scan APKs for signs of "Master Key" exploit

This exploit is old, and was fixed in 4.4.  But it was easy to exploit,
so it is still worth scanning for it.  It is also easy to scan for, since
valid APKs should not have files with duplicate names.  In theory, this
could look for duplicate file names for any file, but this limits the
false positives by only checking names of files related to executing code.

fdroidclient#40

fdroidserver/update.py

@@ -470,13 +470,24 @@ def sha256sum(filename):
     return sha.hexdigest()
 
 
-def has_old_openssl(filename):
-    '''checks for known vulnerable openssl versions in the APK'''
+def has_known_vulnerability(filename):
+    """checks for known vulnerabilities in the APK
+
+    Checks OpenSSL .so files in the APK to see if they are a known vulnerable
+    version.  Google also enforces this:
+    https://support.google.com/faqs/answer/6376725?hl=en
+
+    Checks whether there are more than one classes.dex or AndroidManifest.xml
+    files, which is invalid and an essential part of the "Master Key" attack.
+
+    http://www.saurik.com/id/17
+    """
 
     # statically load this pattern
-    if not hasattr(has_old_openssl, "pattern"):
-        has_old_openssl.pattern = re.compile(b'.*OpenSSL ([01][0-9a-z.-]+)')
+    if not hasattr(has_known_vulnerability, "pattern"):
+        has_known_vulnerability.pattern = re.compile(b'.*OpenSSL ([01][0-9a-z.-]+)')
 
+    files_in_apk = set()
     with zipfile.ZipFile(filename) as zf:
         for name in zf.namelist():
             if name.endswith('libcrypto.so') or name.endswith('libssl.so'):
@@ -485,7 +496,7 @@ def has_old_openssl(filename):
                     chunk = lib.read(4096)
                     if chunk == b'':
                         break
-                    m = has_old_openssl.pattern.search(chunk)
+                    m = has_known_vulnerability.pattern.search(chunk)
                     if m:
                         version = m.group(1).decode('ascii')
                         if version.startswith('1.0.1') and version[5] >= 'r' \
@@ -495,6 +506,11 @@ def has_old_openssl(filename):
                             logging.warning('"%s" contains outdated %s (%s)', filename, name, version)
                             return True
                         break
+            elif name == 'AndroidManifest.xml' or name == 'classes.dex' or name.endswith('.so'):
+                if name in files_in_apk:
+                    return True
+                files_in_apk.add(name)
+
     return False
 
 
@@ -1172,7 +1188,7 @@ def scan_apk(apkcache, apkfilename, repodir, knownapks, use_date_from_apk):
         if not common.verify_apk_signature(apkfile):
             return True, None, False
 
-        if has_old_openssl(apkfile):
+        if has_known_vulnerability(apkfile):
             apk['antiFeatures'].add('KnownVuln')
 
         apkzip = zipfile.ZipFile(apkfile, 'r')