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A webhook handler can pass its tests and still perform the same business action twice. The usual gap is that tests validate one valid request, while real delivery involves retries, concurrent attempts, or a response lost after the handler has already committed its work. Signature verification authenticates a request; it does not make processing exactly once.
The title describes a general engineering failure pattern, not a verified incident at a named company. The practical fix is to verify the signed bytes, distinguish replay protection from duplicate-event handling, and make event processing safe to repeat.
How a valid webhook can trigger the same action twice
- A provider sends a valid event, and the receiver verifies its signature.
- The handler commits an effect, such as recording a payment, changing a database row, or sending a notification.
- The response is delayed or lost, so the sender cannot tell that the work succeeded and retries the delivery.
- The retry is also authentic. If the receiver has no durable guard for the event and the effect is not idempotent, the handler performs it again.
Each request can be individually valid while the sequence is unsafe. A response timeout does not undo a database commit or retract a notification. Provider retry and redelivery rules vary, so consult the sender’s current documentation rather than assuming one universal retry schedule or response policy.
Why tests and code review miss the failure
A test that sends one request and checks for a successful HTTP status can prove that the happy path works. It does not prove that a second delivery is harmless, that two deliveries arriving together cannot both pass a “not seen” check, or that a retry after a crash leaves state correct.
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Review has a similar blind spot when it follows the intended flow from request to effect but does not examine delivery sequences and failure boundaries. The useful question is not merely “Does this request validate?” but “What happens if this same event arrives again before, during, or after the effect?” No prevalence rate is established for how often this class of bug escapes testing or review.
What the security checks do—and do not do
Verify the exact raw request body
Calculate and check the provider’s signature over the exact bytes it signed, before parsing, normalizing, or rewriting the payload. Middleware that parses a body can change the representation used for verification. GitHub notes that modifying the payload or headers before verification can cause verification failures, and Shopify’s delivery-verification guidance warns that body-parser middleware can alter the input needed for HMAC verification.
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Use a constant-time comparison for the calculated and supplied signature; ordinary string equality can reveal information through timing differences. Follow the sender’s documented algorithm, header format, and secret-handling guidance. GitHub’s delivery-validation documentation describes HMAC validation and preserving payload integrity.
Use freshness checks against stale replays
Where the provider signs an attempt timestamp or specifies a freshness rule, validate it using that provider’s requirements. This can reject an old captured request, but it is not a substitute for duplicate-event handling: a legitimate retry may have a fresh attempt timestamp while representing the same underlying event.
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Deduplicate on the stable event identity
After authentication, use the provider’s stable event or delivery identifier to recognize a repeated event. Attempt timestamps and event IDs answer different questions: freshness asks whether this attempt is recent enough; deduplication asks whether this event has already been processed. Standard Webhooks documents this distinction alongside signing metadata and idempotency guidance in its specification.
Make duplicate handling durable and safe under concurrency
Persist the event claim in durable storage with an atomic uniqueness mechanism, such as a unique database constraint. A separate “check whether seen” followed by “insert if not” is unsafe: two concurrent deliveries can both observe that the ID is absent and both proceed.
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The claim also has to relate safely to the business effect. If the process crashes after recording an event but before applying its effect, a retry must not silently lose the work. If it applies the effect first and crashes before recording the claim, a retry can repeat it. Use a transaction when the claim and effect share a transactional store. For work that crosses system boundaries or runs asynchronously, use a durable inbox/outbox or an equivalent pattern that fits the system, and make downstream operations idempotent where possible. A queue by itself does not guarantee exactly-once effects.
- For a duplicate already completed, skip the business effect and return the provider-appropriate success response so the sender does not retry needlessly.
- For an event currently being processed, coordinate attempts through a durable state transition or lock with suitable recovery behavior; do not let both execute the effect.
- For a failed or interrupted event, retain enough state to retry or reconcile it without treating an incomplete claim as successful completion.
Acknowledge promptly without losing work
Provider deadlines affect whether a sender retries. GitHub’s webhook best-practices guidance says: “Your server should respond with a 2XX response within 10 seconds of receiving a webhook delivery.” This is GitHub-specific operational guidance, not a universal deadline. GitHub recommends queueing work so the receiver can acknowledge promptly while processing continues; its documentation also covers redelivery and delivery IDs in webhook best practices.
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Queueing is safe only if accepting the event and placing it into durable work storage are coordinated. Acknowledge after durable acceptance, not after placing work only in volatile memory. Use the provider’s current rules for response codes, timeout, retry cadence, and manual redelivery.
Test the delivery sequences that matter
Assert final state and the number of business effects, not just the HTTP response. The OWASP draft Webhook Security Guidelines includes checks for invalid or missing signatures, replay, duplicate event IDs, and oversized payloads.
Quick Recap
- Repeated delivery: send the same authenticated event twice; verify one effect and the expected final state.
- Concurrent duplicate: deliver the same event ID simultaneously; verify only one attempt can claim and execute it.
- Freshness boundaries: test a valid signed attempt within the permitted window and a stale replay outside it.
- Invalid signature: send an invalid signature for a body whose event ID was otherwise processed; verify the request is rejected before deduplication or business handling can authorize it.
- Response lost after commit: commit the effect, simulate a lost acknowledgement, then retry; verify no duplicate effect occurs.
- Partial failure and restart: fail between the durable claim, business effect, and acknowledgement; restart the worker and verify recovery does not lose or repeat work.
- Input limits: test missing signature data and oversized payloads according to the provider’s and application’s limits.
A practical review checklist
- Does verification use the exact raw bytes, the correct provider algorithm, and a constant-time comparison?
- Is freshness checked separately from stable event-ID deduplication?
- Is the event claim durable and atomic under concurrent delivery?
- Can a crash at every boundary between claim, effect, queueing, and acknowledgement be recovered safely?
- Are downstream effects idempotent or otherwise protected against retries?
- Does the receiver meet the specific provider’s acknowledgement deadline and retry semantics?
- Do tests simulate duplicate, concurrent, replayed, interrupted, and restarted delivery sequences and count actual effects?
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