# CVE-2026-3471: Mattermost Desktop App denial of service via invalid pop-up URL

Source: https://junglewise.ai/threats/cve-2026-3471-mattermost-desktop-app-denial-of-service-via-invalid-pop-up-url  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-05-18. Updated: 2026-05-18  
License: CC BY 4.0 with a link back

CVE-2026-3471 · Severity: medium · CVSS 6.5 · Published 2026-05-18

Mattermost is a collaboration platform used for team communication. A vulnerability in the Mattermost Desktop application allows a malicious server owner to repeatedly crash a user's application by triggering invalid pop-up windows. This results in a denial-of-service for the user, disrupting their ability to communicate and collaborate.

Technologies: [Mattermost Desktop App](https://junglewise.ai/threats/technologies/desktop-app). Vendors: [Mattermost](https://junglewise.ai/threats/vendors/mattermost).

## Quick answers

**Is CVE-2026-3471 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-3471 being exploited in the wild.

**How severe is CVE-2026-3471?** Medium severity, with a CVSS score of 6.5, published 18 May 2026.

**Which products and versions are affected?** Mattermost Desktop App <=6.1, 6.0.1, 5.4.13.0.

**Is there a fix for CVE-2026-3471?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-3471 attacked?** It is attacked over the network, in a network appliance.

**Am I affected by CVE-2026-3471?** Check whether you run Mattermost Desktop App at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

The Mattermost Desktop App contains a vulnerability classified under CWE-939 (Improper Authorization in Handler for Custom URL Scheme). The application fails to properly validate or restrict URLs when opening new windows via the window.open() method. A malicious server owner can exploit this by calling window.open('javascript:alert()'), which causes the desktop client to crash. This is a remote attack requiring user interaction (connecting to the malicious server) and results in a high impact on application availability.

## Affected products

- Mattermost Desktop App <=6.1, 6.0.1, 5.4.13.0

## Timeline

- 2026-05-18: disclosed
- 2026-05-18: advisory

## References

- [https://mattermost.com/security-updates](https://mattermost.com/security-updates)

## Related threats

- [Mattermost Desktop App URL scheme validation bypass](https://junglewise.ai/threats/cve-2026-75588-mattermost-desktop-app-url-scheme-validation-bypass)
- [Mattermost Desktop App IPC validation bypass in leaveCall handler](https://junglewise.ai/threats/cve-2026-12284-mattermost-desktop-app-ipc-validation-bypass-in-leavecall-handler)
- [Mattermost Desktop App content restriction bypass](https://junglewise.ai/threats/cve-2026-75025-mattermost-desktop-app-content-restriction-bypass)
- [Mattermost Desktop App denial of service via malformed payload](https://junglewise.ai/threats/cve-2026-9602-mattermost-desktop-app-denial-of-service-via-malformed-payload)
- [Mattermost Desktop App denial of service via malicious image link](https://junglewise.ai/threats/cve-2026-8075-mattermost-desktop-app-denial-of-service-via-malicious-image-link)