What is CVE-2026-4339?

CVE-2026-4339 is a medium-severity vulnerability in Mattermost, classified under Server-Side Request Forgery (SSRF). CVSS score: 6.5/10. Published 2026-06-26.

How severe is CVE-2026-4339?

Medium severity. CVSS v3 base score is 6.5 out of 10.

SSRF in Mattermost

CVE-2026-4339

Mattermost versions 10.11.x <= 10.11.18, 11.6.x <= 11.6.3, 11.5.x <= 11.5.6 fail to validate attachment URLs against internal or private IP ranges in the Mattermost Agents plugin MCP server which allows an attacker with access to the MCP s…

Vulnerability class: SSRF (Server-Side Request Forgery)

CVSS v3 metric

CVSS v3 base score 6.5 (Medium). Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N.

Affected products

Mattermost — versions 10.11.0, 11.6.0, 11.5.0

Weakness classification (CWE)

CWE-918 — Server-Side Request Forgery (SSRF)

References

responsibledisclosure@mattermost.com (vendor-advisory)

Frequently asked questions

What is CVE-2026-4339?: CVE-2026-4339 is a medium-severity vulnerability in Mattermost, classified under Server-Side Request Forgery (SSRF). CVSS score: 6.5/10. Published 2026-06-26.
How severe is CVE-2026-4339?: Medium severity. CVSS v3 base score is 6.5 out of 10.