{"id":"CVE-2026-109553","kind":"vulnerability","aliases":["CVE-2026-109553"],"related":[],"title":"Gitea: SSH certificate principals let attackers log in as other users","description":"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.","summary":"Gitea matched SSH certificate principals against the contents of every stored public key, not only principal keys. With a trusted user certificate authority and arbitrary principals allowed, an attacker can register another user's key line as their principal and authenticate as that user over SSH.","severity":"high","cvss":7.5,"epss":null,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["git","gitea","package","self-hosted","ssh"],"not_affected_if":["default-config","no-user-ca"],"how_to_check":"- Check whether `SSH_TRUSTED_USER_CA_KEYS` is set in the Gitea configuration file (app.ini or environment).\n- Check whether `SSH_AUTHORIZED_PRINCIPALS_ALLOW` is set to `anything`.\n- Check the installed Gitea version with `gitea --version`.","fix":"- Upgrade Gitea to the patched release listed in the advisory (fixed version unknown from the sources).\n- Set `SSH_AUTHORIZED_PRINCIPALS_ALLOW` to a restrictive value instead of `anything` until upgraded.\n- Remove `SSH_TRUSTED_USER_CA_KEYS` if no user certificate authority is needed.","status":"open","published_at":"2026-10-18T19:16:32+00:00","first_seen_at":"2026-10-23T21:57:17+00:00","triaged_at":"2026-10-25T03:23:30+00:00","updated_at":"2026-10-25T03:23:30+00:00","category":"package","confidence":0.6,"triage":{"at":"2026-10-25T03:23:30Z","model":"claude-haiku-5-5","output":{"fix":"- Upgrade Gitea to the patched release listed in the advisory (fixed version unknown from the sources).\n- Set `SSH_AUTHORIZED_PRINCIPALS_ALLOW` to a restrictive value instead of `anything` until upgraded.\n- Remove `SSH_TRUSTED_USER_CA_KEYS` if no user certificate authority is needed.","reach":"network","title":"Gitea: SSH certificate principals let attackers log in as other users","impact":"high","summary":"Gitea matched SSH certificate principals against the contents of every stored public key, not only principal keys. With a trusted user certificate authority and arbitrary principals allowed, an attacker can register another user's key line as their principal and authenticate as that user over SSH.","category":"package","nicknames":[],"who_cares":["gitea","self-hosted","ssh","git"],"confidence":0.6,"needs_auth":"yes","worst_case":"Attacker authenticates as another user and reaches their repositories.","analyst_note":"Confirm the patched Gitea version and the fix release from the NVD entry or Gitea release notes; sources did not list a fixed version.","how_to_check":"- Check whether `SSH_TRUSTED_USER_CA_KEYS` is set in the Gitea configuration file (app.ini or environment).\n- Check whether `SSH_AUTHORIZED_PRINCIPALS_ALLOW` is set to `anything`.\n- Check the installed Gitea version with `gitea --version`.","needs_analyst":true,"not_affected_if":["default-config","no-user-ca"],"who_is_affected":"Only Gitea instances that trust a user certificate authority through SSH_TRUSTED_USER_CA_KEYS and accept arbitrary principals through SSH_AUTHORIZED_PRINCIPALS_ALLOW=anything. Instances that do not trust a user certificate authority are not affected, and the sources do not give further detail on other configurations.","user_interaction":"no","needs_special_setup":"yes","severity_if_unknown":"high"},"cost_usd":0.000699,"input_tokens":3424,"output_tokens":714,"severity_before":"high"},"analyst_status":null,"analyst_note":null,"search":"'-109553':14A,17A '-2026':13A,16A 'accept':106C 'affect':173C 'allow':51B,112C 'anoth':56B,118C 'anyth':113C 'api':154C 'arbitrari':49B,107C 'attack':6A,53B 'authent':65B,143C 'author':47B,99C,110C,137C,170C 'built':158C 'built-in':157C 'ca':103C 'certif':3A,28B,46B,78C,98C,133C,169C 'content':32B,82C 'could':116C,138C 'cve':12A,15A 'everi':34B 'git':18B 'gitea':1A,19B,25B,71C,94C 'holder':130C 'host':23B 'http':152C 'instanc':162C 'instead':88C 'key':37B,41B,59B,87C,92C,104C,122C,149C 'let':5A 'line':60B,123C 'list':139C 'log':7A 'match':26B 'owner':146C 'packag':20B 'princip':4A,29B,40B,50B,63B,74C,91C,108C,111C,127C 'public':36B,86C,121C 'regist':55B,117C 'resolv':72C 'search':80C 'self':22B 'self-host':21B 'server':161C 'sign':134C 'signatur':153C 'ssh':2A,24B,27B,70B,77C,100C,109C,160C 'store':35B,85C 'trust':44B,95C,101C,166C 'user':11A,45B,57B,68B,97C,102C,115C,119C,168C","nicknames":[],"mitigations":[],"who_affected":"Only Gitea instances that trust a user certificate authority through SSH_TRUSTED_USER_CA_KEYS and accept arbitrary principals through SSH_AUTHORIZED_PRINCIPALS_ALLOW=anything. Instances that do not trust a user certificate authority are not affected, and the sources do not give further detail on other configurations.","tier":"B","verified":{},"analyst_revisit_at":null,"first_public_at":"2026-10-18T19:16:32+00:00","kev":null,"cvss_vector":"CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H","cvss_by":"134c704f-9b21-4f2e-91b3-4a467353bcc0","poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-109553","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-109553","severity":null,"published_at":"2026-10-18T19:16:32+00:00","modified_at":"2026-10-19T21:17:04+00:00","first_seen_at":"2026-10-23T21:57:17+00:00","fetched_at":"2026-10-23T22:28:56+00:00","note":null}],"documents":[],"notice":"Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai"}