CVE-2026-4339 Vulnerability Analysis & Exploit Details

CVE-2026-4339
Vulnerability Scoring

6.5
/10
High Risk

If left unpatched, CVE-2026-4339 could lead to major system disruptions or data loss.

Attack Complexity Details

  • Attack Complexity: Low
    Exploits can be performed without significant complexity or special conditions.
  • Attack Vector: Local
    Vulnerability requires local system access.
  • Privileges Required: Low
    Some privileges are necessary to exploit the vulnerability.
  • Scope: Changed
    Successful exploitation can impact components beyond the vulnerable component.
  • User Interaction: None
    No user interaction is necessary for exploitation.

CVE-2026-4339 Details

Status: Analyzed

Last updated: 🕖 29 Jun 2026, 19:26 UTC
Originally published on: 🕒 26 Jun 2026, 15:16 UTC

Time between publication and last update: 3 days

CVSS Release: version 3

CVSS3 Source

responsibledisclosure@mattermost.com

CVSS3 Type

Secondary

CVSS3 Vector

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

CVE-2026-4339 Vulnerability Summary

CVE-2026-4339: 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

Assessing the Risk of CVE-2026-4339

Access Complexity Graph

The exploitability of CVE-2026-4339 depends on two key factors: attack complexity (the level of effort required to execute an exploit) and privileges required (the access level an attacker needs).

Exploitability Analysis for CVE-2026-4339

CVE-2026-4339 presents an accessible attack vector with minimal effort required. Restricting access controls and implementing security updates are critical to reducing exploitation risks.

Understanding AC and PR

A lower complexity and fewer privilege requirements make exploitation easier. Security teams should evaluate these aspects to determine the urgency of mitigation strategies, such as patch management and access control policies.

Attack Complexity (AC) measures the difficulty in executing an exploit. A high AC means that specific conditions must be met, making an attack more challenging, while a low AC means the vulnerability can be exploited with minimal effort.

Privileges Required (PR) determine the level of system access necessary for an attack. Vulnerabilities requiring no privileges are more accessible to attackers, whereas high privilege requirements limit exploitation to authorized users with elevated access.

CVSS Score Breakdown Chart

Above is the CVSS Sub-score Breakdown for CVE-2026-4339, illustrating how Base, Impact, and Exploitability factors combine to form the overall severity rating. A higher sub-score typically indicates a more severe or easier-to-exploit vulnerability.

CIA Impact Analysis

Below is the Impact Analysis for CVE-2026-4339, showing how Confidentiality, Integrity, and Availability might be affected if the vulnerability is exploited. Higher values usually signal greater potential damage.

  • Confidentiality: High
    Exploiting CVE-2026-4339 can result in unauthorized access to sensitive data, severely compromising data privacy.
  • Integrity: None
    CVE-2026-4339 poses no threat to data integrity.
  • Availability: None
    CVE-2026-4339 does not impact system availability.

CVE-2026-4339 References

External References

CWE Common Weakness Enumeration

CWE-918

CAPEC Common Attack Pattern Enumeration and Classification

  • Server Side Request Forgery CAPEC-664 An adversary exploits improper input validation by submitting maliciously crafted input to a target application running on a server, with the goal of forcing the server to make a request either to itself, to web services running in the server’s internal network, or to external third parties. If successful, the adversary’s request will be made with the server’s privilege level, bypassing its authentication controls. This ultimately allows the adversary to access sensitive data, execute commands on the server’s network, and make external requests with the stolen identity of the server. Server Side Request Forgery attacks differ from Cross Site Request Forgery attacks in that they target the server itself, whereas CSRF attacks exploit an insecure user authentication mechanism to perform unauthorized actions on the user's behalf.

Vulnerable Configurations

  • cpe:2.3:a:mattermost:mattermost_server:10.11.0:-:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.0:-:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.0:rc1:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.0:rc1:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.0:rc2:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.0:rc2:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.0:rc3:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.0:rc3:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.1:-:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.1:-:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.1:rc1:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.1:rc1:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.2:-:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.2:-:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.2:rc1:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.2:rc1:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.2:rc2:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.2:rc2:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.3:*:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.3:*:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.4:-:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.4:-:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.4:rc1:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.4:rc1:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.4:rc2:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.4:rc2:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.4:rc3:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.4:rc3:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.5:*:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.5:*:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.6:*:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.6:*:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.7:*:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.7:*:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.8:*:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.8:*:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.9:*:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.9:*:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.9:-:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.9:-:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.9:rc1:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.9:rc1:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.10:*:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.10:*:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.10:-:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.10:-:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.11:-:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.11:-:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.11:rc2:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.11:rc2:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.12:*:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.12:*:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.13:-:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.13:-:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.13:rc1:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.13:rc1:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.14:-:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.14:-:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.14:rc1:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.14:rc1:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.15:-:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.15:-:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.15:rc1:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.15:rc1:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.16:*:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.16:*:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.17:*:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.17:*:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:10.11.18:*:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:10.11.18:*:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:11.5.0:-:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:11.5.0:-:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:11.5.0:rc1:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:11.5.0:rc1:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:11.5.0:rc2:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:11.5.0:rc2:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:11.5.0:rc3:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:11.5.0:rc3:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:11.5.1:*:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:11.5.1:*:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:11.5.1:-:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:11.5.1:-:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:11.5.2:-:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:11.5.2:-:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:11.5.2:rc1:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:11.5.2:rc1:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:11.5.2:rc2:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:11.5.2:rc2:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:11.5.2:rc3:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:11.5.2:rc3:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:11.5.3:*:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:11.5.3:*:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:11.5.4:*:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:11.5.4:*:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:11.5.5:*:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:11.5.5:*:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:11.5.6:*:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:11.5.6:*:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:11.6.0:-:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:11.6.0:-:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:11.6.0:rc1:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:11.6.0:rc1:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:11.6.0:rc2:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:11.6.0:rc2:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:11.6.0:rc3:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:11.6.0:rc3:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:11.6.0:rc4:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:11.6.0:rc4:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:11.6.1:-:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:11.6.1:-:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:11.6.1:rc1:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:11.6.1:rc1:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:11.6.2:*:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:11.6.2:*:*:*:*:*:*:*
  • cpe:2.3:a:mattermost:mattermost_server:11.6.3:*:*:*:*:*:*:*
    cpe:2.3:a:mattermost:mattermost_server:11.6.3:*:*:*:*:*:*:*

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