{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-27656","assignerOrgId":"9302f53e-dde5-4bf3-b2f2-a83f91ac0eee","state":"PUBLISHED","assignerShortName":"Mattermost","dateReserved":"2026-02-23T22:07:32.808Z","datePublished":"2026-03-25T16:28:29.739Z","dateUpdated":"2026-03-26T13:19:52.338Z"},"containers":{"cna":{"affected":[{"defaultStatus":"unaffected","product":"Mattermost","vendor":"Mattermost","versions":[{"lessThanOrEqual":"11.4.0","status":"affected","version":"11.4.0","versionType":"semver"},{"lessThanOrEqual":"11.3.1","status":"affected","version":"11.3.0","versionType":"semver"},{"lessThanOrEqual":"11.2.3","status":"affected","version":"11.2.0","versionType":"semver"},{"lessThanOrEqual":"10.11.11","status":"affected","version":"10.11.0","versionType":"semver"},{"version":"11.5.0","status":"unaffected"},{"version":"11.4.1","status":"unaffected"},{"version":"11.3.2","status":"unaffected"},{"version":"11.2.4","status":"unaffected"},{"version":"10.11.12","status":"unaffected"}]}],"credits":[{"lang":"en","type":"finder","value":"Christopher Poile"}],"descriptions":[{"lang":"en","value":"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"}],"metrics":[{"cvssV3_1":{"attackVector":"NETWORK","attackComplexity":"HIGH","privilegesRequired":"HIGH","userInteraction":"REQUIRED","scope":"UNCHANGED","confidentialityImpact":"HIGH","integrityImpact":"HIGH","availabilityImpact":"NONE","version":"3.1","vectorString":"CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:H/I:H/A:N","baseSeverity":"MEDIUM","baseScore":5.7},"format":"CVSS","scenarios":[{"lang":"en","value":"GENERAL"}]}],"problemTypes":[{"descriptions":[{"lang":"en","type":"CWE","description":"CWE-303: Incorrect Implementation of Authentication Algorithm","cweId":"CWE-303"}]}],"references":[{"url":"https://mattermost.com/security-updates","name":"MMSA-2026-00590","tags":["vendor-advisory"]}],"solutions":[{"value":"Update Mattermost to versions 11.5.0, 11.4.1, 11.3.2, 11.2.4, 10.11.12 or higher.","lang":"en"}],"source":{"advisory":"MMSA-2026-00590","defect":["https://mattermost.atlassian.net/browse/MM-67181"],"discovery":"{\"self\"=>\"https://mattermost.atlassian.net/rest/api/2/customFieldOption/10557\", \"value\"=>\"Internal\", \"id\"=>\"10557\"}"},"title":"Account Takeover via Substring Matching in OpenID Connect Authentication","providerMetadata":{"orgId":"9302f53e-dde5-4bf3-b2f2-a83f91ac0eee","shortName":"Mattermost","dateUpdated":"2026-03-25T16:28:29.739Z"},"x_generator":{"engine":"cvelib 1.8.0"}},"adp":[{"metrics":[{"other":{"type":"ssvc","content":{"timestamp":"2026-03-26T03:55:33.452102Z","id":"CVE-2026-27656","options":[{"Exploitation":"none"},{"Automatable":"no"},{"Technical Impact":"total"}],"role":"CISA Coordinator","version":"2.0.3"}}}],"title":"CISA ADP Vulnrichment","providerMetadata":{"orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","shortName":"CISA-ADP","dateUpdated":"2026-03-26T13:19:52.338Z"}}]}}