Symfony: Mailomat Mailer Webhook Parser Reads the HMAC Algorithm from the Request: Signature Algorithm Downgrade

Description

Symfony\Component\Mailer\Bridge\Mailomat\Webhook\MailomatRequestParser::validateSignature() parses the X-MOM-Webhook-Signature request header as algo=signature and passes the wire-supplied $algo directly to hash_hmac() when verifying the request against the configured webhook secret. The request therefore selects the HMAC primitive used to authenticate it.

PHP's hash_hmac() enforces only that the chosen algorithm is HMAC-compatible. That set still includes primitives with known cryptanalysis (md4, md5, ripemd128, tiger128,3, … — e.g. existential forgery of HMAC-MD4, Contini & Yin, ASIACRYPT 2006). This is the canonical algorithm-confusion shape, analogous to JWT alg=none / alg=HS256 downgrades: any future cryptographic weakness in any HMAC primitive PHP exposes becomes immediately exploitable against a Mailomat webhook receiver, the moment an attacker is in a position to compute a signature for that primitive, without a code change on the Symfony side.

Mailomat's documented webhook security pins SHA-256; the parser did not.

Resolution

MailomatRequestParser::validateSignature() now requires the signature header to be of the form sha256=<hex> and verifies the signature with HMAC-SHA256 keyed by the configured secret using a constant-time comparison. Any other algorithm declared on the wire (including the HMAC primitives PHP would otherwise accept) is rejected.

The patch for this issue is available here for branch 7.4 (and forward-ported to 8.0 and 8.1).

Credits

Symfony would like to thank Omar Alshammari, Essam Alanazi and Alwaleed Alshammari for reporting the issue and Nicolas Grekas for providing the fix.