CVE-2026-27656 — Account Takeover via Substring Matching in OpenID Connect Authentication

CVE-2026-27656

Account Takeover via Substring Matching in OpenID Connect Authentication PUBLISHED

Description (CNA)

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 (CNA)

Vendor / product	Versions
Mattermost / Mattermost	affected 11.4.0 through 11.4.0; affected 11.3.0 through 11.3.1; affected 11.2.0 through 11.2.3; affected 10.11.0 through 10.11.11; unaffected 11.5.0; unaffected 11.4.1; unaffected 11.3.2; unaffected 11.2.4; unaffected 10.11.12

CWE

CWE-303

References

https://mattermost.com/security-updates

ADP enrichment

CISA-ADP

No English description supplied.

No references supplied.