diff --git a/packages/w3c/websub/src/index.test.ts b/packages/w3c/websub/src/index.test.ts
index b5134e7f..2acb7103 100644
--- a/packages/w3c/websub/src/index.test.ts
+++ b/packages/w3c/websub/src/index.test.ts
@@ -178,5 +178,17 @@ describe('WebSub helpers', () => {
       secret,
       signatureHeader: `sha256=${'0'.repeat(signature.length)}`,
     })).toBe(false);
+
+    for (const malformed of [
+      `sha256=${signature}zz`,
+      `sha256=${signature}f`,
+      `sha256=${signature}=ignored`,
+    ]) {
+      expect(verifyDistributionSignature({
+        body,
+        secret,
+        signatureHeader: malformed,
+      })).toBe(false);
+    }
   });
 });
diff --git a/packages/w3c/websub/src/index.ts b/packages/w3c/websub/src/index.ts
index e3f143b9..7cd297a9 100644
--- a/packages/w3c/websub/src/index.ts
+++ b/packages/w3c/websub/src/index.ts
@@ -201,10 +201,13 @@ export function verifyDistributionSignature(input: {
   signatureHeader?: string | null;
 }): boolean {
   if (!input.signatureHeader) return false;
-  const [algorithm, signature] = input.signatureHeader.split('=', 2);
+  const match = /^([a-z0-9]+)=([0-9a-fA-F]+)$/.exec(input.signatureHeader);
+  const algorithm = match?.[1];
+  const signature = match?.[2];
   if (!algorithm || !signature || !SUPPORTED_SIGNATURE_ALGORITHMS.has(algorithm)) return false;
 
   const expected = createHmac(algorithm, input.secret).update(input.body).digest('hex');
+  if (signature.length !== expected.length) return false;
   const expectedBytes = Buffer.from(expected, 'hex');
   const actualBytes = Buffer.from(signature, 'hex');
   return expectedBytes.length === actualBytes.length && timingSafeEqual(expectedBytes, actualBytes);