PT-2026-27989 · Mattermost · Mattermost

Christopher Poile · Published 2026-03-25 · Updated 2026-03-25 · CVE-2026-27656

Positive Technologies (AI Summary)

CVSS v3.1

6.1

Medium

Base vector

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

Name of the Vulnerable Software and Affected Versions Mattermost versions 11.4.0 and earlier, 11.3.1 and earlier, 11.2.3 and earlier, 10.11.11 and earlier

Description The software does not properly validate user identity within the OpenID IsSameUser() comparison logic. This flaw, stemming from overly permissive substring matching in the user discovery flow, could allow an attacker to compromise user accounts.

Recommendations Update Mattermost to a version later than 11.4.0. Update Mattermost to a version later than 11.3.1. Update Mattermost to a version later than 11.2.3. Update Mattermost to a version later than 10.11.11.

Fix

Weakness Enumeration

CWE-303

Related Identifiers

CVE-2026-27656

GHSA-FG35-5RF6-QG3G

Affected Products

Mattermost

References · 6

• https://mattermost.com/security-updates · Vendor Advisory
• https://osv.dev/vulnerability/GHSA-fg35-5rf6-qg3g · Vendor Advisory
• https://nvd.nist.gov/vuln/detail/CVE-2026-27656 · Security Note
• https://github.com/mattermost/mattermost · Note
• https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2026-27656 · Note
• https://twitter.com/CVEnew/status/2036911848105402670 · Twitter Post