Is CVE-2026-6517 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-6517 being exploited in the wild.

How severe is CVE-2026-6517?

Medium severity, with a CVSS score of 6.3, published 15 June 2026.

Which products and versions are affected?

Mattermost Desktop App <=6.1, 5.5.13.0.

Is there a fix for CVE-2026-6517?

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-2026-6517 attacked?

It is attacked over the network, in a network appliance.

Am I affected by CVE-2026-6517?

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.

CVE-2026-6517: Mattermost Desktop App NTLM credential leakage via embedded images

Q: Is there a fix for CVE-2026-6517?

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.

Q: How is CVE-2026-6517 attacked?

It is attacked over the network, in a network appliance.

Q: Am I affected by CVE-2026-6517?

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.

CVE-2026-6517 · Severity: medium · CVSS 6.3 · Published 2026-06-15

Mattermost is a collaboration and messaging platform used by organizations for internal communication. A security flaw in the Mattermost Desktop App allows an attacker to steal a user's Windows login credentials (NTLM hashes) by embedding a malicious image in a chat message. This occurs because the application incorrectly sends sensitive authentication data to external servers, potentially allowing an attacker to gain unauthorized access to the victim's corporate account or network.

Technologies: Mattermost Desktop App. Vendors: Mattermost.

Quick answers

Is CVE-2026-6517 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-6517 being exploited in the wild.
How severe is CVE-2026-6517?: Medium severity, with a CVSS score of 6.3, published 15 June 2026.
Which products and versions are affected?: Mattermost Desktop App <=6.1, 5.5.13.0.
Is there a fix for CVE-2026-6517?: 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-2026-6517 attacked?: It is attacked over the network, in a network appliance.
Am I affected by CVE-2026-6517?: 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 (versions <=6.1 and 5.5.13.0) contains an insufficient credential protection vulnerability (CWE-522). The application fails to properly restrict the list of trusted domains allowed to receive NTLM authentication hashes. On Mattermost servers where the image proxy is disabled, a remote attacker with low privileges can embed a specially crafted image link in a message. When a victim views the message, the Desktop App attempts to fetch the image and automatically forwards the user's NTLM credentials to the attacker-controlled external web server. This requires the attacker to have an account on the same Mattermost server as the victim.

Affected products

Mattermost Desktop App <=6.1, 5.5.13.0

Timeline

2026-06-15: disclosed: CVE published and advisory issued by Mattermost.

References

https://mattermost.com/security-updates

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