CVE-2026-1628: Mattermost allows external websites to open within the app, exposing preload functionality to non-trusted sites.

Severity: medium (CVSS 4.6 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.1% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-03-02 · updated 2026-06-17
Weakness: CWE-829

Description

Mattermost Desktop App versions <=5.13.3 fail to attach listeners restricting navigation to external sites within the Mattermost app which allows a malicious server to expose preload script functionality to untrusted servers via having a user open an external link in their Mattermost server. Mattermost Advisory ID: MMSA-2026-00596

Affected products

Mattermost Desktop: before 5.13.4 — fixed in 5.13.4

How to fix

Upgrade to a fixed version: Mattermost Desktop 5.13.4. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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