. WebChannelParent accepted a principal supplied in a content message and used it for callback and permission checks instead of deriving the sender principal from the window actor. Exploitation requires prior arbitrary code execution in a hostile content process and can invoke privileged parent-process WebChannel handlers.