CVE-2026-4339

SSRF via unvalidated attachment URLs in Mattermost Agents plugin MCP server

CVSS 6.5 MEDIUMEPSS 0.1%CWE-918

Vexday Risk Score

13Low

SSVC decision (CISA)

Track

No exploitation signal → monitor

CVSS 6.5EPSS 0.1%KEV nãoPoC —Nuclei —Metasploit —Patch referenciado

Lifecycle

26 Jun 2026Published on NVD

Recommendation: Monitor — no exploitation signal at the moment.

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

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N

Affected products

Mattermost · Mattermost

Want to know if your infrastructure is exposed to this?

Talk to TrueHacking →

References

https://mattermost.com/security-updates