CVE-2026-4339: Mattermost Server SSRF Vulnerability

CVE-2026-4339 Overview

CVE-2026-4339 is a Server-Side Request Forgery (SSRF) vulnerability [CWE-918] in the Mattermost Agents plugin Model Context Protocol (MCP) server. The flaw affects Mattermost Server versions 10.11.x <= 10.11.18, 11.6.x <= 11.6.3, and 11.5.x <= 11.5.6. The MCP server running in stdio mode fails to validate attachment URLs against internal or private IP ranges. An authenticated attacker with access to the MCP server can supply internal URLs as file attachments in post creation requests. This allows the attacker to force the server to make requests to internal network resources and exfiltrate data from services that are not exposed externally.

Critical Impact
An authenticated local attacker can leverage the Mattermost Agents MCP server to reach internal-only services and exfiltrate sensitive data via SSRF.

Affected Products

Mattermost Server 10.11.x through 10.11.18
Mattermost Server 11.6.x through 11.6.3
Mattermost Server 11.5.x through 11.5.6

Discovery Timeline

2026-06-26 - CVE-2026-4339 published to the National Vulnerability Database
2026-06-29 - Last updated in NVD database
Advisory - Tracked by Mattermost as MMSA-2026-00635

Technical Details for CVE-2026-4339

Vulnerability Analysis

The vulnerability resides in the attachment handling logic of the Mattermost Agents plugin, which exposes a Model Context Protocol (MCP) server. When operating in stdio mode, the MCP server accepts file attachment URLs supplied inside post creation requests. The server fetches these URLs server-side to retrieve attachment content. Because the URL validation routine does not reject destinations that resolve to internal, private, loopback, or link-local IP ranges, an attacker can direct the fetcher to arbitrary internal endpoints. The response content is then returned through the attachment pipeline, effectively turning the Mattermost host into a proxy against protected network segments.

Root Cause

The root cause is missing egress URL validation in the MCP server attachment handler. The code path parses attacker-controlled URLs and issues outbound HTTP requests without enforcing an allow list of destinations or blocking RFC 1918, loopback (127.0.0.0/8), and metadata service ranges (for example, 169.254.169.254). This is a classic SSRF pattern classified under [CWE-918].

Attack Vector

Exploitation requires local access to the MCP server in stdio mode and authenticated privileges to create posts with file attachments. The attacker crafts a post creation request that specifies an internal URL — such as an internal admin console, database HTTP interface, or cloud instance metadata endpoint — as an attachment source. The Mattermost server fetches the URL and delivers the response content back to the attacker via the attachment mechanism. Refer to the Mattermost Security Updates advisory for detailed technical guidance.

Detection Methods for CVE-2026-4339

Indicators of Compromise

Post creation API calls containing attachment URLs pointing to private IP ranges such as 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, or 127.0.0.0/8.
Requests from the Mattermost server process to cloud metadata endpoints, most notably 169.254.169.254.
Attachments in posts whose stored source URL references internal hostnames not intended for user consumption.

Detection Strategies

Inspect Mattermost application logs for attachment fetch operations targeting non-public destinations.
Correlate outbound HTTP connections from the Mattermost host with the user identity that initiated the originating post.
Enable Agents plugin audit logging and review MCP server activity when running in stdio mode.

Monitoring Recommendations

Monitor egress traffic from Mattermost servers using network firewall or proxy logs and flag connections to internal subnets.
Alert on any requests from the Mattermost service account to cloud metadata IPs.
Track unusual spikes in attachment creation volume by a single account, which may indicate scripted SSRF probing.

How to Mitigate CVE-2026-4339

Immediate Actions Required

Upgrade Mattermost Server to a fixed release beyond 10.11.18, 11.6.3, or 11.5.6 as documented by Mattermost.
Restrict access to the Agents plugin MCP server and disable stdio mode if not required.
Apply network egress filtering on Mattermost hosts to block outbound traffic to internal management interfaces and metadata endpoints.

Patch Information

Mattermost has released security updates addressing MMSA-2026-00635. Administrators should consult the Mattermost Security Updates page for the specific fixed version corresponding to their deployment branch and apply the upgrade promptly.

Workarounds

Disable the Mattermost Agents plugin until the upgrade is applied.
Enforce host-based firewall rules that deny the Mattermost process from connecting to RFC 1918 ranges and 169.254.169.254.
Limit the set of users authorized to interact with the MCP server to trusted administrative accounts only.

bash

# Example iptables rules to block Mattermost egress to internal ranges
iptables -A OUTPUT -m owner --uid-owner mattermost -d 169.254.169.254 -j REJECT
iptables -A OUTPUT -m owner --uid-owner mattermost -d 10.0.0.0/8 -j REJECT
iptables -A OUTPUT -m owner --uid-owner mattermost -d 172.16.0.0/12 -j REJECT
iptables -A OUTPUT -m owner --uid-owner mattermost -d 192.168.0.0/16 -j REJECT