Vulnerability · Published Jun 26, 2026 · Assigned by Mattermost

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

Medium 6.5

In plain English

A medium-severity server-side request forgery in Mattermost from Mattermost. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.1% (higher than 3% of all known vulnerabilities).

Severity (CVSS 3.1)6.5Medium

Exploit odds (30 days)0.1%top 96.7% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

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

Affected products (as reported)

Vendor	Product	Versions
Mattermost	Mattermost	10.11.0 – through 10.11.1811.6.0 – through 11.6.311.5.0 – through 11.5.6

Weakness: CWE-918 · Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N

References

MMSA-2026-00635 vendor-advisory