CVE-2026-10103: Authenticated remote cluster can modify or delete posts it does not own in Mattermost Connected Workspaces shared channels

Severity: medium (CVSS 4.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-07-13 · updated 2026-07-14
Weakness: CWE-639

Description

Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 fail to verify post ownership in the shared channel inbound sync handler, which allows an authenticated remote cluster to modify or delete posts authored by local users or other remotes via crafted sync messages referencing arbitrary post IDs in channels shared with that remote.. Mattermost Advisory ID: MMSA-2026-00689

Affected products

Mattermost: from 10.11.0 before 10.11.20; from 11.6.0 before 11.6.5; from 11.7.0 before 11.7.3 — fixed in 10.11.20, 11.6.5, 11.7.3

How to fix

Upgrade to a fixed version: Mattermost 10.11.20, 11.6.5, 11.7.3. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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