{"id":"CVE-2026-18551","kind":"vulnerability","aliases":["CVE-2026-18551"],"related":[],"title":"Mattermost: guest accounts can gain team or channel admin rights","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 combination of guest and admin scheme roles which allows an authenticated attacker to escalate a guest account to team or channel admin via a crafted PUT request to the member roles API endpoint.. Mattermost Advisory ID: MMSA-2026-00752","summary":"Mattermost fails to validate combined guest and admin scheme roles, so an authenticated guest user can send a crafted PUT request to the member roles API. That lets the guest escalate to team or channel admin.","severity":"medium","cvss":6.5,"epss":null,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["guest-accounts","mattermost","package","self-hosted"],"not_affected_if":["fixed-version","no-guest-accounts"],"how_to_check":"- Check the running Mattermost server version with `mattermost version` or the System Console, and compare against the affected ranges: 11.7.x up to 11.7.9, 11.8.x up to 11.8.5, 11.9.x up to 11.9.1, and 11.10.0.\n- Review whether guest accounts exist on the server in the System Console under Users, filtered by the guest role.","fix":"- Upgrade Mattermost to a fixed release listed in advisory MMSA-2026-00752 for your release line (unknown exact fixed versions in the sources).\n- Until patched, review team and channel admin assignments for guest accounts and remove any elevated roles.","status":"open","published_at":"2026-10-16T07:17:12+00:00","first_seen_at":"2026-10-23T21:57:12+00:00","triaged_at":"2026-10-25T03:58:01+00:00","updated_at":"2026-10-25T03:58:01+00:00","category":"package","confidence":0.7,"triage":{"at":"2026-10-25T03:58:01Z","model":"claude-haiku-5-5","output":{"fix":"- Upgrade Mattermost to a fixed release listed in advisory MMSA-2026-00752 for your release line (unknown exact fixed versions in the sources).\n- Until patched, review team and channel admin assignments for guest accounts and remove any elevated roles.","reach":"network","title":"Mattermost: guest accounts can gain team or channel admin rights","impact":"medium","summary":"Mattermost fails to validate combined guest and admin scheme roles, so an authenticated guest user can send a crafted PUT request to the member roles API. That lets the guest escalate to team or channel admin.","category":"package","nicknames":[],"who_cares":["mattermost","self-hosted","guest-accounts"],"confidence":0.7,"needs_auth":"yes","worst_case":"A guest account becomes team or channel admin on the server.","analyst_note":"Confirm the fixed version numbers for each release line in advisory MMSA-2026-00752 and whether default configs allow guest accounts.","how_to_check":"- Check the running Mattermost server version with `mattermost version` or the System Console, and compare against the affected ranges: 11.7.x up to 11.7.9, 11.8.x up to 11.8.5, 11.9.x up to 11.9.1, and 11.10.0.\n- Review whether guest accounts exist on the server in the System Console under Users, filtered by the guest role.","needs_analyst":true,"not_affected_if":["no-guest-accounts","fixed-version"],"who_is_affected":"Self-hosted or managed Mattermost servers in the listed version ranges that use guest accounts are exposed, since the attacker must already hold a guest login. Servers without guest accounts, or running a version outside the affected ranges, can skip this.","user_interaction":"no","needs_special_setup":"unknown","severity_if_unknown":"medium"},"cost_usd":0.000657,"input_tokens":3409,"output_tokens":632,"severity_before":"medium"},"analyst_status":null,"analyst_note":null,"search":"'-00752':118C '-18551':13A,16A '-2026':12A,15A,117C '..':112C '11.10':63C '11.10.0':65C '11.7':66C '11.7.9':68C '11.8':72C '11.8.5':74C '11.9':69C '11.9.1':71C 'account':3A,19B,95C 'admin':9A,32B,60B,83C,100C 'advisori':114C 'allow':87C 'api':50B,110C 'attack':90C 'authent':37B,89C 'channel':8A,59B,99C 'combin':29B,79C 'craft':43B,103C 'cve':11A,14A 'endpoint':111C 'escal':55B,92C 'fail':26B,75C 'gain':5A 'guest':2A,18B,30B,38B,54B,81C,94C 'guest-account':17B 'host':24B 'id':115C 'let':52B 'mattermost':1A,20B,25B,61C,113C 'member':48B,108C 'mmsa':116C 'packag':21B 'put':44B,104C 'request':45B,105C 'right':10A 'role':34B,49B,85C,109C 'scheme':33B,84C 'self':23B 'self-host':22B 'send':41B 'team':6A,57B,97C 'user':39B 'valid':28B,77C 'version':62C 'via':101C 'x':64C,67C,70C,73C","nicknames":[],"mitigations":[],"who_affected":"Self-hosted or managed Mattermost servers in the listed version ranges that use guest accounts are exposed, since the attacker must already hold a guest login. Servers without guest accounts, or running a version outside the affected ranges, can skip this.","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:H/UI:N/S:U/C:H/I:H/A:N","cvss_by":"responsibledisclosure@mattermost.com","poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-18551","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-18551","severity":null,"published_at":"2026-10-16T07:17:12+00:00","modified_at":"2026-10-20T14:16:59+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"}