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 hashhmac() when verifying the request against the configured webhook secret. The request therefore selects the HMAC primitive used to authenticate it.
PHP's hashhmac() 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.
Description
The Mailtrap mailer bridge ships a webhook request parser used to authenticate and decode the event callbacks Mailtrap POSTs to an application's webhook endpoint. Its doParse(Request $request, #[\SensitiveParameter] string $secret) method receives the configured webhook secret but never reads it; it decodes and returns the payload unconditionally, ignoring the X-Mt-Signature HMAC header Mailtrap sends with each request.
As a result, an application that wires up the Mailtrap webhook endpoint accepts any POST to that URL, even when a signing secret is configured (the recommended setup). An attacker who knows the endpoint exists can submit forged event payloads, fake delivery / bounce / open / click / spam events, leading to suppression-list corruption, delivery-metrics fraud, etc.
Resolution
MailtrapRequestParser::doParse() now requires and verifies the X-Mt-Signature header, an HMAC-SHA256 of the raw request body keyed with the configured secret, before decoding the payload, using a constant-time comparison.
When no secret is configured the behaviour is unchanged: signature verification remains opt-in, but it is now actually enforced once opted in.
The patch for this issue is available here for branch 7.4.
Credits
Symfony would like to thank Himanshu Anand for reporting the issue and Alexandre Daubois providing the fix.