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Description

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to validate that a remote cluster has access to a channel before processing membership removal requests during shared channel membership sync, which allows a malicious remote cluster to remove any user from any channel, including private channels, via crafted membership sync messages targeting channels the remote cluster is not authorized to access. Mattermost Advisory ID: MMSA-2026-00576

PUBLISHED Reserved 2026-03-10 | Published 2026-05-18 | Updated 2026-05-18 | Assigner Mattermost




MEDIUM: 4.3CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N

Problem types

CWE-863: Incorrect Authorization

Product status

Default status
unaffected

11.5.0 (semver)
affected

10.11.0 (semver)
affected

11.4.0 (semver)
affected

11.6.0
unaffected

11.5.2
unaffected

10.11.14
unaffected

11.4.4
unaffected

Credits

daw10 finder

References

mattermost.com/security-updates (MMSA-2026-00576) vendor-advisory

cve.org (CVE-2026-28759)

nvd.nist.gov (CVE-2026-28759)

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