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CVE-2026-4339— SSRF via unvalidated attachment URLs in Mattermost Agents plugin MCP server

Quick assessment

Affected
Mattermost Mattermost
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Mattermost是美国Mattermost公司开源的一个开源协作平台。 Mattermost 10.11.3及之前的10.11.x版本、11.6.3及之前的11.6.x版本和11.5.6及之前的11.5.x版本存在服务端请求伪造漏洞，该漏洞源于Mattermost Agents插件MCP服务器未能验证附件URL，可能导致具有MCP服务器stdio模式访问权限的攻击者执行服务端请求伪造，并通过在帖子创建请求中提供内部URL作为文件附件来渗漏内部网络服务数据。

CVSS 6.5 · Medium EPSS 0.14% · P3

Possible ATT&CK Techniques 1 AI

T1113 · Screen Capture

Affected Version Matrix 7

VendorProduct Version RangeStatus
Mattermost Mattermost 10.11.0≤ 10.11.18 affected
11.6.0≤ 11.6.3 affected
11.5.0≤ 11.5.6 affected
11.7.0 unaffected
10.11.19 unaffected
11.6.4 unaffected
11.5.7 unaffected
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I. Basic Information for CVE-2026-4339

Vulnerability Information
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Vulnerability Title
SSRF via unvalidated attachment URLs in Mattermost Agents plugin MCP server
Source: CVE Program / CVE List V5
Vulnerability 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
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
服务端请求伪造(SSRF)
Source: CVE Program / CVE List V5
Vulnerability Title
Mattermost 服务端请求伪造漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Mattermost是美国Mattermost公司开源的一个开源协作平台。 Mattermost 10.11.3及之前的10.11.x版本、11.6.3及之前的11.6.x版本和11.5.6及之前的11.5.x版本存在服务端请求伪造漏洞，该漏洞源于Mattermost Agents插件MCP服务器未能验证附件URL，可能导致具有MCP服务器stdio模式访问权限的攻击者执行服务端请求伪造，并通过在帖子创建请求中提供内部URL作为文件附件来渗漏内部网络服务数据。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
Mattermost Mattermost 10.11.0 ~ 10.11.18 -

II. Public POCs for CVE-2026-4339

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III. Intelligence Information for CVE-2026-4339

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Same Patch Batch · Mattermost · 2026-06-26 · 4 CVEs total

CVE-2026-9699 6.8 MEDIUM Mattermost Agents plugin logs unsanitized OpenAI API keys on authentication errors
CVE-2026-13426 5.4 MEDIUM Client4 fails to validate path parameters
CVE-2026-3472 3.5 LOW Markdown image rendering bypass in AI bot tool result posts in Mattermost

IV. Related Vulnerabilities

Same product: Mattermost
Same vendor: Mattermost
Same weakness: CWE-918

V. Comments for CVE-2026-4339

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