Is CVE-2026-4339 in the CISA KEV catalog?

No. CVE-2026-4339 is not currently on the CISA Known Exploited Vulnerabilities list.

What is the CVSS severity of CVE-2026-4339?

CVE-2026-4339 has a CVSS v3 base score of 6.5 (MEDIUM).

What is the EPSS score for CVE-2026-4339?

CVE-2026-4339 has an EPSS score of 0.10% (1th percentile), estimating the probability of exploitation in the next 30 days.

How do I fix CVE-2026-4339?

Upgrade mattermost server to 11.6.4 or later.

CVE & CISA-KEV Catalog

CVE-2026-4339

MEDIUM

6.5

CVSS v3

NVD

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

CWE-918Server-Side Request Forgery (SSRF)

How to fix

Remediation Available

mattermost serverNVD

Affected:>= 11.6.0, < 11.6.4Fixed in:11.6.4CVE-2026-4339derived from NVD

Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.

CVSS v3 Vector

Exploitability

Attack VectorLocal

Attack ComplexityLow

Privileges RequiredLow

User InteractionNone

ScopeChanged

Impact

ConfidentialityHigh

IntegrityNone

AvailabilityNone

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

Exploit Intelligence

0.10%probability of exploitation in 30 days

1stpercentile

Low risk: more likely to be exploited than 1% of all known CVEs.

References

Vendor Advisory1

mattermost.com

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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-06-29.