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CVE-2026-4339

SSRF via unvalidated attachment URLs in Mattermost Agents plugin MCP server

Published: Jun 26, 2026Updated: Jun 29, 2026 Sources: CVE List NVDCWE-918
6.5MEDIUM
NVD MITRE
EPSS Score
0.1%
Top 98.8%
CISA KEV
Not in KEV
Exploits
No Known Exploits
Remediation
Patch Available
Self-Check
OpenVAS

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

No summary for this CVE yet.

CVSS Vector Breakdown

AV:LAC:LPR:LUI:NS:CC:HI:NA:N
Exploitability
AV:LAttack Vector
Local
AC:LAttack Complexity
Low
PR:LPrivileges Required
Low
UI:NUser Interaction
None
Scope
S:CScope
Changed
Impact
C:HConfidentiality
High
I:NIntegrity
None
A:NAvailability
None
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Weaknesses

CWE-918

Affected Products

Mattermost
commercial·US
mattermost serverOn-premWeb App
Communications / messaging-chat
MattermostOn-premWeb App
Communications / messaging-chat

Exploitability

Official Patch Available
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Attack Graph

Products CVE Techniques Tactics

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MITRE ATT&CK

2 techniques
Command and Control
Initial Access
View detailed technique mapping

References

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

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