{"id":"CVE-2026-19603","kind":"vulnerability","aliases":["CVE-2026-19603"],"related":[],"title":"Mattermost: user status API leaks private channel IDs to authenticated users","description":"Mattermost versions 11.9.x <= 11.9.1, 11.8.x <= 11.8.5, 11.7.x <= 11.7.10, 11.10.x <= 11.10.1 fail to sanitize the active channel field returned by the user status API endpoints, which allows an authenticated user to learn the channel ID of a private channel another user is actively viewing via the status endpoints, without being a member of that channel. Mattermost Advisory ID: MMSA-2026-00765","summary":"Mattermost versions 11.7.x through 11.10.1 do not sanitize the active channel field returned by the user status API endpoints. An authenticated user can learn the channel ID of a private channel another user is currently viewing, without being a member of that channel.","severity":"medium","cvss":4.3,"epss":null,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["chat","mattermost","package","self-hosted"],"not_affected_if":["managed-hosting"],"how_to_check":"- Run `mmctl version` or check the server version in the System Console and compare it against the affected ranges: 11.7.x up to 11.7.10, 11.8.x up to 11.8.5, 11.9.x up to 11.9.1, 11.10.x up to 11.10.1.","fix":"- Upgrade self-hosted Mattermost to a release that includes the fix for MMSA-2026-00765 in each affected line (the fixed version numbers are unknown from the sources; check the Mattermost advisory MMSA-2026-00765).","status":"open","published_at":"2026-10-16T07:17:12+00:00","first_seen_at":"2026-10-23T21:57:12+00:00","triaged_at":"2026-10-25T04:41:13+00:00","updated_at":"2026-10-25T04:41:13+00:00","category":"package","confidence":0.6,"triage":{"at":"2026-10-25T04:41:13Z","model":"claude-haiku-5-5","output":{"fix":"- Upgrade self-hosted Mattermost to a release that includes the fix for MMSA-2026-00765 in each affected line (the fixed version numbers are unknown from the sources; check the Mattermost advisory MMSA-2026-00765).","reach":"network","title":"Mattermost: user status API leaks private channel IDs to authenticated users","impact":"low","summary":"Mattermost versions 11.7.x through 11.10.1 do not sanitize the active channel field returned by the user status API endpoints. An authenticated user can learn the channel ID of a private channel another user is currently viewing, without being a member of that channel.","category":"package","nicknames":[],"who_cares":["mattermost","self-hosted","chat"],"confidence":0.6,"needs_auth":"yes","worst_case":"Authenticated users learn private channel IDs they are not members of.","analyst_note":"Fixed version numbers for each affected line are not in the sources; check MMSA-2026-00765 on the Mattermost security updates page.","how_to_check":"- Run `mmctl version` or check the server version in the System Console and compare it against the affected ranges: 11.7.x up to 11.7.10, 11.8.x up to 11.8.5, 11.9.x up to 11.9.1, 11.10.x up to 11.10.1.","needs_analyst":true,"not_affected_if":["managed-hosting"],"who_is_affected":"Self-hosted Mattermost servers on the affected 11.x release lines are exposed, since any logged-in user can query the status endpoints. Teams on Mattermost Cloud are handled by the vendor, and teams on versions outside the listed ranges are not affected per the sources.","user_interaction":"no","needs_special_setup":"unknown","severity_if_unknown":"medium"},"cost_usd":0.000647,"input_tokens":3416,"output_tokens":610,"severity_before":"medium"},"analyst_status":null,"analyst_note":null,"search":"'-00765':131C '-19603':14A,17A '-2026':13A,16A,130C '11.10':79C '11.10.1':29B,81C '11.7':26B,76C '11.7.10':78C '11.8':73C '11.8.5':75C '11.9':70C '11.9.1':72C 'activ':34B,86C,113C 'advisori':127C 'allow':97C 'anoth':56B,110C 'api':4A,42B,94C 'authent':10A,45B,99C 'channel':7A,35B,50B,55B,67B,87C,104C,109C,125C 'chat':18B 'current':59B 'cve':12A,15A 'endpoint':43B,95C,118C 'fail':82C 'field':36B,88C 'host':23B 'id':8A,51B,105C,128C 'leak':5A 'learn':48B,102C 'mattermost':1A,19B,24B,68C,126C 'member':64B,122C 'mmsa':129C 'packag':20B 'privat':6A,54B,108C 'return':37B,89C 'sanit':32B,84C 'self':22B 'self-host':21B 'status':3A,41B,93C,117C 'user':2A,11A,40B,46B,57B,92C,100C,111C 'version':25B,69C 'via':115C 'view':60B,114C 'without':61B,119C 'x':27B,71C,74C,77C,80C","nicknames":[],"mitigations":[],"who_affected":"Self-hosted Mattermost servers on the affected 11.x release lines are exposed, since any logged-in user can query the status endpoints. Teams on Mattermost Cloud are handled by the vendor, and teams on versions outside the listed ranges are not affected per the sources.","tier":"C","verified":{},"analyst_revisit_at":null,"first_public_at":"2026-10-16T07:17:12+00:00","kev":null,"cvss_vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N","cvss_by":"responsibledisclosure@mattermost.com","poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-19603","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-19603","severity":null,"published_at":"2026-10-16T07:17:12+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"}