fix(dictionary): reject zips with corrupted central directory records

- Walk every central-directory record and validate its signature and variable-length field bounds so an in-place corrupted record is rejected even when the archive's overall size still matches
- Add tests covering a corrupted record signature and an out-of-bounds name length

src/main/character-dictionary-runtime/zip.test.ts

@@ -143,6 +143,21 @@ test('readDictionaryZipRevision reads the built revision and rejects foreign arc
       assert.equal(readDictionaryZipRevision(cutPath), null, `cut of ${missingBytes} bytes`);
     }
 
+    // Same size, corrupt directory: a record overwritten in place has to be rejected too.
+    const centralStart = archive.readUInt32LE(archive.length - 22 + 16);
+    const brokenSignaturePath = path.join(dir, 'broken-signature.zip');
+    const brokenSignature = Buffer.from(archive);
+    brokenSignature.writeUInt32LE(0xdeadbeef, centralStart);
+    fs.writeFileSync(brokenSignaturePath, brokenSignature);
+    assert.equal(readDictionaryZipRevision(brokenSignaturePath), null);
+
+    const brokenLengthPath = path.join(dir, 'broken-length.zip');
+    const brokenLength = Buffer.from(archive);
+    // Name length that runs the walk past the end of the directory.
+    brokenLength.writeUInt16LE(0xffff, centralStart + 28);
+    fs.writeFileSync(brokenLengthPath, brokenLength);
+    assert.equal(readDictionaryZipRevision(brokenLengthPath), null);
+
     const foreignPath = path.join(dir, 'foreign.zip');
     fs.writeFileSync(foreignPath, Buffer.from('not a zip at all', 'utf8'));
     assert.equal(readDictionaryZipRevision(foreignPath), null);

src/shared/stored-zip.ts

@@ -152,6 +152,50 @@ const LOCAL_FILE_HEADER_SIGNATURE = 0x04034b50;
 const LOCAL_FILE_HEADER_SIZE = 30;
 const END_OF_CENTRAL_DIRECTORY_SIGNATURE = 0x06054b50;
 const END_OF_CENTRAL_DIRECTORY_SIZE = 22;
+const CENTRAL_FILE_HEADER_SIGNATURE = 0x02014b50;
+const CENTRAL_FILE_HEADER_SIZE = 46;
+// 65535 entries with names of a few dozen bytes stay far under this; the cap only stops a corrupt
+// record length from asking for an allocation the size of the archive.
+const MAX_CENTRAL_DIRECTORY_BYTES = 16 * 1024 * 1024;
+
+/**
+ * Walks every declared central-directory record, checking each signature and keeping the
+ * variable-length name/extra/comment fields inside the directory. The walk has to land exactly on
+ * the end of the directory, so a record that was overwritten in place fails even though the file
+ * kept its size.
+ */
+function isCentralDirectoryIntact(
+  fd: number,
+  centralStart: number,
+  centralSize: number,
+  entryCount: number,
+): boolean {
+  if (centralSize === 0 || centralSize > MAX_CENTRAL_DIRECTORY_BYTES) {
+    return false;
+  }
+  const central = Buffer.alloc(centralSize);
+  if (fs.readSync(fd, central, 0, centralSize, centralStart) !== centralSize) {
+    return false;
+  }
+
+  let cursor = 0;
+  for (let index = 0; index < entryCount; index += 1) {
+    if (cursor + CENTRAL_FILE_HEADER_SIZE > centralSize) {
+      return false;
+    }
+    if (central.readUInt32LE(cursor) !== CENTRAL_FILE_HEADER_SIGNATURE) {
+      return false;
+    }
+    const nameLength = central.readUInt16LE(cursor + 28);
+    const extraLength = central.readUInt16LE(cursor + 30);
+    const commentLength = central.readUInt16LE(cursor + 32);
+    cursor += CENTRAL_FILE_HEADER_SIZE + nameLength + extraLength + commentLength;
+    if (cursor > centralSize) {
+      return false;
+    }
+  }
+  return cursor === centralSize;
+}
 
 /**
  * Start of the central directory, or null when the archive is not a complete one of ours. The
@@ -171,7 +215,8 @@ function readCentralDirectoryStart(fd: number, fileSize: number): number | null
   if (end.readUInt32LE(0) !== END_OF_CENTRAL_DIRECTORY_SIGNATURE || end.readUInt16LE(20) !== 0) {
     return null;
   }
-  if (end.readUInt16LE(10) === 0) {
+  const entryCount = end.readUInt16LE(10);
+  if (entryCount === 0) {
     return null;
   }
   const centralSize = end.readUInt32LE(12);
@@ -179,6 +224,9 @@ function readCentralDirectoryStart(fd: number, fileSize: number): number | null
   if (centralStart + centralSize !== endOffset) {
     return null;
   }
+  if (!isCentralDirectoryIntact(fd, centralStart, centralSize, entryCount)) {
+    return null;
+  }
   return centralStart;
 }