CVE-2017-12880

In PyJWT 1.5.0 and below 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.

Source: CVE-2017-12880