{"cve":"CVE-2026-27656","nvd":{"published":"2026-03-25T17:16:56.797","modified":"2026-06-17T10:27:28.430","status":"Analyzed","description":"Mattermost versions 11.4.x <= 11.4.0, 11.3.x <= 11.3.1, 11.2.x <= 11.2.3, 10.11.x <= 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","cvss_v4":null,"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:U/C:H/I:H/A:N","score":6.1},"cwes":["CWE-303"],"references":["https://mattermost.com/security-updates"],"products":["mattermost:mattermost_server"],"source":"nvd","synced_at":"2026-10-19T22:28:18.033120106+00:00"},"epss":{"cve":"CVE-2026-27656","score":0.00183,"percentile":0.07893,"date":"2026-10-31"},"kev":null,"osv":[{"id":"GHSA-fg35-5rf6-qg3g","url":"https://osv.dev/vulnerability/GHSA-fg35-5rf6-qg3g","summary":"Mattermost allows attackers to take over arbitrary user accounts via overly permissive substring matching flaw","modified":"2026-06-25T19:56:08.822530656Z","packages":[{"ecosystem":"Go","name":"github.com/mattermost/mattermost-server","purl":"pkg:golang/github.com/mattermost/mattermost-server","ranges":[">=11.4.0-rc1 <11.4.1"],"fixed":["11.4.1"]},{"ecosystem":"Go","name":"github.com/mattermost/mattermost-server","purl":"pkg:golang/github.com/mattermost/mattermost-server","ranges":[">=11.3.0-rc1 <11.3.2"],"fixed":["11.3.2"]},{"ecosystem":"Go","name":"github.com/mattermost/mattermost-server","purl":"pkg:golang/github.com/mattermost/mattermost-server","ranges":[">=11.2.0-rc1 <11.2.4"],"fixed":["11.2.4"]},{"ecosystem":"Go","name":"github.com/mattermost/mattermost-server","purl":"pkg:golang/github.com/mattermost/mattermost-server","ranges":[">=10.11.0-rc1 <10.11.12"],"fixed":["10.11.12"]},{"ecosystem":"Go","name":"github.com/mattermost/mattermost/server/v8","purl":"pkg:golang/github.com/mattermost/mattermost/server/v8","ranges":[">=8.0.0-20260105080200-d27a2195068d <8.0.0-20260217110922-b7d4a1f1f59b"],"fixed":["8.0.0-20260217110922-b7d4a1f1f59b"]}]},{"id":"GO-2026-5360","url":"https://osv.dev/vulnerability/GO-2026-5360","summary":"Mattermost allows attackers to take over arbitrary user accounts via overly permissive substring matching flaw in github.com/mattermost/mattermost-server","modified":"2026-06-25T19:45:34.725735565Z","packages":[{"ecosystem":"Go","name":"github.com/mattermost/mattermost-server","purl":"pkg:golang/github.com/mattermost/mattermost-server","ranges":[">=10.11.0-rc1+incompatible <10.11.12+incompatible",">=11.2.0-rc1+incompatible <11.2.4+incompatible",">=11.3.0-rc1+incompatible <11.3.2+incompatible",">=11.4.0-rc1+incompatible <11.4.1+incompatible"],"fixed":["10.11.12+incompatible","11.2.4+incompatible","11.3.2+incompatible","11.4.1+incompatible"]},{"ecosystem":"Go","name":"github.com/mattermost/mattermost-server/v5","purl":"pkg:golang/github.com/mattermost/mattermost-server/v5","ranges":["all versions"],"fixed":[]},{"ecosystem":"Go","name":"github.com/mattermost/mattermost-server/v6","purl":"pkg:golang/github.com/mattermost/mattermost-server/v6","ranges":["all versions"],"fixed":[]},{"ecosystem":"Go","name":"github.com/mattermost/mattermost/server/v8","purl":"pkg:golang/github.com/mattermost/mattermost/server/v8","ranges":[">=8.0.0-20260105080200-d27a2195068d <8.0.0-20260217110922-b7d4a1f1f59b"],"fixed":["8.0.0-20260217110922-b7d4a1f1f59b"]}]}],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}