diff --git a/jose/jwe.py b/jose/jwe.py
index 09e5c32..d104a7a 100644
--- a/jose/jwe.py
+++ b/jose/jwe.py
@@ -140,6 +140,14 @@ def decrypt(jwe_str, key):
         try:
             cek_bytes = key.unwrap_key(encrypted_key)
 
+            # An unwrap that returns the wrong number of bytes is a padding
+            # failure that did not raise: PKCS1v15 implementations may return
+            # arbitrary bytes from their constant-time path instead of raising.
+            # Treat it exactly like a raised error so that length errors stay
+            # indistinguishable from format and padding errors (RFC 7516 §11.5).
+            if len(cek_bytes) != len(_get_random_cek_bytes_for_enc(enc)):
+                raise JWEError("Invalid CEK length")
+
             # Record whether the CEK could be successfully determined for this
             # recipient or not.
             cek_valid = True
diff --git a/tests/test_jwe.py b/tests/test_jwe.py
index 6ab9971..9f650ec 100644
--- a/tests/test_jwe.py
+++ b/tests/test_jwe.py
@@ -1,4 +1,5 @@
 import json
+import os
 
 import pytest
 
@@ -7,7 +8,7 @@
 from jose.constants import ALGORITHMS, ZIPS
 from jose.exceptions import JWEError, JWEParseError
 from jose.jwk import AESKey, RSAKey
-from jose.utils import base64url_decode
+from jose.utils import base64url_decode, base64url_encode
 
 backends = []
 try:
@@ -357,6 +358,38 @@ def test_non_json_header_is_parse_error(self):
         with pytest.raises(JWEParseError):
             jwe.decrypt(jwe_str, "key")
 
+    @pytest.mark.skipif(RSAKey is None, reason="No RSA backend")
+    def test_rsa1_5_malformed_keys_are_indistinguishable(self):
+        """RFC 7516 §11.5: format, padding and length errors must not be distinguishable.
+
+        PKCS1v15 unwrapping does not raise for every malformed encrypted key —
+        the constant-time path can return arbitrary bytes instead. Those must be
+        rejected the same way a raised error is, or the resulting length-specific
+        message becomes a padding oracle.
+        """
+        header = base64url_encode(json.dumps({"alg": "RSA1_5", "enc": "A256GCM"}).encode())
+
+        def malformed_token():
+            return b".".join(
+                [
+                    header,
+                    base64url_encode(os.urandom(256)),
+                    base64url_encode(os.urandom(12)),
+                    base64url_encode(os.urandom(32)),
+                    base64url_encode(os.urandom(16)),
+                ]
+            ).decode()
+
+        messages = set()
+        for _ in range(50):
+            with pytest.raises(JWEError) as exc:
+                jwe.decrypt(malformed_token(), PRIVATE_KEY_PEM)
+            messages.add(str(exc.value))
+
+        assert messages == {
+            "Invalid JWE Auth Tag"
+        }, f"malformed keys produced distinguishable errors: {sorted(messages)}"
+
 
 class TestEncrypt:
     @pytest.mark.skipif(AESKey is None, reason="No AES backend")