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TI-2026-03777 Aliases CVE-2026-4339GHSA-p3qg-h7r3-79xr

SSRF via unvalidated attachment URLs in Mattermost Agents plugin MCP server

Priority LOW 28CWE-918Published · Updated

Key facts

Attack vector
Local
Privileges required
Low
User interaction
None
Attack complexity
Low
Impact type
Information disclosure
Confidentiality · integrity · availability
High · None · None
Fix status
Fixed

Attack profile from CVSS 3.1 · CNA. Fix status from CNA. Impact type from CWE-918 and the CVSS impact.

Summary

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

Why it matters now

Exploitation None reported CISA SSVC · not in CISA KEV
EPSS 0.00139 3rd percentileScored 2026-11-07
Fix Fixed Mattermost (Mattermost): 11.7.0, 10.11.19, 11.6.4 +1

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation
None
Automatable
No
Technical impact
Partial

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

VendorProductAffected versionsFixed inStatusSource
MattermostMattermost10.11.0 – 10.11.18; 11.6.0 – 11.6.3; 11.5.0 – 11.5.611.7.0, 10.11.19, 11.6.4, 11.5.7Patch availableCNA
  • Mattermost VendorMattermost Affected 10.11.0 – 10.11.18; 11.6.0 – 11.6.3; 11.5.0 – 11.5.6 Fixed in 11.7.0, 10.11.19, 11.6.4, 11.5.7 Patch available CNA

CPE configurations NVD · 1

CPEVersionsRoleVendor
cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*10.11.0 to before 10.11.19; 11.5.0 to before 11.5.7; 11.6.0 to before 11.6.4VulnerableMattermost
  • cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:* Versions 10.11.0 to before 10.11.19; 11.5.0 to before 11.5.7; 11.6.0 to before 11.6.4 Vulnerable ·Mattermost

Remediation

  • Fix · CNA Mattermost (Mattermost) Fixed in 11.7.0, 10.11.19, 11.6.4, 11.5.7

Timeline

  1. 2026-10-02 Threatist record created from CVE-2026-4339
  2. 2026-10-03 Alias added: GHSA-p3qg-h7r3-79xr

References

Advisory 1

  • TI-2025-18933 Arbitrary file read in Mattermost Boards via import & export board archive CVSS 3.1CRITICAL 9.9
  • TI-2025-04465 Path Traversal Leading to RCE by Any Authenticated Mattermost User CVSS 3.1CRITICAL 9.9
  • TI-2017-14068 An issue was discovered in Mattermost Server before 3.6.2. CVSS 3.1CRITICAL 9.8
  • TI-2017-14060 An issue was discovered in Mattermost Server before 3.8.2, 3.7.5, and 3.6.7. CVSS 3.1CRITICAL 9.8
  • TI-2017-14048 An issue was discovered in Mattermost Server before 4.1.0, 4.0.4, and 3.10.3. CVSS 3.1CRITICAL 9.8
  • TI-2025-14360 Arbitrary file read via block duplication in Mattermost Boards CVSS 3.1CRITICAL 9.9
See all 505 →

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences