CVE-2024-39777: Malicious remote can invite itself to an arbitrary local channel

Severity: critical (CVSS 9.6 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.4% 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 disallow unsolicited invites to expose access to local channels, when shared channels are enabled, which allows a malicious remote to send an invite with the ID of an existing local channel, and that local channel will then become shared without the consent of the local admin.

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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