Mattermost 11.4.0, 11.3.1, 11.2.3, 10.11.11 OpenID IsSameUser SubstrMatch PrivEsc
CVE-2026-27656 Published on March 25, 2026
Account Takeover via Substring Matching in OpenID Connect Authentication
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
Vulnerability Analysis
CVE-2026-27656 is exploitable with network access, requires user interaction and user privileges. This vulnerability is consided to have a high level of attack complexity. The potential impact of an exploit of this vulnerability is considered to have a high impact on confidentiality and integrity, and no impact on availability.
Weakness Type
Incorrect Implementation of Authentication Algorithm
The requirements for the software dictate the use of an established authentication algorithm, but the implementation of the algorithm is incorrect. This incorrect implementation may allow authentication to be bypassed.
Products Associated with CVE-2026-27656
Want to know whenever a new CVE is published for MatterMost? stack.watch will email you.
Affected Versions
Mattermost:- Version 11.4.0, <= 11.4.0 is affected.
- Version 11.3.0, <= 11.3.1 is affected.
- Version 11.2.0, <= 11.2.3 is affected.
- Version 10.11.0, <= 10.11.11 is affected.
- Version 11.5.0 is unaffected.
- Version 11.4.1 is unaffected.
- Version 11.3.2 is unaffected.
- Version 11.2.4 is unaffected.
- Version 10.11.12 is unaffected.
Exploit Probability
EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.