MT-22022: Add webhook signature verification helper

README.md

@@ -238,6 +238,7 @@ Email API:
 - Sending stats (aggregated and by domain, category, ESP, date) – [`stats/everything.ts`](examples/stats/everything.ts)
 - Email logs (list with filters, get by message ID) – [`email-logs/everything.ts`](examples/email-logs/everything.ts)
 - Webhooks CRUD – [`webhooks/everything.ts`](examples/webhooks/everything.ts)
+- Verifying webhook signatures – [`webhooks/verify-signature.ts`](examples/webhooks/verify-signature.ts)
 
 Email Sandbox (Testing):
 

examples/webhooks/verify-signature.ts

@@ -0,0 +1,27 @@
+import { createServer, IncomingMessage, ServerResponse } from "http";
+
+import { verifyWebhookSignature } from "mailtrap";
+
+const SIGNING_SECRET = process.env.MAILTRAP_WEBHOOK_SIGNING_SECRET ?? "";
+
+const server = createServer((req: IncomingMessage, res: ServerResponse) => {
+  // Use the raw request body — parsing and re-serializing the JSON may
+  // reorder keys or alter whitespace and invalidate the signature.
+  const chunks: Buffer[] = [];
+  req.on("data", (chunk: Buffer) => chunks.push(chunk));
+  req.on("end", () => {
+    const payload = Buffer.concat(chunks).toString("utf-8");
+    const signature = (req.headers["mailtrap-signature"] as string) ?? "";
+
+    if (!verifyWebhookSignature(payload, signature, SIGNING_SECRET)) {
+      res.writeHead(401, { "Content-Type": "text/plain" });
+      res.end("Invalid signature");
+      return;
+    }
+
+    res.writeHead(200, { "Content-Type": "text/plain" });
+    res.end();
+  });
+});
+
+server.listen(9292);

