CVE-2026-4274
Received Received - Intake
Improper Access Control in Mattermost Team Membership Sync

Publication date: 2026-03-26

Last updated on: 2026-03-26

Assigner: Mattermost, Inc.

Description
Mattermost versions 11.2.x <= 11.2.2, 10.11.x <= 10.11.10, 11.4.x <= 11.4.0, 11.3.x <= 11.3.1 fail to restrict team-level access when processing membership sync from a remote cluster, which allows a malicious remote cluster to grant a user access to an entire private team instead of only the shared channel via sending crafted membership sync messages that trigger team membership assignment. Mattermost Advisory ID: MMSA-2026-00574
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Meta Information
Published
2026-03-26
Last Modified
2026-03-26
Generated
2026-06-16
AI Q&A
2026-03-26
EPSS Evaluated
2026-06-15
NVD
CVE-2026-4274
EUVD
EUVD-2026-16162
Affected Vendors & Products
Showing 4 associated CPEs
Vendor Product Version / Range
mattermost mattermost_server From 10.11.0 (inc) to 10.11.11 (exc)
mattermost mattermost_server From 11.2.0 (inc) to 11.2.3 (exc)
mattermost mattermost_server From 11.3.0 (inc) to 11.3.2 (exc)
mattermost mattermost_server From 11.4.0 (inc) to 11.4.1 (exc)
Helpful Resources
Exploitability
CWE
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KEV
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CWE ID Description
CWE-863 The product performs an authorization check when an actor attempts to access a resource or perform an action, but it does not correctly perform the check.
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Executive Summary

This vulnerability affects certain versions of Mattermost where the system fails to properly restrict team-level access during membership synchronization from a remote cluster.

Specifically, a malicious remote cluster can exploit this flaw by sending crafted membership sync messages that incorrectly assign a user access to an entire private team instead of limiting access to only the shared channel.

Impact Analysis

This vulnerability can lead to unauthorized access where a user gains access to an entire private team without proper permission.

Such unauthorized access could expose sensitive information shared within private teams, potentially leading to data leakage or privacy breaches.

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