CVE-2024-39274: Malicious remote can add users to arbitrary teams and channels

Severity: medium (CVSS 6.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2024-08-01 · updated 2026-06-17
Weakness: CWE-284

Description

Mattermost versions 9.9.x <= 9.9.0, 9.5.x <= 9.5.6, 9.7.x <= 9.7.5 and 9.8.x <= 9.8.1 fail to properly validate that the channel that comes from the sync message is a shared channel, when shared channels are enabled, which allows a malicious remote to add users to arbitrary teams and channels

Affected products

Mattermost App Framework: from 9.5.0 before 9.5.7; from 9.7.0 before 9.7.6; from 9.8.0 before 9.8.2; 9.9.0 — fixed in 9.5.7, 9.7.6, 9.8.2

How to fix

Upgrade to a fixed version: Mattermost App Framework 9.5.7, 9.7.6, 9.8.2. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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