{"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": "2026-03-25T16:28:29.739Z"}, "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"}}}}

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{"cveTags": [], "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"}], "id": "CVE-2026-27656", "lastModified": "2026-03-25T17:16:56.797", "metrics": {"cvssMetricV31": [{"cvssData": {"attackComplexity": "HIGH", "attackVector": "NETWORK", "availabilityImpact": "NONE", "baseScore": 5.7, "baseSeverity": "MEDIUM", "confidentialityImpact": "HIGH", "integrityImpact": "HIGH", "privilegesRequired": "HIGH", "scope": "UNCHANGED", "userInteraction": "REQUIRED", "vectorString": "CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:H/I:H/A:N", "version": "3.1"}, "exploitabilityScore": 0.5, "impactScore": 5.2, "source": "responsibledisclosure@mattermost.com", "type": "Secondary"}]}, "published": "2026-03-25T17:16:56.797", "references": [{"source": "responsibledisclosure@mattermost.com", "url": "https://mattermost.com/security-updates"}], "sourceIdentifier": "responsibledisclosure@mattermost.com", "vulnStatus": "Received", "weaknesses": [{"description": [{"lang": "en", "value": "CWE-303"}], "source": "responsibledisclosure@mattermost.com", "type": "Secondary"}]}

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