CVE-2026-6517 Mattermost Desktop App fails to restrict the allow list of domains which NTLM credentials are passed

Q: Is CVE-2026-6517 being exploited?

There is no known exploitation of CVE-2026-6517 in the wild and no public exploit code.

Q: How do I fix CVE-2026-6517?

No fix has been published for CVE-2026-6517 yet. Limit exposure of the affected software until one is available.

Medium 6.3 Published 15 Jun 2026 Updated 15 Jun 2026

Attackers can intercept NTLM credentials by embedding images routing to external servers, affecting users on servers without image proxy enabled.

Exploited in the wild: Not known
Login needed: Any signed-in user
Attackable: Over the internet
Victim action needed: Not needed
Fix: None listed

What is CVE-2026-6517?

CVE-2026-6517 is a medium-severity vulnerability in Mattermost, published on 15 Jun 2026. Any signed-in user can exploit it over the network, with no action from the victim.

Official description

Mattermost Desktop App versions <=6.1 5.5.13.0 fail to restrict the allow list of domains to which NTLM credentials were forwarded to in the Mattermost Desktop App which allows any user on a server without the image proxy enabled to intercept other users credentials via embedding an image that routes to an external web server. Mattermost Advisory ID: MMSA-2026-00651

Weakness: CWE-522

Which versions are affected by CVE-2026-6517?

It affects 1 product.

Products include Mattermost Mattermost Desktop.

How do I fix CVE-2026-6517?

No fix has been published for CVE-2026-6517 yet. Limit exposure of the affected software until one is available.

Is CVE-2026-6517 being exploited?

There is no known exploitation of CVE-2026-6517 in the wild and no public exploit code.

Sources

Also known as GHSA-2wmq-3m94-g4jr.