{"id":"CVE-2026-18799","kind":"vulnerability","aliases":["CVE-2026-18799"],"related":[],"title":"Mattermost: Slack import lets team admins impersonate existing users","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 restrict Slack import account binding to system administrator-initiated imports which allows an authenticated attacker with team admin privileges to force-add existing Mattermost users to their team and forge posts attributed to those users' identities via a crafted Slack export containing target users' email addresses.. Mattermost Advisory ID: MMSA-2026-00757","summary":"Mattermost's Slack import does not limit account binding to imports started by a system administrator. A team admin can craft a Slack export containing other users' email addresses to force-add those users to their team and post messages attributed to them.","severity":"medium","cvss":4.3,"epss":null,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["chat","mattermost","package","self-hosted"],"not_affected_if":["default-config","versions-outside-range"],"how_to_check":"- Check the running Mattermost server version with `mattermost version` or the System Console; affected builds are 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.x up to `11.10.0`.\n- Check whether team admins can run Slack imports in the System Console under Data Management.","fix":"- Upgrade Mattermost to a fixed release listed in advisory MMSA-2026-00757 (the fixed versions are not stated in the sources; check the advisory).\n- Until upgraded, restrict team admin use of Slack import.","status":"open","published_at":"2026-10-16T08:17:13+00:00","first_seen_at":"2026-10-23T21:57:12+00:00","triaged_at":"2026-10-25T04:41:09+00:00","updated_at":"2026-10-25T04:41:09+00:00","category":"package","confidence":0.6,"triage":{"at":"2026-10-25T04:41:09Z","model":"claude-haiku-5-5","output":{"fix":"- Upgrade Mattermost to a fixed release listed in advisory MMSA-2026-00757 (the fixed versions are not stated in the sources; check the advisory).\n- Until upgraded, restrict team admin use of Slack import.","reach":"network","title":"Mattermost: Slack import lets team admins impersonate existing users","impact":"medium","summary":"Mattermost's Slack import does not limit account binding to imports started by a system administrator. A team admin can craft a Slack export containing other users' email addresses to force-add those users to their team and post messages attributed to them.","category":"package","nicknames":[],"who_cares":["mattermost","self-hosted","chat"],"confidence":0.6,"needs_auth":"yes","worst_case":"Attacker impersonates other users and posts messages under their names","analyst_note":"Confirm the fixed versions and whether team admins can run Slack imports by default; see the Mattermost advisory MMSA-2026-00757 and the NVD entry.","how_to_check":"- Check the running Mattermost server version with `mattermost version` or the System Console; affected builds are 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.x up to `11.10.0`.\n- Check whether team admins can run Slack imports in the System Console under Data Management.","needs_analyst":true,"not_affected_if":["default-config","versions-outside-range"],"who_is_affected":"Self-hosted or managed Mattermost deployments on the affected version ranges where team admins can run Slack imports. Exposure requires a team admin account, so instances where only system administrators can import data are at lower risk. Deployments on versions outside the listed ranges can skip it.","user_interaction":"no","needs_special_setup":"unknown","severity_if_unknown":"medium"},"cost_usd":0.000663,"input_tokens":3437,"output_tokens":638,"severity_before":"medium"},"analyst_status":null,"analyst_note":null,"search":"'-00757':134C '-18799':12A,15A '-2026':11A,14A,133C '..':128C '11.10':66C '11.10.0':68C '11.7':69C '11.7.9':71C '11.8':75C '11.8.5':77C '11.9':72C '11.9.1':74C 'account':29B,83C 'add':54B,103C 'address':50B,127C 'admin':6A,40B,98C 'administr':37B,88C 'administrator-initi':87C 'advisori':130C 'allow':92C 'attack':95C 'attribut':63B,113C 'authent':94C 'bind':30B,84C 'chat':16B 'contain':46B,123C 'craft':42B,120C 'cve':10A,13A 'email':49B,126C 'exist':8A,104C 'export':45B,122C 'fail':78C 'forc':53B,102C 'force-add':52B,101C 'forg':111C 'host':21B 'id':131C 'ident':117C 'imperson':7A 'import':3A,25B,32B,82C,90C 'initi':89C 'let':4A 'limit':28B 'mattermost':1A,17B,22B,64C,105C,129C 'messag':62B 'mmsa':132C 'packag':18B 'post':61B,112C 'privileg':99C 'restrict':80C 'self':20B 'self-host':19B 'slack':2A,24B,44B,81C,121C 'start':33B 'system':36B,86C 'target':124C 'team':5A,39B,59B,97C,109C 'user':9A,48B,56B,106C,116C,125C 'version':65C 'via':118C 'x':67C,70C,73C,76C","nicknames":[],"mitigations":[],"who_affected":"Self-hosted or managed Mattermost deployments on the affected version ranges where team admins can run Slack imports. Exposure requires a team admin account, so instances where only system administrators can import data are at lower risk. Deployments on versions outside the listed ranges can skip it.","tier":"C","verified":{},"analyst_revisit_at":null,"first_public_at":"2026-10-16T08:17:13+00:00","kev":null,"cvss_vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N","cvss_by":"responsibledisclosure@mattermost.com","poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-18799","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-18799","severity":null,"published_at":"2026-10-16T08:17:13+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"}