Prevent from Base64 Malleability

src/joserfc/util.py

@@ -4,7 +4,7 @@
 import struct
 import binascii
 import json
-
+from .errors import DecodeError
 
 def to_bytes(x: Any, charset: str = "utf-8", errors: str = "strict") -> bytes:
     if isinstance(x, bytes):
@@ -21,10 +21,23 @@ def to_str(x: bytes | str, charset: str = "utf-8") -> str:
         return x.decode(charset)
     return x
 
+def __is_urlsafe_b64_encoding_non_canonical(s: bytes) -> bool:
+    # https://github.com/FrancoisCapon/Base64SteganographyTools/blob/main/tools/b64_print_regular_characters.sh
+    p = len(s) % 4 # padding?
+    if p == 0:
+        return False
+    p = 4 - p # number of padding characters
+    if p == 2 and s[-1] in b"AQgw":
+        return False
+    if p == 1 and s[-1] in b"AEIMQUYcgkosw048":
+        return False
+    return True
 
 def urlsafe_b64decode(s: bytes) -> bytes:
     if b"+" in s or b"/" in s:
         raise binascii.Error
+    if __is_urlsafe_b64_encoding_non_canonical(s):
+        raise DecodeError
     s += b"=" * (-len(s) % 4)
     return base64.b64decode(s, b"-_", validate=True)
 

tests/jws/test_compact.py

@@ -73,3 +73,12 @@ def test_strict_check_header(self):
 
         registry = JWSRegistry(strict_check_header=False)
         serialize_compact(header, b"hi", key, registry=registry)
+
+    def test_non_canonical_signature_encoding(self):
+        text = "eyJhbGciOiJIUzI1NiJ9.eyJ1c2VyIjoiYWRtaW4ifQ.VI29GgHzuh2xfF0bkRYvZIsSuQnbTXSIvuRyt7RDrwo"[:-1] + "p"
+        self.assertRaises(
+            DecodeError,
+            deserialize_compact,
+            text,
+            OctKey.import_key("secret")
+        )

tests/test_util.py

@@ -1,7 +1,7 @@
 import binascii
 from unittest import TestCase
 from joserfc import util
-
+from joserfc import errors
 
 class TestUtil(TestCase):
     def test_to_bytes(self):
@@ -23,3 +23,7 @@ def test_json_b64encode(self):
     def test_urlsafe_b64decode(self):
         self.assertEqual(util.urlsafe_b64decode(b"_foo123-"), b"\xfd\xfa(\xd7m\xfe")
         self.assertRaises(binascii.Error, util.urlsafe_b64decode, b"+foo123/")
+        for c in "RSTUVWXYZabdef": # A -> QQ==
+            self.assertRaises(errors.DecodeError, util.urlsafe_b64decode, b"Q" + c.encode())
+        for c in "FGH": # AAAAAAAAAAAAAA -> QUFBQUFBQUFBQUFBQUE=
+            self.assertRaises(errors.DecodeError, util.urlsafe_b64decode, b"QUFBQUFBQUFBQUFBQU" + c.encode())