CVE-2026-27656

Name: CVE-2026-27656
Creator: NVD / NIST
Published: 2026-03-25T17:16:56.797+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

MEDIUMCVSS 6.1/10EPSS 0.18%

Last modified Jun 17, 2026

CVE-2026-27656 is a medium-severity vulnerability rated 6.1/10 on the CVSS scale. 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. EPSS estimates a 0.18% chance of exploitation in the next 30 days.

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

Metrics

CVSS 3.1

6.1/10

CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:U/C:H/I:H/A:N

EPSS Probability

0.18%

7.7th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-303

Affected Software

Vendor	Product	Versions
Mattermost	Mattermost Server	>= 10.11.0, < 10.11.12
Mattermost	Mattermost Server	>= 11.2.0, < 11.2.4
Mattermost	Mattermost Server	>= 11.3.0, < 11.3.2
Mattermost	Mattermost Server	>= 11.4.0, < 11.4.1

References

https://mattermost.com/security-updatesVendor Advisory

Timeline

Published: Mar 25, 2026
Last Modified: Jun 17, 2026
Status: Analyzed

Frequently Asked Questions

What is CVE-2026-27656?

How severe is CVE-2026-27656?

CVE-2026-27656 has a CVSS score of 6.1/10 (MEDIUM severity). The EPSS model estimates a 0.18% probability of exploitation in the next 30 days.

How do I fix CVE-2026-27656?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2026-27656?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST