The tough library, prior to 0.7.1, does not properly verify the uniqueness of keys in the signatures provided to meet the threshold of cryptographic signatures. It allows someone with access to a valid signing key to create multiple valid signatures in order to circumvent TUF requiring a minimum threshold of unique keys before the metadata is considered valid.

GHSA-5q2r-92f9-4m49
theupdateframework/python-tuf#974
https://nvd.nist.gov/vuln/detail/CVE-2020-6174