CVE-2026-4339

🔶 medium

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

CVSS Score

6.5

Medium

EPSS Score

0.1

Exploit Probability

Published Date

2026-06-26

First Seen: 2026-06-27

📊 Relative Risk Intelligence

This CVE is Lower Risk - more severe than 47.6% of all 338,292 vulnerabilities in our database.

#177,181

Below average severity

Severity Percentile

🎯 CISA SSVC Assessment Updated: Jun 26, 2026

🔍 Exploitation Status

None

No known exploits

⚙️ Automatable

Requires human interaction

💥 Technical Impact

Partial

Limited system impact

🏆 Discovered By

s00me00ne

SSVC data provided by CISA

Last Modified 2026-06-29

Source NVD 🔗

CVSS Vector 3.1 CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N

CWE IDs (Weakness Types)

CWE-918

📦 Affected Products 3

Vendor	Product	Status	Affected Versions
mattermost	mattermost_server	Affected	> 10.11.0 < 10.11.19
mattermost	mattermost_server	Affected	> 11.5.0 < 11.5.7
mattermost	mattermost_server	Affected	> 11.6.0 < 11.6.4

🔗 References 1

https://mattermost.com/security-updates

Vendor Advisory

🔗 Related CVEs 6

CVE ID	Severity	CVSS	EPSS	Summary	Published
CVE-2026-10085	🔶 medium	5.4	0.2	Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 fail to restrict the group_constrained chann...	2026-07-13
CVE-2026-10103	🔶 medium	4.3	0.2	Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 fail to verify post ownership in the shared ...	2026-07-13
CVE-2026-10106	🔶 medium	6.5	0.2	Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 fail to verify that the channel referenced i...	2026-07-13
CVE-2026-6850	🔶 medium	6.5	0.2	Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 fail to validate the length and content of m...	2026-07-13
CVE-2026-9571	🔶 medium	5.9	0.2	Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 fail to invalidate OAuth refresh tokens upon...	2026-07-13
CVE-2026-9597	🔶 medium	5.4	0.1	Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4 fail to verify whether a guest account is deactivated before crea...	2026-07-13

These CVEs affect the same products