Mattermost 10.11.x <= 10.11.9 Channel Membership Leak via /common_teams
CVE-2026-20796 Published on February 13, 2026
Time-of-check time-of-use vulnerability in common teams API
Mattermost versions 10.11.x <= 10.11.9 fail to properly validate channel membership at the time of data retrieval which allows a deactivated user to learn team names they should not have access to via a race condition in the /common_teams API endpoint.. Mattermost Advisory ID: MMSA-2025-00549
Vulnerability Analysis
CVE-2026-20796 is exploitable with network access, and requires small amount of 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 small impact on confidentiality, a small impact on integrity and availability.
Weakness Type
What is a TOCTTOU Vulnerability?
The software checks the state of a resource before using that resource, but the resource's state can change between the check and the use in a way that invalidates the results of the check. This can cause the software to perform invalid actions when the resource is in an unexpected state. This weakness can be security-relevant when an attacker can influence the state of the resource between check and use. This can happen with shared resources such as files, memory, or even variables in multithreaded programs.
CVE-2026-20796 has been classified to as a TOCTTOU vulnerability or weakness.
Products Associated with CVE-2026-20796
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Affected Versions
Mattermost:- Version 10.11.0, <= 10.11.9 is affected.
- Version 11.3.0 is unaffected.
- Version 10.11.10 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.