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 M
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
Details
Original advisory: https://nvd.nist.gov/vuln/detail/CVE-2026-4339
Exploitation outlook
EPSS (FIRST.org) estimates each CVE’s probability of exploitation in the next 30 days — here is the CSIRTS.com read on those numbers.
- Low exploitation riskCVE-2026-43390.10% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 1% of all scored CVEs.
Referenced CVEs
| CVE | CSIRTS overview | External |
|---|---|---|
| CVE-2026-4339 | coverage & exploitation status | NVD · CVE.org |
Recent advisories for Mattermost
A cluster of recent advisories against the same product widens the attack surface — attackers routinely chain freshly published CVEs on one product, so review these together.
- mediumGHSA-4m3v-q747-pc6h: OpenClaw: Mattermost slash token revocation could lag until monitor refreshghsa · 2026-07-02
- mediumGHSA-gp79-m99v-gjmh: OpenClaw: Mattermost handlers could fall open when channel type was missingghsa · 2026-07-02
- unknownMultiple vulnerabilities in Mattermost Server (June 29, 2026)cert-fr-avis · 2026-06-29
- mediumCVE-2026-9699: Mattermost Plugins versions <=11.6 10.18.11 11.3.6 11.6.5.0 fail to sanitize error responses fr…nvd · 2026-06-26
- lowCVE-2026-3472: Mattermost versions 10.11.x <= 10.11.18, 11.6.x <= 11.6.3, 11.5.x <= 11.5.6 fail to properly ap…nvd · 2026-06-26
- mediumCVE-2026-13426: The Mattermost Go module github.com/mattermost/mattermost/server/public versions < v0.1.22 fai…nvd · 2026-06-26
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