sbomify Insights

Search by CVE or GitHub advisory id, package name or title.

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

Medium 6.5 Published Updated

An attacker with MCP server access can perform SSRF to exfiltrate data from internal network services via unvalidated attachment URLs.

Exploited in the wild
Not known
Login needed
Any signed-in user
Attackable
Local or same network only
Victim action needed
Not needed
Fix
None listed

What is CVE-2026-4339?

CVE-2026-4339 is a medium-severity server-side request forgery vulnerability in Mattermost Mattermost Server, published on 26 Jun 2026. It can only be exploited from the same machine or local network, with no action from the victim.

Official 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

Weakness: CWE-918

Which versions are affected by CVE-2026-4339?

It affects 1 product.

Products include Mattermost Mattermost Server.

How do I fix CVE-2026-4339?

No fix has been published for CVE-2026-4339 yet. Limit exposure of the affected software until one is available.

Upgrade, and send outbound requests through an allow-list that blocks internal addresses. More about Server-side request forgery.

Is CVE-2026-4339 being exploited?

There is no known exploitation of CVE-2026-4339 in the wild and no public exploit code.

All Mattermost Mattermost Server vulnerabilities

Sources

Also known as GHSA-p3qg-h7r3-79xr.

Is CVE-2026-4339 in your software?

Upload an SBOM and sbomify checks it against every vulnerability here, every day.

Check your SBOMs for free