Gitea incorrectly interprets SSH certificate principals by scanning all stored public keys instead of matching against the designated principal key. When the instance trusts a user certificate authority and allows arbitrary principals, a malicious user can register or supply another user’s public key line as their own principal. Through the HTTP signature API or the built‑in SSH server, the attacker can then authenticate as the target user, achieving full impersonation of legitimate accounts. This results in unauthorized access to repositories and potentially sensitive data, violating confidentiality, integrity, and availability of the service.

Based on the description, the flaw impacts all Gitea installations that trust a user certificate authority and enable arbitrary principal acceptance. Versions prior to 28.2.0 are vulnerable; the 28.2.0 release and later include the necessary verification fix. Instances that do not trust a user certificate authority are not affected.

Exploitation requires only a certificate signed by the trusted CA or the ability to submit a valid public key line belonging to another user, and does not necessitate local privileges. The attacker can carry out the attack remotely via the HTTP signature endpoint or by connecting to the built‑in SSH server. The CVSS score of 7.5 indicates a high severity, and the EPSS score of less than 1% shows low but non‑zero exploitation probability. The vulnerability is not listed in CISA’s KEV catalog. The combination of remote attack surface, impersonation capability, and high severity makes this a critical risk that should be patched promptly.