diff --git a/fdroidserver/update.py b/fdroidserver/update.py
index e548df43..f90e4993 100644
--- a/fdroidserver/update.py
+++ b/fdroidserver/update.py
@@ -496,8 +496,10 @@ 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
@@ -506,6 +508,13 @@ def has_known_vulnerability(filename):
     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():
diff --git a/tests/janus.apk b/tests/janus.apk
new file mode 100644
index 00000000..ed4eed96
Binary files /dev/null and b/tests/janus.apk differ
diff --git a/tests/run-tests b/tests/run-tests
index 696bcd75..af29f471 100755
--- a/tests/run-tests
+++ b/tests/run-tests
@@ -9,7 +9,7 @@ echo_header() {
 copy_apks_into_repo() {
     set +x
     find $APKDIR -type f -name '*.apk' -print0 | while IFS= read -r -d '' f; do
-        echo $f | grep -F -v -e unaligned -e unsigned -e badsig -e badcert -e bad-unicode || continue
+        echo $f | grep -F -v -e unaligned -e unsigned -e badsig -e badcert -e bad-unicode -e janus.apk || continue
         apk=`$aapt dump badging "$f" | sed -n "s,^package: name='\(.*\)' versionCode='\([0-9][0-9]*\)' .*,\1_\2.apk,p"`
         test "$f" -nt repo/$apk && rm -f repo/$apk  # delete existing if $f is newer
         if [ ! -e repo/$apk ] && [ ! -e archive/$apk ]; then
diff --git a/tests/update.TestCase b/tests/update.TestCase
index e49e1bf0..db463a89 100755
--- a/tests/update.TestCase
+++ b/tests/update.TestCase
@@ -601,6 +601,35 @@ class UpdateTest(unittest.TestCase):
         self.assertEqual('urzip', data['Name'])
         self.assertEqual('urzip', data['Summary'])
 
+    def test_has_known_vulnerability(self):
+        good = [
+            'org.bitbucket.tickytacky.mirrormirror_1.apk',
+            'org.bitbucket.tickytacky.mirrormirror_2.apk',
+            'org.bitbucket.tickytacky.mirrormirror_3.apk',
+            'org.bitbucket.tickytacky.mirrormirror_4.apk',
+            'org.dyndns.fules.ck_20.apk',
+            'urzip.apk',
+            'urzip-badcert.apk',
+            'urzip-badsig.apk',
+            'urzip-release.apk',
+            'urzip-release-unsigned.apk',
+            'repo/com.politedroid_3.apk',
+            'repo/com.politedroid_4.apk',
+            'repo/com.politedroid_5.apk',
+            'repo/com.politedroid_6.apk',
+            'repo/obb.main.oldversion_1444412523.apk',
+            'repo/obb.mainpatch.current_1619_another-release-key.apk',
+            'repo/obb.mainpatch.current_1619.apk',
+            'repo/obb.main.twoversions_1101613.apk',
+            'repo/obb.main.twoversions_1101615.apk',
+            'repo/obb.main.twoversions_1101617.apk',
+            'repo/urzip-; Рахма́нинов, [rɐxˈmanʲɪnəf] سيرجي_رخمانينوف 谢尔盖·.apk',
+        ]
+        for f in good:
+            self.assertFalse(fdroidserver.update.has_known_vulnerability(f))
+        with self.assertRaises(fdroidserver.exception.FDroidException):
+            fdroidserver.update.has_known_vulnerability('janus.apk')
+
 
 if __name__ == "__main__":
     parser = optparse.OptionParser()