CVE-2024-39839: Remote username set to an arbitrary string by remote user

Severity: medium (CVSS 4.3 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, 9.8.x <= 9.8.1 fail to disallow users to set their own remote username, when shared channels were enabled, which allows a user on a remote to set their remote username prop to an arbitrary string, which would be then synced to the local server as long as the user hadn't been synced before.

Affected products

Mattermost: 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 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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