# CVE-2026-27656

## Summary

- **CVE ID:** CVE-2026-27656
- **Severity:** MEDIUM
- **CVSS Score:** 5.7 (CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:H/I:H/A:N)
- **CWE:** CWE-303
- **Published:** Mar 25, 2026
- **Last Modified:** Mar 26, 2026

## 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 Products

- Mattermost — Mattermost (11.4.0)
- Mattermost — Mattermost (11.3.0)
- Mattermost — Mattermost (11.2.0)
- Mattermost — Mattermost (10.11.0)
- Mattermost — Mattermost (11.5.0)
- Mattermost — Mattermost (11.4.1)
- Mattermost — Mattermost (11.3.2)
- Mattermost — Mattermost (11.2.4)
- Mattermost — Mattermost (10.11.12)

## References

- [CNA](https://mattermost.com/security-updates)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.18%
- **EPSS Percentile:** 7.6

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-11._