{"id":"CVE-2026-18223","kind":"vulnerability","aliases":["CVE-2026-18223"],"related":[],"title":"Mattermost: invitation tokens redeemable by any account, exposing private channels","description":"Mattermost versions 11.10.x <= 11.10.0, 11.7.x <= 11.7.9, 11.9.x <= 11.9.1, 11.8.x <= 11.8.5 fail to validate the invited recipient's email address when redeeming team and guest invitation tokens, which allows an authenticated attacker to join a team or gain unauthorized access to private channels via a forwarded or leaked invitation link.. Mattermost Advisory ID: MMSA-2026-00747","summary":"Mattermost fails to check that the signed-in user's email matches the invited recipient when redeeming team and guest invitation tokens. An authenticated user holding a forwarded or leaked invite link can join a team or reach private channels they were not invited to.","severity":"medium","cvss":5.0,"epss":null,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["mattermost","package","self-hosted","team-chat"],"not_affected_if":["default-config"],"how_to_check":null,"fix":"- Upgrade Mattermost to a fixed release for your line: `11.10.1` or later for 11.10.x, `11.7.10` or later for 11.7.x, `11.9.2` or later for 11.9.x, `11.8.6` or later for 11.8.x (inferred from the affected ranges; confirm against MMSA-2026-00747)","status":"open","published_at":"2026-10-16T08:17:12+00:00","first_seen_at":"2026-10-23T21:57:12+00:00","triaged_at":"2026-10-25T04:31:03+00:00","updated_at":"2026-10-25T04:31:03+00:00","category":"package","confidence":0.6,"triage":{"at":"2026-10-25T04:31:03Z","model":"claude-haiku-5-5","output":{"fix":"- Upgrade Mattermost to a fixed release for your line: `11.10.1` or later for 11.10.x, `11.7.10` or later for 11.7.x, `11.9.2` or later for 11.9.x, `11.8.6` or later for 11.8.x (inferred from the affected ranges; confirm against MMSA-2026-00747)","reach":"network","title":"Mattermost: invitation tokens redeemable by any account, exposing private channels","impact":"medium","summary":"Mattermost fails to check that the signed-in user's email matches the invited recipient when redeeming team and guest invitation tokens. An authenticated user holding a forwarded or leaked invite link can join a team or reach private channels they were not invited to.","category":"package","nicknames":[],"who_cares":["mattermost","self-hosted","team-chat"],"confidence":0.6,"needs_auth":"yes","worst_case":"Authenticated user joins teams or reads private channels without invitation","analyst_note":"Confirm fixed versions and how_to_check from the Mattermost advisory MMSA-2026-00747; the sources give only the affected ranges.","how_to_check":"unknown","needs_analyst":true,"not_affected_if":["default-config"],"who_is_affected":"Self-hosted or managed Mattermost servers on the affected 11.7, 11.8, 11.9 and 11.10 release ranges that issue team or guest invitations. The attacker needs a valid Mattermost account, so servers with open self-registration or many users are more exposed. Installs on other versions, or instances that do not use invitations, are less exposed.","user_interaction":"no","needs_special_setup":"no","severity_if_unknown":"medium"},"cost_usd":0.000633,"input_tokens":3417,"output_tokens":582,"severity_before":"medium"},"analyst_status":null,"analyst_note":null,"search":"'-00747':129C '-18223':13A,16A '-2026':12A,15A,128C '..':123C '11.10':72C '11.10.0':74C '11.7':75C '11.7.9':77C '11.8':81C '11.8.5':83C '11.9':78C '11.9.1':80C 'access':112C 'account':7A 'address':92C 'advisori':125C 'allow':101C 'attack':104C 'authent':49B,103C 'channel':10A,65B,115C 'chat':24B 'check':28B 'cve':11A,14A 'email':36B,91C 'expos':8A 'fail':26B,84C 'forward':53B,118C 'gain':110C 'guest':45B,97C 'hold':51B 'host':21B 'id':126C 'invit':2A,39B,46B,56B,69B,88C,98C,121C 'join':59B,106C 'leak':55B,120C 'link':57B,122C 'match':37B 'mattermost':1A,17B,25B,70C,124C 'mmsa':127C 'packag':18B 'privat':9A,64B,114C 'reach':63B 'recipi':40B,89C 'redeem':4A,42B,94C 'self':20B 'self-host':19B 'sign':32B 'signed-in':31B 'team':23B,43B,61B,95C,108C 'team-chat':22B 'token':3A,47B,99C 'unauthor':111C 'user':34B,50B 'valid':86C 'version':71C 'via':116C 'x':73C,76C,79C,82C","nicknames":[],"mitigations":[],"who_affected":"Self-hosted or managed Mattermost servers on the affected 11.7, 11.8, 11.9 and 11.10 release ranges that issue team or guest invitations. The attacker needs a valid Mattermost account, so servers with open self-registration or many users are more exposed. Installs on other versions, or instances that do not use invitations, are less exposed.","tier":"C","verified":{},"analyst_revisit_at":null,"first_public_at":"2026-10-16T08:17:12+00:00","kev":null,"cvss_vector":"CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:U/C:H/I:L/A:N","cvss_by":"responsibledisclosure@mattermost.com","poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-18223","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-18223","severity":null,"published_at":"2026-10-16T08:17:12+00:00","modified_at":"2026-10-16T19:23:18+00:00","first_seen_at":"2026-10-23T21:57:12+00:00","fetched_at":"2026-10-23T22:28:55+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"}