Description

Key confusion through non-blocklisted public key formats

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...
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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...
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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

Are there any links users can visit to find out more?

For more information

If you have any questions or comments about this advisory:

Open an issue in https://github.com/jpadilla/pyjwt

Email José Padilla: pyjwt at jpadilla dot com

Mitigation

Aliases

References