Two gaps:
1. /api/v1/admin/apps/{id}/triggers/email accepted `inbound_secret: null`
and treated it as "this trigger is unsigned". The runtime then
skipped HMAC verification entirely. URL discovery (logs, ops
dashboards, bruteforce on the 122-bit TriggerId space) then fan-out-
amplified into the outbox for free — anonymous DoS via the
dispatcher.
2. Even signed triggers had no per-trigger rate limit on signature-
verify failures, so an attacker who knew the URL could pump unsigned
POSTs to force Argon2-equivalent work (HMAC verify + nonce-dedup +
stored-secret decrypt) at line rate.
Fix:
* triggers_api::create_email_trigger now requires a non-empty
inbound_secret. 400 on missing / empty / whitespace-only.
* email_inbound_api::receive_inbound_email returns 401 immediately when
the resolved target has no inbound_secret_encrypted column (pre-fix
rows). The previous `if let Some(ct), Some(nonce)` branch is gone.
* New `BadSignatureLimiter`: per-`(app_id, trigger_id)` sliding-window
bucket (10 fails / 60 s). On lockout the receiver returns 429 with
Retry-After instead of 401. record_failure runs on both the
"no-secret" 401 path and any verify_signature failure.
Test fallout:
* email_inbound integration tests that relied on None secret: removed
the now-impossible `unsigned_trigger_accepts_without_signature` test,
replaced with `create_without_inbound_secret_is_rejected` covering
null / "" / " "; updated `malformed_body_is_422` and
`cross_app_path_is_404` to pass + sign with a secret.
* Two new unit tests pin the limiter behavior (burst lockout + per-
trigger independence).
Audit refs: security_audit/08_dos_resource.md#h-3,
security_audit/09_external_integrations.md#f-ext-m-001.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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