Fixed #18144 -- Restored compatibility with SHA1 hashes with empty salt.

Thanks dahool for the report and initial version of the patch.

django/conf/global_settings.py

@@ -510,6 +510,7 @@ PASSWORD_HASHERS = (
     'django.contrib.auth.hashers.BCryptPasswordHasher',
     'django.contrib.auth.hashers.SHA1PasswordHasher',
     'django.contrib.auth.hashers.MD5PasswordHasher',
+    'django.contrib.auth.hashers.UnsaltedSHA1PasswordHasher',
     'django.contrib.auth.hashers.UnsaltedMD5PasswordHasher',
     'django.contrib.auth.hashers.CryptPasswordHasher',
 )

django/contrib/auth/hashers.py

@@ -127,9 +127,14 @@ def identify_hasher(encoded):
     get_hasher() to return hasher. Raises ValueError if
     algorithm cannot be identified, or if hasher is not loaded.
     """
+    # Ancient versions of Django created plain MD5 passwords and accepted
+    # MD5 passwords with an empty salt.
     if ((len(encoded) == 32 and '$' not in encoded) or
             (len(encoded) == 37 and encoded.startswith('md5$$'))):
         algorithm = 'unsalted_md5'
+    # Ancient versions of Django accepted SHA1 passwords with an empty salt.
+    elif len(encoded) == 46 and encoded.startswith('sha1$$'):
+        algorithm = 'unsalted_sha1'
     else:
         algorithm = encoded.split('$', 1)[0]
     return get_hasher(algorithm)
@@ -350,14 +355,48 @@ class MD5PasswordHasher(BasePasswordHasher):
         ])
 
 
+class UnsaltedSHA1PasswordHasher(BasePasswordHasher):
+    """
+    Very insecure algorithm that you should *never* use; stores SHA1 hashes
+    with an empty salt.
+
+    This class is implemented because Django used to accept such password
+    hashes. Some older Django installs still have these values lingering
+    around so we need to handle and upgrade them properly.
+    """
+    algorithm = "unsalted_sha1"
+
+    def salt(self):
+        return ''
+
+    def encode(self, password, salt):
+        assert salt == ''
+        hash = hashlib.sha1(force_bytes(password)).hexdigest()
+        return 'sha1$$%s' % hash
+
+    def verify(self, password, encoded):
+        encoded_2 = self.encode(password, '')
+        return constant_time_compare(encoded, encoded_2)
+
+    def safe_summary(self, encoded):
+        assert encoded.startswith('sha1$$')
+        hash = encoded[6:]
+        return SortedDict([
+            (_('algorithm'), self.algorithm),
+            (_('hash'), mask_hash(hash)),
+        ])
+
+
 class UnsaltedMD5PasswordHasher(BasePasswordHasher):
     """
-    I am an incredibly insecure algorithm you should *never* use;
-    stores unsalted MD5 hashes without the algorithm prefix.
+    Incredibly insecure algorithm that you should *never* use; stores unsalted
+    MD5 hashes without the algorithm prefix, also accepts MD5 hashes with an
+    empty salt.
 
-    This class is implemented because Django used to store passwords
-    this way. Some older Django installs still have these values
-    lingering around so we need to handle and upgrade them properly.
+    This class is implemented because Django used to store passwords this way
+    and to accept such password hashes. Some older Django installs still have
+    these values lingering around so we need to handle and upgrade them
+    properly.
     """
     algorithm = "unsalted_md5"
 
@@ -365,6 +404,7 @@ class UnsaltedMD5PasswordHasher(BasePasswordHasher):
         return ''
 
     def encode(self, password, salt):
+        assert salt == ''
         return hashlib.md5(force_bytes(password)).hexdigest()
 
     def verify(self, password, encoded):

django/contrib/auth/tests/hashers.py

@@ -60,7 +60,7 @@ class TestUtilsHashPass(unittest.TestCase):
         self.assertEqual(identify_hasher(encoded).algorithm, "md5")
 
     def test_unsalted_md5(self):
-        encoded = make_password('lètmein', 'seasalt', 'unsalted_md5')
+        encoded = make_password('lètmein', '', 'unsalted_md5')
         self.assertEqual(encoded, '88a434c88cca4e900f7874cd98123f43')
         self.assertTrue(is_password_usable(encoded))
         self.assertTrue(check_password('lètmein', encoded))
@@ -72,6 +72,17 @@ class TestUtilsHashPass(unittest.TestCase):
         self.assertTrue(check_password('lètmein', alt_encoded))
         self.assertFalse(check_password('lètmeinz', alt_encoded))
 
+    def test_unsalted_sha1(self):
+        encoded = make_password('lètmein', '', 'unsalted_sha1')
+        self.assertEqual(encoded, 'sha1$$6d138ca3ae545631b3abd71a4f076ce759c5700b')
+        self.assertTrue(is_password_usable(encoded))
+        self.assertTrue(check_password('lètmein', encoded))
+        self.assertFalse(check_password('lètmeinz', encoded))
+        self.assertEqual(identify_hasher(encoded).algorithm, "unsalted_sha1")
+        # Raw SHA1 isn't acceptable
+        alt_encoded = encoded[6:]
+        self.assertFalse(check_password('lètmein', alt_encoded))
+
     @skipUnless(crypt, "no crypt module to generate password.")
     def test_crypt(self):
         encoded = make_password('lètmei', 'ab', 'crypt')