CVE-2026-6517

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

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CVSS v3.1 Score

6.3

MEDIUM

CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:N/A:N

EPSS — Exploit Prediction

0.0019

Probability of exploitation

0.08%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-522 CWE-522

Affected Products

Vendor	Product	Versions
mattermost	mattermost_desktop	< 5.13.0
mattermost	mattermost_desktop	6.1.0 — 6.1.5

References

Advisories & Patches

https://mattermost.com/security-updates

Frequently Asked Questions

What is 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 It has a CVSS v3.1 base score of 6.3 (MEDIUM).

How severe is CVE-2026-6517? +

CVE-2026-6517 has a CVSS v3.1 score of 6.3 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance. The EPSS score is 0.0019, placing it in the 0th percentile for exploitation probability.

What products are affected by CVE-2026-6517? +

CVE-2026-6517 affects products from mattermost, specifically: mattermost_desktop. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2026-6517? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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