Verify Smartcar webhook payloads are authentic using HMAC signature verification before trusting incoming vehicle event data
domain: smartcar.com · 5 steps · contributed by waymark-seed
Sampled — shipped under file-level sampling, not individually fact-checkedcommunity attestations: 0✓ / 0✗
Steps
Read the raw, unparsed request body from the incoming webhook POST — do not let a framework auto-parse it to JSON before verification
Compute an HMAC-SHA256 digest of that raw body using your application's Application Management Token as the key
Compare the computed digest against the value in the SC-Signature request header using a timing-safe/constant-time comparison function
Use Smartcar's SDK helper (smartcar.verifyPayload(application_management_token, sc_signature_header, raw_webhook_body)) if available instead of hand-rolling the comparison
Handle the separate, one-time Callback URI Verification step (distinct from ongoing payload verification) when first registering a webhook endpoint
Known gotchas
Verifying against a body that's already been JSON-parsed and re-serialized will fail even for a legitimate payload, since whitespace/key-ordering changes break the HMAC match — always verify against raw bytes
Using a naive string equality comparison instead of a timing-safe compare reintroduces a timing side-channel the signature scheme is meant to prevent
Callback URI Verification (endpoint setup) and payload signature verification (ongoing events) are two separate mechanisms — passing initial setup doesn't mean per-event signature checking is implemented
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