Insufficient authorization in shared channel membership sync grants team-level access instead of channel-level access

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

Problem type

CWE-863: Incorrect Authorization

Affected products

Mattermost

Mattermost

<= 11.2.2 - AFFECTED

<= 10.11.10 - AFFECTED

<= 11.4.0 - AFFECTED

<= 11.3.1 - AFFECTED

11.5.0 - UNAFFECTED

11.2.3 - UNAFFECTED

10.11.11 - UNAFFECTED

11.4.1 - UNAFFECTED

11.3.2 - UNAFFECTED

References

MMSA-2026-00574

https://mattermost.com/security-updates

vendor-advisory

GitHub Security Advisories

GHSA-g7fp-cqj5-x8hf

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...

https://github.com/advisories/GHSA-g7fp-cqj5-x8hf

nvd.nist.gov

https://nvd.nist.gov/vuln/detail/CVE-2026-4274

mattermost.com

https://mattermost.com/security-updates

github.com

https://github.com/advisories/GHSA-g7fp-cqj5-x8hf

JSON source

https://cveawg.mitre.org/api/cve/CVE-2026-4274

Click to expand

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