CVE-2020-6174: Incorrect threshold signature computation in TUF

Metadadata signature verification, as used in tuf.client.updater, counted each of multiple signatures with identical authorized keyids separately towards the threshold. Therefore, an attacker with access to a valid signing key could create multiple valid signatures in order to meet the minimum threshold of keys before the metadata was considered valid.

The tuf maintainers would like to thank Erik MacLean of Analog Devices, Inc. for reporting this issue.