{"id":"CVE-2026-27656","summary":"Account Takeover via Substring Matching in OpenID Connect Authentication","details":"Mattermost versions 11.4.x \u003c= 11.4.0, 11.3.x \u003c= 11.3.1, 11.2.x \u003c= 11.2.3, 10.11.x \u003c= 10.11.11 fail to properly validate user identity in the OpenID {{IsSameUser()}} comparison logic, which allows an attacker to take over arbitrary user accounts via an overly permissive substring matching flaw in the user discovery flow.. Mattermost Advisory ID: MMSA-2026-00590","aliases":["GHSA-fg35-5rf6-qg3g","GO-2026-5360"],"modified":"2026-07-31T18:29:59.876398786Z","published":"2026-03-25T16:28:29.739Z","related":["openSUSE-SU-2026:21483-1"],"database_specific":{"cna_assigner":"Mattermost","cwe_ids":["CWE-303"],"osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/27xxx/CVE-2026-27656.json","unresolved_ranges":[{"extracted_events":[{"introduced":"11.4.0"},{"last_affected":"11.4.0"},{"introduced":"11.3.0"},{"last_affected":"11.3.1"},{"introduced":"11.2.0"},{"last_affected":"11.2.3"},{"introduced":"10.11.0"},{"last_affected":"10.11.11"}],"source":"AFFECTED_FIELD"}]},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/27xxx/CVE-2026-27656.json"},{"type":"ADVISORY","url":"https://mattermost.com/security-updates"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-27656"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://github.com/mattermost/mattermost","events":[{"introduced":"5b955468ea1ea8b56c0fb10feb258a36a767d7bb"},{"fixed":"e1a78d1e9111eb77008014c8fec345708c74e756"},{"introduced":"ab7b138824c61fe1fe815243d7e2a914dec49c8b"},{"fixed":"eda9f2a1fe66177a6dd6e96d8294e50f9bf4228d"},{"introduced":"d27a2195068ddfa3ce17cf470f9739e4edb94b83"},{"fixed":"b7d4a1f1f59b645a7598181d33cfc537ea9a6c9c"},{"introduced":"2a1ac98bd3fae587927adf9340ba266c8c02018e"},{"fixed":"84f62bb0c14a45a200af7f4dbe7186915865495f"}],"database_specific":{"source":"CPE_RANGE","cpe":"cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*","extracted_events":[{"introduced":"10.11.0"},{"fixed":"10.11.12"},{"introduced":"11.2.0"},{"fixed":"11.2.4"},{"introduced":"11.3.0"},{"fixed":"11.3.2"},{"introduced":"11.4.0"},{"fixed":"11.4.1"}]}}],"versions":["v11.3.1","v11.2.3","v10.11.11-rc2","v10.11.11","v11.4.0-rc3","v11.4.0","mattermost-redux@11.4.0","@mattermost/types@11.4.0","@mattermost/client@11.4.0","v10.11.11-rc1","v11.3.1-rc1","v10.11.10","v11.2.2","v11.3.0-rc4","v11.3.0-rc3","v11.3.0","mattermost-redux@11.3.0","@mattermost/types@11.3.0","@mattermost/client@11.3.0","v10.11.9","v11.2.1","v11.2.1-rc1","v11.2.0","v10.11.9-rc1","v10.11.8","v10.11.7","v10.11.6","v10.11.5","v10.11.4-rc3","v10.11.4","v10.11.4-rc2","v10.11.4-rc1","v10.11.3","v10.11.2-rc2","v10.11.2","v10.11.2-rc1","v10.11.1","v10.11.1-rc1","v10.11.0-rc3","v10.11.0","mattermost-redux@10.11.0","@mattermost/types@10.11.0","@mattermost/client@10.11.0"],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-27656.json"}}],"schema_version":"1.7.5","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:H/I:H/A:N"}]}