CVE-2026-27656

Account Takeover via Substring Matching in OpenID Connect Authentication

State: Published
Severity: 5.7 Medium
Published: Mar 25, 2026
Updated: Mar 26, 2026
Reserved: Feb 23, 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

Scores

CVSS	Score	Vector	Source
3.1	5.7 Medium	CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:H/I:H/A:N	Mattermost

Weaknesses

CWE-303 Incorrect Implementation of Authentication Algorithm

Affected Products

Product	Versions
MattermostMattermost	11.4.0 through 11.4.0Affected 11.3.0 through 11.3.1Affected 11.2.0 through 11.2.3Affected 10.11.0 through 10.11.11Affected 11.5.0Not affected 11.4.1Not affected 11.3.2Not affected 11.2.4Not affected 10.11.12Not affected

References

MMSA-2026-00590vendor-advisory

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