Fix potential DoS issue with p2c header

Unbounded p2c headers may be used to cause an application that accept
PBES algorithms to spend alot of resources running PBKDF2 with a very
high number of iterations.

Clamp the default maximum to 16384 (double the default of 8192).
An application that wants to use more iterations will have to chenge the
jwa default max.

Fixes CVE-2023-6681

Signed-off-by: Simo Sorce <simo@redhat.com>

jwcrypto/jwa.py

@@ -28,6 +28,8 @@
 
 # Implements RFC 7518 - JSON Web Algorithms (JWA)
 
+default_max_pbkdf2_iterations = 16384
+
 
 class JWAAlgorithm(metaclass=ABCMeta):
 
@@ -588,6 +590,9 @@ def __init__(self):
         self.aeskwmap = {128: _A128KW, 192: _A192KW, 256: _A256KW}
 
     def _get_key(self, alg, key, p2s, p2c):
+        if p2c > default_max_pbkdf2_iterations:
+            raise ValueError('Invalid p2c value, too large')
+
         if not isinstance(key, JWK):
             # backwards compatibility for old interface
             if isinstance(key, bytes):

jwcrypto/tests.py

@@ -2099,6 +2099,18 @@ def test_pbes2_hs256_aeskw_custom_params(self):
         key = jwk.JWK.from_password('password')
         self.assertRaises(ValueError, enc.add_recipient, key)
 
+        # Test p2c iteration checks
+        maxiter = jwa.default_max_pbkdf2_iterations
+        p2cenc = jwe.JWE(plaintext='plain',
+                         protected={"alg": "PBES2-HS256+A128KW",
+                                    "enc": "A256CBC-HS512",
+                                    "p2c": maxiter + 1,
+                                    "p2s": base64url_encode("A" * 16)})
+        with self.assertRaisesRegex(ValueError, 'too large'):
+            p2cenc.add_recipient(key)
+        jwa.default_max_pbkdf2_iterations += 2
+        p2cenc.add_recipient(key)
+
 
 class JWATests(unittest.TestCase):
     def test_jwa_create(self):