CVE-2025-49222: Mattermost Shared Channel Upload Type Validation Bypass

Severity: medium (CVSS 6.8 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: 2025-08-21 · updated 2026-06-17
Weakness: CWE-434

Description

Mattermost versions 10.8.x <= 10.8.3, 10.5.x <= 10.5.8, 9.11.x <= 9.11.17, 10.9.x <= 10.9.2, 10.10.x <= 10.10.0 fail to validate upload types in remote cluster upload sessions which allows a system admin to upload non-attachment file types via shared channels that could potentially be placed in arbitrary filesystem directories.

Affected products

Mattermost: from 9.11.0 before 9.11.18; from 10.5.0 before 10.5.9; from 10.8.0 before 10.8.4; from 10.9.0 before 10.9.3; 10.10.0 — fixed in 10.5.9, 10.8.4, 10.9.3, 9.11.18

How to fix

Upgrade to a fixed version: Mattermost 10.5.9, 10.8.4, 10.9.3, 9.11.18. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

Get alerted when CVE-2025-49222 or the next vulnerability affects your stack