What is CVE-2026-6517?

CVE-2026-6517 is a medium-severity vulnerability in Mattermost, classified under Insufficiently Protected Credentials. CVSS score: 6.3/10. Published 2026-06-15.

How severe is CVE-2026-6517?

Medium severity. CVSS v3 base score is 6.3 out of 10.

Vulnerability in Mattermost

CVE-2026-6517

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

Published 2026-06-15 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 6.3 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:N/A:N.

EPSS exploit prediction

EPSS: 0.003 (22.1th percentile), scored 2026-09-30.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 22nd percentile — 22.1% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.002 · newest: 0.003 · change: +0.001

Affected products

Mattermost — versions 0, 6.2.0, 5.13.6.0
Mattermost Desktop

Weakness classification (CWE)

CWE-522 — Insufficiently Protected Credentials

References

mattermost.com/security-updates (vendor-advisory, Vendor Advisory)

Frequently asked questions

What is CVE-2026-6517?: CVE-2026-6517 is a medium-severity vulnerability in Mattermost, classified under Insufficiently Protected Credentials. CVSS score: 6.3/10. Published 2026-06-15.
How severe is CVE-2026-6517?: Medium severity. CVSS v3 base score is 6.3 out of 10.