Gitea's SSH key ingestion endpoint accepts SSH2 format keys and normalizes them before storage. The normalization function concatenates key content line-by-line using an inefficient O(N²) algorithm without limiting input size. An attacker can exploit this by submitting a large number of short lines in a malicious SSH2 key, causing quadratic growth in CPU and memory usage. This leads to resource exhaustion and denial of service. The vulnerability affects versions prior to 1.27.0. The key validity check occurs only after the expensive concatenation, so the server performs the costly operation regardless of key validity. No size constraints exist on the API or web form fields to prevent this attack.

Any authenticated user can exploit this vulnerability to exhaust the server's CPU and memory resources by submitting a specially crafted large SSH2 public key. This results in denial of service by taking the Gitea instance offline due to resource exhaustion. There is no indication of privilege escalation or data compromise from this vulnerability.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Until a fix is available, restrict or monitor authenticated user submissions of SSH keys to prevent excessively large inputs. Consider implementing input size limits or rate limiting on the SSH key ingestion endpoint to mitigate exploitation risk.