CVE-2026-4339: SSRF via unvalidated attachment URLs in Mattermost Agents plugin MCP server

Severity: medium (CVSS 6.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.1% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-06-26 · updated 2026-06-29
Weakness: CWE-918

Description

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 server in stdio mode to perform server-side request forgery (SSRF) and exfiltrate data from internal network services via supplying internal URLs as file attachments in post creation requests.. Mattermost Advisory ID: MMSA-2026-00635

Affected products

Mattermost: from 10.11.0 before 10.11.19; from 11.5.0 before 11.5.7; from 11.6.0 before 11.6.4 — fixed in 10.11.19, 11.5.7, 11.6.4

How to fix

Upgrade to a fixed version: Mattermost 10.11.19, 11.5.7, 11.6.4. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

Get alerted when CVE-2026-4339 or the next vulnerability affects your stack