Gitea resolved the principals of an SSH certificate by searching the content of all stored public keys instead of only principal keys. When Gitea trusts a user certificate authority ("SSH_TRUSTED_USER_CA_KEYS") and accepts arbitrary principals ("SSH_AUTHORIZED_PRINCIPALS_ALLOW=anything"), a user could register another user's public key line as their own principal, or a holder of a certificate signed by that authority could list it, and then authenticate as the owner of that key through the HTTP signature API and the built-in SSH server. Instances that do not trust a user certificate authority are not affected.