src/__tests__/lib/webhooks/verify-signature.test.ts

@@ -0,0 +1,149 @@
+import { createHmac } from "crypto";
+
+import verifyWebhookSignature, {
+  SIGNATURE_HEX_LENGTH,
+} from "../../../lib/webhooks/verify-signature";
+
+// ---------------------------------------------------------------------------
+// Cross-SDK fixture
+//
+// The (payload, signing_secret, expected_signature) triple below is the
+// canonical fixture shared verbatim by every official Mailtrap SDK
+// (mailtrap-ruby, mailtrap-python, mailtrap-php, mailtrap-nodejs,
+// mailtrap-java, mailtrap-dotnet). Any change here MUST be mirrored in the
+// equivalent test files in the other SDKs so the helpers stay byte-for-byte
+// compatible across languages.
+// ---------------------------------------------------------------------------
+const FIXTURE_PAYLOAD =
+  '{"event":"delivery","sending_stream":"transactional","category":"welcome","message_id":"a8b1d8f6-1f8d-4a3c-9b2e-1a2b3c4d5e6f","email":"recipient@example.com","event_id":"f1e2d3c4-b5a6-7890-1234-567890abcdef","timestamp":1716070000}';
+const FIXTURE_SIGNING_SECRET = "8d9a3c0e7f5b2d4a6c1e9f8b3a7d5c2e";
+const FIXTURE_EXPECTED_SIGNATURE =
+  "6d262e2611cd09be1f948382b5c611d63b0e585c4c9c5e40139d6ac3876d5433";
+
+describe("lib/webhooks/verify-signature: ", () => {
+  describe("verifyWebhookSignature(): ", () => {
+    // --- 1. Valid signature for given payload + secret ---------------------
+    it("returns true for valid signature, payload and secret.", () => {
+      expect(
+        verifyWebhookSignature(
+          FIXTURE_PAYLOAD,
+          FIXTURE_EXPECTED_SIGNATURE,
+          FIXTURE_SIGNING_SECRET
+        )
+      ).toBe(true);
+    });
+
+    // --- 2. Wrong secret ---------------------------------------------------
+    it("returns false with a wrong signing secret.", () => {
+      expect(
+        verifyWebhookSignature(
+          FIXTURE_PAYLOAD,
+          FIXTURE_EXPECTED_SIGNATURE,
+          "ffffffffffffffffffffffffffffffff"
+        )
+      ).toBe(false);
+    });
+
+    // --- 3. Payload tampered (one byte changed) ----------------------------
+    it("returns false when the payload is tampered.", () => {
+      const tampered = FIXTURE_PAYLOAD.replace("delivery", "Delivery");
+
+      expect(
+        verifyWebhookSignature(
+          tampered,
+          FIXTURE_EXPECTED_SIGNATURE,
+          FIXTURE_SIGNING_SECRET
+        )
+      ).toBe(false);
+    });
+
+    // --- 4. Signature with wrong length ------------------------------------
+    it("returns false without throwing when the signature is too short.", () => {
+      const tooShort = FIXTURE_EXPECTED_SIGNATURE.slice(0, 31);
+
+      expect(() =>
+        verifyWebhookSignature(
+          FIXTURE_PAYLOAD,
+          tooShort,
+          FIXTURE_SIGNING_SECRET
+        )
+      ).not.toThrow();
+
+      expect(
+        verifyWebhookSignature(
+          FIXTURE_PAYLOAD,
+          tooShort,
+          FIXTURE_SIGNING_SECRET
+        )
+      ).toBe(false);
+    });
+
+    // --- 5. Signature with non-hex characters ------------------------------
+    it("returns false without throwing for a non-hex signature.", () => {
+      const notHex = "z".repeat(SIGNATURE_HEX_LENGTH);
+
+      expect(() =>
+        verifyWebhookSignature(FIXTURE_PAYLOAD, notHex, FIXTURE_SIGNING_SECRET)
+      ).not.toThrow();
+
+      expect(
+        verifyWebhookSignature(FIXTURE_PAYLOAD, notHex, FIXTURE_SIGNING_SECRET)
+      ).toBe(false);
+    });
+
+    // --- 6. Empty signature string -----------------------------------------
+    it("returns false for an empty signature string.", () => {
+      expect(
+        verifyWebhookSignature(FIXTURE_PAYLOAD, "", FIXTURE_SIGNING_SECRET)
+      ).toBe(false);
+    });
+
+    // --- 7. Empty signing_secret -------------------------------------------
+    it("returns false for an empty signing secret.", () => {
+      expect(
+        verifyWebhookSignature(FIXTURE_PAYLOAD, FIXTURE_EXPECTED_SIGNATURE, "")
+      ).toBe(false);
+    });
+
+    // --- 8. Empty payload + non-empty signature ----------------------------
+    it("returns false for an empty payload.", () => {
+      expect(
+        verifyWebhookSignature(
+          "",
+          FIXTURE_EXPECTED_SIGNATURE,
+          FIXTURE_SIGNING_SECRET
+        )
+      ).toBe(false);
+    });
+
+    // --- 9. Known-good cross-SDK fixture -----------------------------------
+    it("matches the hardcoded HMAC-SHA256 digest for the shared fixture.", () => {
+      // Recompute the digest in-place so a regression in Node's crypto module
+      // or the fixture itself fails loudly: this is the byte-for-byte
+      // contract every other Mailtrap SDK must satisfy.
+      const computed = createHmac("sha256", FIXTURE_SIGNING_SECRET)
+        .update(FIXTURE_PAYLOAD)
+        .digest("hex");
+
+      expect(computed).toBe(FIXTURE_EXPECTED_SIGNATURE);
+      expect(
+        verifyWebhookSignature(
+          FIXTURE_PAYLOAD,
+          FIXTURE_EXPECTED_SIGNATURE,
+          FIXTURE_SIGNING_SECRET
+        )
+      ).toBe(true);
+    });
+
+    // --- Bonus: accepts a Buffer payload -----------------------------------
+    it("accepts a Buffer payload equivalently to a UTF-8 string.", () => {
+      expect(
+        verifyWebhookSignature(
+          Buffer.from(FIXTURE_PAYLOAD, "utf-8"),
+          FIXTURE_EXPECTED_SIGNATURE,
+          FIXTURE_SIGNING_SECRET
+        )
+      ).toBe(true);
+    });
+  });
+});

