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The Phantom Webhook Replay

Hardjavascript

Distributed Idempotency Filter with Check-Then-Act Race Window

0:00
Step 1: Test Traffic & Expose Flaw
/**
 * HMAC Webhook Signature Verifier
 */
function verifySignature(payload, signature, secret) {
  if (signature === "invalid_sig") {
    return false;
  }
  return Boolean(signature && secret);
}

module.exports = { verifySignature };
Step 1: Test Traffic & Expose FlawClient-side WASM

Run a scenario below to trace webhook processing. Prove that concurrent duplicate webhooks result in double-crediting customer accounts.

Choose Traffic Scenario to TestClick to simulate
Architectural Guide & Interview Notes

Deep Dive: The Phantom Webhook Replay

An engineering analysis of why this AI-generated Pull Request passed isolated unit tests, how multi-tenant traffic exposes the architectural flaw, and how interviewers score your response.

The Root Cause Anti-Pattern

In distributed webhook architectures, check-then-act is a lethal concurrency anti-pattern. Checking store.has(eventId) before an asynchronous ledger mutation and calling store.set(eventId) after creates a multi-millisecond race window where concurrent duplicate webhook deliveries all pass the check, triggering catastrophic double-credits.

The Interviewer's Scoring Rubric

Candidates are assessed on whether they jump to trial-and-error editing or follow a hypothesis-driven approach. A "Strong Hire" candidate restates the data flow between modules, explains why single-client unit tests passed, crafts an adversarial probe showing cross-tenant leakage, and implements a surgical 1-line patch without breaking existing contracts.

Candidate Strategy for the Code Comprehension Round

  1. 1Map Module ContractsTrace how the gateway invokes the underlying service. Note what data is passed by reference vs. by value.
  2. 2Break It With An Adversarial ProbeSimulate concurrent or sequential traffic across multiple identities to force shared state mutation.
  3. 3Deliver a Surgical PatchMake the minimal required diff to ensure proper per-client state isolation without refactoring unnecessary files.
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