CVE-2026-4339CWE-918

SSRF via unvalidated attachment URLs in Mattermost Agents plugin MCP server

Medium · published June 26, 2026

CVSS v3.1
6.5
EPSS
0%
Percentile
3.0
In the wild
Unconfirmed
What it is

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

The record
Technical detail
CVSS v3.1
6.5 · MEDIUM
Vector
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N
CVSS v4.0
Not supplied
EPSS
0.00139 · 3.0th percentile
Weakness
CWE-918 · Server-Side Request Forgery (SSRF)
Published
2026-06-26T14:44Z
EPSS history
Timeline
  • 26 JUN 14:44Z
    SSRF via unvalidated attachment URLs in Mattermost Agents plugin MCP server
    cvelistv5