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

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

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 26 Jun 2026
- **Updated:** 26 Jun 2026
- **Also known as:** GHSA-p3qg-h7r3-79xr

## 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 (Server-Side Request Forgery)

## Which versions are affected by CVE-2026-4339?

It affects 1 product.


Products: 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.

## Is CVE-2026-4339 being exploited?

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

## Other vulnerabilities in Mattermost Mattermost Server

- [CVE-2025-12419](https://insights.sbomify.com/CVE-2025-12419/): Account takeover on OAuth/OpenID-enabled servers (critical 9.9)
- [CVE-2025-12421](https://insights.sbomify.com/CVE-2025-12421/): Account Takeover via Code Exchange Endpoint (critical 9.9)
- [CVE-2025-4981](https://insights.sbomify.com/CVE-2025-4981/): Path Traversal Leading to RCE by Any Authenticated Mattermost User (critical 9.9)
- [CVE-2025-20051](https://insights.sbomify.com/CVE-2025-20051/): Arbitrary file read via block duplication in Mattermost Boards (critical 9.9)
- [CVE-2025-25279](https://insights.sbomify.com/CVE-2025-25279/): Arbitrary file read in Mattermost Boards via import & export board archive (critical 9.9)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-4339 and fix it if it is affected.

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

Affected product: Mattermost Mattermost Server. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-4339/ (Markdown: https://insights.sbomify.com/CVE-2026-4339/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-4339
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-4339
- OSV: https://osv.dev/vulnerability/CVE-2026-4339

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-4339/
Summaries are generated and may contain mistakes; the official description above is the record.