CVE-2026-4339

SSRF via unvalidated attachment URLs in Mattermost Agents plugin MCP server

MEDIUM

6.5

CVSS Score

Published: June 26, 2026 at 02:44 PM UTC

Updated: July 1, 2026 at 11:22 PM UTC

2 products affected across 1 vendorMattermost ServerMattermost

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Severity

MEDIUM6.5

Exploit prediction

EPSS 0.1%

SSVC

No known exploit

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

Affected

Our own answer: ranges merged, affected separated from platform-only, with confidence and review flags.

Mattermost Server · Mattermost

95%

10.11.0 ≤ v < 10.11.1911.5.0 ≤ v < 11.5.711.6.0 ≤ v < 11.6.4

Affected range stops below 10.11.19, so it looks safe — our inference, not a vendor statement

Affected range stops below 11.5.7, so it looks safe — our inference, not a vendor statement

Affected range stops below 11.6.4, so it looks safe — our inference, not a vendor statement

Mattermost · Mattermost

60%

10.11.0 ≤ v ≤ 10.11.1811.5.0 ≤ v ≤ 11.5.611.6.0 ≤ v ≤ 11.6.310.11.0 11.5.0 11.6.0

Vendor states this is fixed in 10.11.19 (unaffected)

Vendor states this is fixed in 11.5.7 (unaffected)

Vendor states this is fixed in 11.6.4 (unaffected)

Vendor states this is fixed in 11.7.0 (unaffected)

Remediation

No guide yet

No remediation guidance has been written for this CVE yet. What the record does show:

Patch and advisory references

https://mattermost.com/security-updates

References (1)

Vendor Advisory

Source: cverecord

https://mattermost.com/security-updates

SSVC — Action Prioritization

SSVCNo known exploitNot automatablePartial impact

SSVC signals action urgency (what to patch first), separate from CVSS which measures severity.

CVSS

6.5MEDIUMMattermost

Attack Vector: LOCAL
Complexity: LOW
Privileges: LOW
User Interaction: NONE
Scope: CHANGED
Confidentiality: HIGH
Integrity: NONE
Availability: NONE

Vector?

Quick Info

StatusANALYZED

CWE

CWE-918

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