{"id":"CVE-2026-27656","state":"PUBLISHED","title":"Account Takeover via Substring Matching in OpenID Connect Authentication","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","publishedAt":"2026-03-25T16:28:29.739Z","updatedAt":"2026-03-26T13:19:52.338Z","severity":{"score":5.7,"label":"Medium","version":"3.1","vector":"CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:H/I:H/A:N","source":"Mattermost"},"assigner":"Mattermost","products":["Mattermost Mattermost"],"affected":[{"vendor":"Mattermost","product":"Mattermost","defaultStatus":"unaffected","versions":[{"version":"11.4.0","status":"affected","lessThanOrEqual":"11.4.0","versionType":"semver"},{"version":"11.3.0","status":"affected","lessThanOrEqual":"11.3.1","versionType":"semver"},{"version":"11.2.0","status":"affected","lessThanOrEqual":"11.2.3","versionType":"semver"},{"version":"10.11.0","status":"affected","lessThanOrEqual":"10.11.11","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"}]}],"references":["https://mattermost.com/security-updates"],"weaknesses":["CWE-303"],"ingestedAt":"2026-10-29T20:11:31.369Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/27xxx/CVE-2026-27656.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-27656","state":"PUBLISHED","datePublished":"2026-03-25T16:28:29.739Z","dateUpdated":"2026-03-26T13:19:52.338Z","assignerShortName":"Mattermost","assignerOrgId":"9302f53e-dde5-4bf3-b2f2-a83f91ac0eee","dateReserved":"2026-02-23T22:07:32.808Z"},"containers":{"cna":{"title":"Account Takeover via Substring Matching in OpenID Connect Authentication","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"}],"affected":[{"vendor":"Mattermost","product":"Mattermost","defaultStatus":"unaffected","versions":[{"version":"11.4.0","status":"affected","lessThanOrEqual":"11.4.0","versionType":"semver"},{"version":"11.3.0","status":"affected","lessThanOrEqual":"11.3.1","versionType":"semver"},{"version":"11.2.0","status":"affected","lessThanOrEqual":"11.2.3","versionType":"semver"},{"version":"10.11.0","status":"affected","lessThanOrEqual":"10.11.11","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"}]}],"references":[{"url":"https://mattermost.com/security-updates","name":"MMSA-2026-00590","tags":["vendor-advisory"]}],"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":[{"cweId":"CWE-303","lang":"en","type":"CWE","description":"CWE-303: Incorrect Implementation of Authentication Algorithm"}]}],"providerMetadata":{"shortName":"Mattermost","orgId":"9302f53e-dde5-4bf3-b2f2-a83f91ac0eee","dateUpdated":"2026-03-25T16:28:29.739Z"},"credits":[{"lang":"en","type":"finder","value":"Christopher Poile"}],"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\"}"},"x_generator":{"engine":"cvelib 1.8.0"}},"adp":[{"title":"CISA ADP Vulnrichment","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"}}}],"providerMetadata":{"shortName":"CISA-ADP","orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","dateUpdated":"2026-03-26T13:19:52.338Z"}}]}}}