Action Mailbox: Possible signature replay in Action Mailbox Mailgun ingress

Impact

The Action Mailbox Mailgun ingress authenticates a request with an HMAC of the timestamp and token parameters concatenated without a separator, and ActionMailbox::Ingresses::Mailgun::InboundEmailsController::Authenticator#recent? rejects only timestamps more than 2 minutes in the past. Because the signature does not fix the boundary between the two values, a captured signed request can be presented with a timestamp far in the future, which the ingress then accepts indefinitely. An attacker who obtains one signed Mailgun webhook request, but not the signing key, could submit arbitrary inbound email that Action Mailbox processes as genuine, possibly leading to forged messages attributed to the sender of the attacker's choosing.

Applications that receive inbound email through the Action Mailbox Mailgun ingress are affected.

Details

The fixed releases also reject a Mailgun request whose timestamp is more than 2 minutes ahead of the server's clock, as they already did for one more than 2 minutes behind it. An application whose clock runs more than 2 minutes behind Mailgun's will now reject genuine webhooks and should synchronize its clock.

Releases

The fixed releases are available at the normal locations.

Patches for those who cannot upgrade at once:

• 8.1: https://github.com/rails/rails/commit/c3a295c5c22266e8ec4d0e664e6923a299235e7a.patch
• 8.0: https://github.com/rails/rails/commit/4aa2ec66822bc4eb8b341a0608f1c90a3e371a34.patch

Credit

This issue was responsibly reported by HackerOne researcher davutselcuk.