Review reviewHigh

CVE-2026-6517

Mattermost Mattermost, mattermost desktop

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

CVSS: 7.7
EPSS: 0.19%
CISA KEV: Not listed
Published: 2026.06.15

PRIORITY ASSESSMENT

Review review

The CVSS severity warrants an early asset and exposure review.

Known exploitationNot established by KEV

Exploit probability0.19%

Technical severityCVSS 7.7

Vulnerability overview

Affected product and versions

Product: Mattermost Mattermost, mattermost desktop
Affected versions: <= 5.13.0, >= 6.1.0 <= 6.1.5
Fixed versions: No verified fixed-version field is available yet

Recommended response sequence

Confirm exposure before applying a vendor-supported change.

Full remediation guide

1
Identify
Confirm that Mattermost Mattermost, mattermost desktop and an affected version are present.
2
Prioritize
Combine exploitation signals with asset exposure and business criticality.
3
Remediate
Follow the vendor advisory or supported update path and preserve rollback options.
4
Verify
Recheck the version, service health, access paths, and relevant logs.

Technical data

CVSS vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N
CWE: CWE-522