CVE-2026-4339 - Vulnerability Details - OpenCVE

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

No CVSS v4.0

Attack Vector Local

Attack Complexity Low

Privileges Required Low

Scope Changed

Confidentiality Impact High

Integrity Impact None

Availability Impact None

User Interaction None

No CVSS v3.0

No CVSS v2

This CVE is not in the KEV list.

No EPSS score available.

Exploitation none

Automatable no

Technical Impact partial

No data.

No data.

No data.

OpenCVE Enrichment is a feature of OpenCVE that uses AI to automatically link vendors and products to CVEs. Learn more on GitHub.

Vendors	Products

References

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

History

Fri, 26 Jun 2026 16:30:00 +0000

Type	Values Removed	Values Added
Metrics		ssvc `{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}`

Fri, 26 Jun 2026 15:15:00 +0000

Type	Values Removed	Values Added
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
Title		SSRF via unvalidated attachment URLs in Mattermost Agents plugin MCP server
Weaknesses		CWE-918
References		https://mattermost.com/security-updates
Metrics		cvssV3_1 `{'score': 6.5, 'vector': 'CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N'}`

MITRE

Status: PUBLISHED

Assigner: Mattermost

Published: 2026-06-26T14:44:58.655Z

Updated: 2026-06-26T15:40:33.300Z

Reserved: 2026-03-17T14:57:10.575Z

Link: CVE-2026-4339

Vulnrichment

Updated: 2026-06-26T15:40:28.917Z

NVD

No data.

Redhat

No data.

OpenCVE Enrichment

Updated: 2026-06-26T16:30:03Z

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