update: reject APKs with invalid file sig, probably Janus exploits

[fdroidserver.git] / fdroidserver / update.py

diff --git a/fdroidserver/update.py b/fdroidserver/update.py
index 02382d2b941933de287f9dd4ae86a28f22e27b6b..f90e49932258315d985a2286bcf76a7fe674cdb5 100644 (file)
--- a/fdroidserver/update.py
+++ b/fdroidserver/update.py
@@ -496,14 +496,25 @@ def has_known_vulnerability(filename):
 
     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
+
+    Janus is similar to Master Key but is perhaps easier to scan for.
+    https://www.guardsquare.com/en/blog/new-android-vulnerability-allows-attackers-modify-apps-without-affecting-their-signatures
     """
 
+    found_vuln = False
+
     # statically load this pattern
     if not hasattr(has_known_vulnerability, "pattern"):
         has_known_vulnerability.pattern = re.compile(b'.*OpenSSL ([01][0-9a-z.-]+)')
 
+    with open(filename.encode(), 'rb') as fp:
+        first4 = fp.read(4)
+    if first4 != b'\x50\x4b\x03\x04':
+        raise FDroidException(_('{path} has bad file signature "{pattern}", possible Janus exploit!')
+                              .format(path=filename, pattern=first4.decode().replace('\n', ' ')) + '\n'
+                              + 'https://www.guardsquare.com/en/blog/new-android-vulnerability-allows-attackers-modify-apps-without-affecting-their-signatures')
+
     files_in_apk = set()
     with zipfile.ZipFile(filename) as zf:
         for name in zf.namelist():
@@ -524,14 +535,15 @@ def has_known_vulnerability(filename):
                         else:
                             logging.warning(_('"{path}" contains outdated {name} ({version})')
                                             .format(path=filename, name=name, version=version))
-                            return True
+                            found_vuln = True
                         break
             elif name == 'AndroidManifest.xml' or name == 'classes.dex' or name.endswith('.so'):
                 if name in files_in_apk:
-                    return True
+                    logging.warning(_('{apkfilename} has multiple {name} files, looks like Master Key exploit!')
+                                    .format(apkfilename=filename, name=name))
+                    found_vuln = True
                 files_in_apk.add(name)
-
-    return False
+    return found_vuln
 
 
 def insert_obbs(repodir, apps, apks):