Moderate severityNVD Advisory· Published Mar 25, 2026· Updated Mar 26, 2026
Account Takeover via Substring Matching in OpenID Connect Authentication
CVE-2026-27656
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
Affected packages
Versions sourced from the GitHub Security Advisory.
| Package | Affected versions | Patched versions |
|---|---|---|
github.com/mattermost/mattermost-serverGo | >= 11.4.0-rc1, < 11.4.1 | 11.4.1 |
github.com/mattermost/mattermost-serverGo | >= 11.3.0-rc1, < 11.3.2 | 11.3.2 |
github.com/mattermost/mattermost-serverGo | >= 11.2.0-rc1, < 11.2.4 | 11.2.4 |
github.com/mattermost/mattermost-serverGo | >= 10.11.0-rc1, < 10.11.12 | 10.11.12 |
github.com/mattermost/mattermost/server/v8Go | >= 8.0.0-20260105080200-d27a2195068d, < 8.0.0-20260217110922-b7d4a1f1f59b | 8.0.0-20260217110922-b7d4a1f1f59b |
Affected products
1- Range: 11.4.0
Patches
0No patches discovered yet.
Vulnerability mechanics
AI mechanics synthesis has not run for this CVE yet.
References
3- github.com/advisories/GHSA-fg35-5rf6-qg3gghsaADVISORY
- mattermost.com/security-updatesghsavendor-advisoryWEB
- nvd.nist.gov/vuln/detail/CVE-2026-27656ghsaADVISORY
News mentions
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