AIKIDO-2026-10891

TL;DR

SigningSerializer verifies message signatures only after calling the inner serializer. When the inner serializer is PhpSerializer, an unsigned or invalidly signed payload can be unserialized before the signature check rejects it. Pre-fix applications that rely on signed Messenger messages can still instantiate attacker-controlled serialized payloads if an attacker can inject into the transport. The fix determines message type without instantiating the payload and verifies signatures before decoding signed message types.