Advisories for Pypi/PyJWT package

PyJWT is a Python implementation of RFC 7519. 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, or specify a single algorithm. The issue is not that big as algorithms=jwt.algorithms.get_default_algorithms() has to be used. Users …

In PyJWT, the invalid_strings check in HMACAlgorithm.prepare_key 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.

It is possible for an attacker to bypass verification when "a token digitally signed with an asymetric key (RS/ES family) of algorithms but instead the attacker send a token digitally signed with a symmetric algorithm (HS* family)". It is also possible for an attacker to create his own signed token with any payload he wants and have it considered valid using the "none" algorithm.