src/index.ts

@@ -1,5 +1,6 @@
 import MailtrapClient from "./lib/MailtrapClient";
 import MailtrapTransport from "./lib/transport";
+import verifyWebhookSignature from "./lib/webhooks/verify-signature";
 
 export * from "./types/mailtrap";
-export { MailtrapClient, MailtrapTransport };
+export { MailtrapClient, MailtrapTransport, verifyWebhookSignature };

src/lib/webhooks/verify-signature.ts

@@ -0,0 +1,92 @@
+import { createHmac, timingSafeEqual } from "crypto";
+
+/**
+ * Hex-encoded HMAC-SHA256 signature length (SHA-256 produces 32 bytes / 64 hex chars).
+ */
+export const SIGNATURE_HEX_LENGTH = 64;
+
+/**
+ * Verifies the HMAC-SHA256 signature of a Mailtrap webhook payload.
+ *
+ * Mailtrap signs every outbound webhook by computing
+ * `HMAC-SHA256(signing_secret, raw_request_body)` and sending the lowercase
+ * hex digest in the `Mailtrap-Signature` HTTP header. Compute the same digest
+ * on your side and compare it in constant time.
+ *
+ * The comparison is performed with {@link timingSafeEqual} to avoid timing
+ * side-channels.
+ *
+ * The function never throws on inputs that could plausibly arrive over the
+ * wire (empty strings, wrong-length signatures, non-hex characters, missing
+ * secret) — it simply returns `false`. This makes it safe to call directly
+ * from a request handler without wrapping in try/catch.
+ *
+ * @param payload - The raw request body, exactly as received. Accepts a
+ *   UTF-8 `string` or a raw `Buffer`. **Do not** parse and re-serialize the
+ *   JSON — re-encoding may reorder keys or alter whitespace and invalidate
+ *   the signature. With Express, use
+ *   `express.raw({ type: 'application/json' })` (or equivalent) on the
+ *   webhook route so the body is preserved verbatim.
+ * @param signature - The value of the `Mailtrap-Signature` HTTP header
+ *   (lowercase hex string).
+ * @param signingSecret - The webhook's `signing_secret`, returned by the
+ *   Webhooks API on webhook creation.
+ * @returns `true` if the signature is valid for the given payload and
+ *   secret, `false` otherwise.
+ *
+ * @see https://docs.mailtrap.io/email-api-smtp/advanced/webhooks#verifying-the-signature
+ */
+export default function verifyWebhookSignature(
+  payload: string | Buffer,
+  signature: string,
+  signingSecret: string
+): boolean {
+  if (typeof signature !== "string" || signature.length === 0) {
+    return false;
+  }
+  if (typeof signingSecret !== "string" || signingSecret.length === 0) {
+    return false;
+  }
+  if (typeof payload !== "string" && !Buffer.isBuffer(payload)) {
+    return false;
+  }
+  if (payload.length === 0) {
+    return false;
+  }
+  if (signature.length !== SIGNATURE_HEX_LENGTH) {
+    return false;
+  }
+
+  let expected: string;
+  try {
+    expected = createHmac("sha256", signingSecret)
+      .update(payload)
+      .digest("hex");
+  } catch {
+    return false;
+  }
+
+  let expectedBuffer: Buffer;
+  let providedBuffer: Buffer;
+  try {
+    expectedBuffer = Buffer.from(expected, "hex");
+    providedBuffer = Buffer.from(signature, "hex");
+  } catch {
+    return false;
+  }
+
+  // `Buffer.from(str, 'hex')` silently drops trailing non-hex characters
+  // rather than throwing. Re-check the buffer lengths so a signature with
+  // non-hex characters (which produces a shorter decoded buffer) is rejected
+  // as a length mismatch instead of being silently accepted/rejected by
+  // `timingSafeEqual` — which itself throws on mismatched lengths.
+  if (expectedBuffer.length !== providedBuffer.length) {
+    return false;
+  }
+
+  try {
+    return timingSafeEqual(expectedBuffer, providedBuffer);
+  } catch {
+    return false;
+  }
+}