CVE-2026-4339

Internal service data disclosure via unchecked attachment URLs

Published Jun 26, 2026 · Updated Jun 26, 2026

SSRF in Mattermost 10.11.0 through 10.11.18, 11.5.0 through 11.5.6, and 11.6.0 through 11.6.3 allows local users to read internal data. The Agents plugin MCP server accepts attachment URLs in post-creation requests without rejecting internal or private IP ranges, then fetches them. Access to the MCP server in stdio mode is required; successful requests expose data returned by internal network services without modifying it.

CVSS severity6.5
Medium
EPSS probability0.14%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

SSRF in Mattermost 10.11.0 through 10.11.18, 11.5.0 through 11.5.6, and 11.6.0 through 11.6.3 allows local users to read internal data. The Agents plugin MCP server accepts attachment URLs in post-creation requests without rejecting internal or private IP ranges, then fetches them. Access to the MCP server in stdio mode is required; successful requests expose data returned by internal network services without modifying it.

The record

CVE
CVE-2026-4339
Published
Jun 26, 2026
Updated
Jun 26, 2026
Vendor
Mattermost
Product
Mattermost
Classifications
CWE-918, CAPEC-664
Attack vector
local
Privileges
authenticated

Timeline

How it unfolded

  1. Jun 26, 2026CVE publishedPublication date reported by the CVE source.
  2. Jun 26, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=10.11.0 <=10.11.18
  2. Affected versionversion=11.5.0 <=11.5.6
  3. Affected versionversion=11.6.0 <=11.6.3

What conditions does exploitation require?

Attack vectorlocal
Required privilegesauthenticated

What is affected?

Mattermost · Mattermostversion=10.11.0 <=10.11.18; version=11.5.0 <=11.5.6; version=11.6.0 <=11.6.3

Published CVSS scores

6.5Mattermost, Inc.CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectorlocal
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-918Server-Side Request Forgery (SSRF)
CAPEC-664Server Side Request Forgery

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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