CVE-2026-15754: Missing per-channel team-scope check in ABAC access control policy unassign allows cross-team policy removal

Severity: medium (CVSS 4.2 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: 2026-08-17 · updated 2026-08-18
Weakness: CWE-863

Description

Mattermost versions 11.7.x <= 11.7.6, 11.8.x <= 11.8.3 The access control policy unassign endpoint fails to re-validate that each target channel still belongs to the requesting admin's team, which allows an authenticated team administrator to remove ABAC (attribute-based access control) policy assignments from channels outside their team via the policy unassign API after a channel has been moved to another team.. Mattermost Advisory ID: MMSA-2026-00718

Affected products

Mattermost: from 11.7.0 before 11.7.7; from 11.8.0 before 11.8.4 — fixed in 11.7.7, 11.8.4

How to fix

Upgrade to a fixed version: Mattermost 11.7.7, 11.8.4. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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