BUG(isJWT): validate decoded header and payload as JSON objects

src/lib/isJWT.js

@@ -1,15 +1,65 @@
 import assertString from './util/assertString';
 import isBase64 from './isBase64';
 
+/* istanbul ignore next */
+function decodeBase64Url(b64) {
+  if (typeof Buffer !== 'undefined') {
+    if (typeof Buffer.from === 'function') {
+      return Buffer.from(b64, 'base64').toString('utf8');
+    }
+    // eslint-disable-next-line no-buffer-constructor
+    return new Buffer(b64, 'base64').toString('utf8');
+  }
+  if (typeof atob === 'function') {
+    const binary = atob(b64);
+    if (typeof TextDecoder !== 'undefined') {
+      const bytes = new Uint8Array(binary.length);
+      for (let i = 0; i < binary.length; i += 1) {
+        bytes[i] = binary.charCodeAt(i);
+      }
+      return new TextDecoder('utf-8').decode(bytes);
+    }
+    let encoded = '';
+    for (let i = 0; i < binary.length; i += 1) {
+      const code = binary.charCodeAt(i).toString(16).padStart(2, '0');
+      encoded += `%${code}`;
+    }
+    return decodeURIComponent(encoded);
+  }
+  return b64;
+}
+
+function tryDecodeJSON(segment) {
+  if (!isBase64(segment, { urlSafe: true })) return false;
+  try {
+    // Normalize base64url alphabet to base64, then restore stripped padding
+    let b64 = segment.replace(/-/g, '+').replace(/_/g, '/');
+    while (b64.length % 4) b64 += '=';
+    const decoded = decodeBase64Url(b64);
+    const parsed = JSON.parse(decoded);
+    if (typeof parsed !== 'object') return false;
+    if (parsed === null) return false;
+    if (Array.isArray(parsed)) return false;
+    return true;
+  } catch (e) {
+    return false;
+  }
+}
+
 export default function isJWT(str) {
   assertString(str);
 
   const dotSplit = str.split('.');
-  const len = dotSplit.length;
 
-  if (len !== 3) {
-    return false;
-  }
+  if (dotSplit.length !== 3) return false;
+
+  const header = dotSplit[0];
+  const payload = dotSplit[1];
+  const signature = dotSplit[2];
+
+  if (!tryDecodeJSON(header)) return false;
+  if (!tryDecodeJSON(payload)) return false;
+  if (!isBase64(signature, { urlSafe: true })) return false;
 
-  return dotSplit.reduce((acc, currElem) => acc && isBase64(currElem, { urlSafe: true }), true);
+  return true;
 }

test/validators.test.js

@@ -5542,13 +5542,27 @@ describe('Validators', () => {
         'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJsb3JlbSI6Imlwc3VtIn0.ymiJSsMJXR6tMSr8G9usjQ15_8hKPDv_CArLhxw28MI',
         'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJkb2xvciI6InNpdCIsImFtZXQiOlsibG9yZW0iLCJpcHN1bSJdfQ.rRpe04zbWbbJjwM43VnHzAboDzszJtGrNsUxaqQ-GQ8',
         'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJqb2huIjp7ImFnZSI6MjUsImhlaWdodCI6MTg1fSwiamFrZSI6eyJhZ2UiOjMwLCJoZWlnaHQiOjI3MH19.YRLPARDmhGMC3BBk_OhtwwK21PIkVCqQe8ncIRPKo-E',
+        'eyJhbGciOiJub25lIn0.eyJpc3MiOiJqb2UiLCJleHAiOjEzMDA4MTkzODAsImh0dHA6Ly9leGFtcGxlLmNvbS9pc19yb290Ijp0cnVlfQ.',
       ],
       invalid: [
         'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9',
         'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NSIsIm5hbWUiOiJKb2huIERvZSIsImlhdCI6MTUxNjIzOTAyMn0',
         'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NSIsIm5hbWUiOiJKb2huIERvZSIsImlhdCI6MTYxNjY1Mzg3Mn0.eyJpc3MiOiJodHRwczovL2V4YW1wbGUuY29tIiwiaWF0IjoxNjE2NjUzODcyLCJleHAiOjE2MTY2NTM4ODJ9.a1jLRQkO5TV5y5ERcaPAiM9Xm2gBdRjKrrCpHkGr_8M',
         '$Zs.ewu.su84',
         'ks64$S/9.dy$§kz.3sd73b',
+        'foo.bar.',
+        '..',
+        '.t.',
+        'foo.bar.baz',
+        'Zm9v.YmFy.',
+        'eyJmb28iOiJiYXIifQ.YmFy.',
+        'Zm9v.eyJiYXIiOiJiYXoifQ.',
+        'W10=.eyJiYXIiOiJiYXoifQ.',
+        'eyJmb28iOiJiYXIifQ.W10=.',
+        'bnVsbA.eyJiYXIiOiJiYXoifQ.',
+        'WzFd.eyJiYXIiOiJiYXoifQ.',
+        'ImhlbGxvIg.eyJiYXIiOiJiYXoifQ.',
+        'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIn0.invalid$sig',
       ],
       error: [
         [],