{"id":"GHSA-fg35-5rf6-qg3g","summary":"Mattermost allows attackers to take over arbitrary user accounts via overly permissive substring matching flaw","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":["CVE-2026-27656","GO-2026-5360"],"modified":"2026-06-25T19:56:26.718374232Z","published":"2026-03-25T18:31:53Z","database_specific":{"github_reviewed_at":"2026-03-31T05:19:17Z","nvd_published_at":"2026-03-25T17:16:56Z","cwe_ids":["CWE-303"],"severity":"MODERATE","github_reviewed":true},"references":[{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-27656"},{"type":"PACKAGE","url":"https://github.com/mattermost/mattermost"},{"type":"WEB","url":"https://mattermost.com/security-updates"}],"affected":[{"package":{"name":"github.com/mattermost/mattermost-server","ecosystem":"Go","purl":"pkg:golang/github.com/mattermost/mattermost-server"},"ranges":[{"type":"SEMVER","events":[{"introduced":"11.4.0-rc1"},{"fixed":"11.4.1"}]}],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/03/GHSA-fg35-5rf6-qg3g/GHSA-fg35-5rf6-qg3g.json"}},{"package":{"name":"github.com/mattermost/mattermost-server","ecosystem":"Go","purl":"pkg:golang/github.com/mattermost/mattermost-server"},"ranges":[{"type":"SEMVER","events":[{"introduced":"11.3.0-rc1"},{"fixed":"11.3.2"}]}],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/03/GHSA-fg35-5rf6-qg3g/GHSA-fg35-5rf6-qg3g.json"}},{"package":{"name":"github.com/mattermost/mattermost-server","ecosystem":"Go","purl":"pkg:golang/github.com/mattermost/mattermost-server"},"ranges":[{"type":"SEMVER","events":[{"introduced":"11.2.0-rc1"},{"fixed":"11.2.4"}]}],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/03/GHSA-fg35-5rf6-qg3g/GHSA-fg35-5rf6-qg3g.json"}},{"package":{"name":"github.com/mattermost/mattermost-server","ecosystem":"Go","purl":"pkg:golang/github.com/mattermost/mattermost-server"},"ranges":[{"type":"SEMVER","events":[{"introduced":"10.11.0-rc1"},{"fixed":"10.11.12"}]}],"database_specific":{"last_known_affected_version_range":"\u003c 10.11.2","source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/03/GHSA-fg35-5rf6-qg3g/GHSA-fg35-5rf6-qg3g.json"}},{"package":{"name":"github.com/mattermost/mattermost/server/v8","ecosystem":"Go","purl":"pkg:golang/github.com/mattermost/mattermost/server/v8"},"ranges":[{"type":"SEMVER","events":[{"introduced":"8.0.0-20260105080200-d27a2195068d"},{"fixed":"8.0.0-20260217110922-b7d4a1f1f59b"}]}],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/03/GHSA-fg35-5rf6-qg3g/GHSA-fg35-5rf6-qg3g.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:H/I:H/A:N"}]}