{"id":"CVE-2026-16453","kind":"vulnerability","aliases":["CVE-2026-16453"],"related":[],"title":"Mattermost: forged SAML RelayState lets users join invite-only teams","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 verify the integrity of the SAML RelayState POST parameter which allows an authenticated SAML user to join any team, including invite-only teams, via forging the team_id or invite_id fields in the RelayState.. Mattermost Advisory ID: MMSA-2026-00727","summary":"Mattermost fails to verify the integrity of the SAML RelayState POST parameter. An authenticated SAML user can forge the team_id or invite_id fields to join any team, including invite-only teams.","severity":"high","cvss":7.1,"epss":null,"exploited":false,"exploited_at":null,"products":[{"name":"mattermost","fixed":["11.7.10","11.8.6","11.9.2","11.10.1","11.11.0"],"affected":">= 11.7.0, <= 11.7.9; >= 11.8.0, <= 11.8.5; >= 11.9.0, <= 11.9.1; 11.10.0","ecosystem":"vendor"}],"ecosystems":[],"tags":["mattermost","package","saml","self-hosted","team-access"],"not_affected_if":["no-saml","patched-version","saml-disabled"],"how_to_check":"Check Mattermost version (System Console > About, or 'mmctl version' / server --version). Affected if running 11.7.0-11.7.9, 11.8.0-11.8.5, 11.9.0-11.9.1 or 11.10.0 AND SAML login is enabled (System Console > Authentication > SAML 2.0). Check team membership audit for unexpected joins of invite-only teams by SAML users.","fix":"Upgrade to 11.11.0, 11.10.1, 11.9.2, 11.8.6 or 11.7.10 (or later on your release line). Fix signs and verifies SAML RelayState with HMAC-SHA256.","status":"open","published_at":"2026-10-16T08:17:12+00:00","first_seen_at":"2026-10-23T21:57:12+00:00","triaged_at":"2026-10-25T03:36:44+00:00","updated_at":"2026-10-25T03:48:55+00:00","category":"package","confidence":0.8,"triage":{"at":"2026-10-25T03:36:44Z","model":"claude-haiku-5-5","output":{"fix":"unknown","reach":"internet","title":"Mattermost: forged SAML RelayState lets users join invite-only teams","impact":"medium","summary":"Mattermost fails to verify the integrity of the SAML RelayState POST parameter. An authenticated SAML user can forge the team_id or invite_id fields to join any team, including invite-only teams.","category":"package","nicknames":[],"who_cares":["mattermost","self-hosted","saml"],"confidence":0.6,"needs_auth":"yes","worst_case":"Authenticated user joins invite-only teams they were never invited to.","analyst_note":"Sources give affected versions only; fixed versions and the MMSA-2026-00727 advisory text should be checked for the patched release numbers.","how_to_check":"unknown","needs_analyst":true,"not_affected_if":["no-saml"],"who_is_affected":"Self-hosted or managed Mattermost deployments on the listed affected versions that use SAML single sign-on and have invite-only teams. Installations without SAML login are not described as affected by the sources.","user_interaction":"no","needs_special_setup":"yes","severity_if_unknown":"high"},"cost_usd":0.000542,"input_tokens":3412,"output_tokens":401,"severity_before":"high"},"analyst_status":"done","analyst_note":"Sources give affected versions only; fixed versions and the MMSA-2026-00727 advisory text should be checked for the patched release numbers.","search":"'-00727':120C '-16453':14A,17A '-2026':13A,16A,119C '..':114C '11.10':64C '11.10.0':66C '11.7':67C '11.7.9':69C '11.8':73C '11.8.5':75C '11.9':70C '11.9.1':72C 'access':27B 'advisori':116C 'allow':88C 'authent':41B,90C 'cve':12A,15A 'fail':29B,76C 'field':52B,110C 'forg':2A,45B,103C 'host':24B 'id':48B,51B,106C,109C,117C 'includ':57B,97C 'integr':33B,80C 'invit':9A,50B,59B,99C,108C 'invite-on':8A,58B,98C 'join':7A,54B,94C 'let':5A 'mattermost':1A,18A,19B,28B,62C,115C 'mmsa':118C 'packag':20B 'paramet':39B,86C 'post':38B,85C 'relayst':4A,37B,84C,113C 'saml':3A,21B,36B,42B,83C,91C 'self':23B 'self-host':22B 'team':11A,26B,47B,56B,61B,96C,101C,105C 'team-access':25B 'user':6A,43B,92C 'verifi':31B,78C 'version':63C 'via':102C 'x':65C,68C,71C,74C","nicknames":[],"mitigations":[],"who_affected":"Self-hosted or managed Mattermost deployments on the listed affected versions that use SAML single sign-on and have invite-only teams. Installations without SAML login are not described as affected by the sources.","tier":"B","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:L/PR:L/UI:N/S:U/C:H/I:L/A:N","cvss_by":"responsibledisclosure@mattermost.com","poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-16453","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-16453","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},{"source":"analyst","external_id":"CVE-2026-16453:797","url":null,"severity":null,"published_at":"2026-10-25T03:48:55+00:00","modified_at":"2026-10-25T03:48:55+00:00","first_seen_at":"2026-10-25T03:48:55+00:00","fetched_at":"2026-10-25T03:48:55+00:00","note":"Fixed versions from Mattermost CNA CVE record (mattermost.com security-updates page is JS-rendered, not readable); fix PR #37837 adds HMAC-SHA256 RelayState signing. No exploitation or PoC found; no social search done (low-profile B). Not marked verified: vendor advisory page itself not read."}],"documents":[{"id":3354,"url":"https://www.cve.org/CVERecord?id=CVE-2026-16453","title":"CVE-2026-16453 (CNA: Mattermost, MMSA-2026-00727)","kind":"advisory","source":"analyst","text":null,"published_at":null,"relation":"advisory","confidence":null,"note":null},{"id":3355,"url":"https://github.com/mattermost/mattermost/pull/37837","title":"MM-69889 Improve handling of RelayState in SAML flow (HMAC-SHA256 signing)","kind":"other","source":"analyst","text":null,"published_at":null,"relation":"fix","confidence":null,"note":null}],"notice":"Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai"}