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VULNERABILITIES

CVE-2026-4339 Details

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

Metrics

CVSS 3.x Severity and Vector Strings:

CNA: [email protected]CVSS-B:6.5 MEDIUMVector:CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N

References to Advisories, Solutions, and Tools

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URLSource(s)Tag(s)
https://mattermost.com/security-updates [email protected]Vendor Advisory

Weakness Enumeration

CWE-IDCWE NameSource
CWE-918Server-Side Request Forgery (SSRF)[email protected]

Affected Products

ProductVersions
mattermost mattermost server
>= 10.11.0, < 10.11.19
>= 11.5.0, < 11.5.7
>= 11.6.0, < 11.6.4

CPE

  • cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*

Remediation

  • No remediation found in references.

Change History

3 change records found show changes

QUICK INFO

CVE Dictionary Entry:
CVE-2026-4339
NVD Published Date:
Jun 26, 2026
NVD Last Modified:
Jun 29, 2026
Source:
[email protected]
CVE-2026-4339 Details - Not Deferred