CVE-2026-3471: Opening a window with {{javascript:alert()}} as URL causes crash in the Mattermost Desktop App

Severity: medium (CVSS 6.5 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: 2026-05-18 · updated 2026-06-17
Weakness: CWE-939

Description

Mattermost Desktop App versions <=6.1 6.0.1 5.4.13.0 fail to prevent an invalid URL from loading in a pop-up window in the Mattermost Desktop App which allows a malicious server owner to repeated crash the application via calling {{window.open('javascript:alert()');}}. Mattermost Advisory ID: MMSA-2026-00618

Affected products

Mattermost Desktop: through 5.4.13.0; from 6.0.0 through 6.0.1; from 6.1.0 before 6.2.0 — fixed in 6.2.0

How to fix

Upgrade to a fixed version: Mattermost Desktop 6.2.0. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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