CVE-2026-4339

MEDIUMPre-NVD 6.56.5—
EchelonGraph scoreHIGH confidence

Score 6.5 from GitHub Security Advisory published 2026-06-26. a secondary CVSS source baseline 6.5; sources differ by 0.0.

Triggered by: GitHub Security Advisory CVSS
Sources: epss, ghsa, secondary
Trending — 3 sources updated this week
6.5
EchelonGraph verdictMonitorLow exploitation likelihood right now — keep watching.
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS: 0%CVSS: 6.5Exploit: NoneExposed: 0

No vendor fix yet — apply a workaround or compensating control (WAF / firewall / segmentation) and watch for a patch.

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

CVSS v3
6.5
EG Score
6.5(high)
EPSS
1.2%
KEV
Not listed

Published

June 26, 2026

Last Modified

June 29, 2026

Advisory Details (1)

Auto-updated Jun 26, 2026
No patch confirmed yet.
generic

Security Updates

https://mattermost.com/security-updates

Vendor Advisories for CVE-2026-4339(1)

These vendors published their own advisory mentioning this CVE — often with vendor-specific remediation steps + affected product lists not in NVD.

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Data Freshness Timeline

(refreshed 19× in last 7d / 30× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

  1. 2026-07-07 07:00 UTCEG score recompute
  2. 2026-07-07 07:00 UTCGHSA enrichment
  3. 2026-07-06 05:29 UTCEG score recompute
  4. 2026-07-06 05:28 UTCGHSA enrichment
  5. 2026-07-06 02:23 UTCEPSS rescore
  6. 2026-07-05 03:58 UTCEG score recompute
  7. 2026-07-05 03:58 UTCGHSA enrichment
  8. 2026-07-05 02:30 UTCEPSS rescore
  9. 2026-07-04 06:31 UTCEPSS rescore
  10. 2026-07-04 02:21 UTCEG score recompute
  11. 2026-07-04 02:21 UTCGHSA enrichment
  12. 2026-07-03 00:49 UTCEG score recompute
  13. 2026-07-03 00:49 UTCGHSA enrichment
  14. 2026-07-01 23:19 UTCEG score recompute
  15. 2026-07-01 23:18 UTCGHSA enrichment
  16. 2026-07-01 15:07 UTCEPSS rescore
  17. 2026-06-30 23:22 UTCEPSS rescore
  18. 2026-06-30 21:47 UTCEG score recompute
  19. 2026-06-30 21:47 UTCGHSA enrichment
  20. 2026-06-29 20:15 UTCEG score recompute
  21. 2026-06-29 20:15 UTCGHSA enrichment
  22. 2026-06-28 18:45 UTCEG score recompute
  23. 2026-06-28 18:45 UTCGHSA enrichment
  24. 2026-06-28 14:07 UTCEPSS rescore
  25. 2026-06-28 04:56 UTCEPSS rescore
Show 5 more
  1. 2026-06-28 04:56 UTCEPSS rescore
  2. 2026-06-27 17:15 UTCEG score recompute
  3. 2026-06-27 17:15 UTCGHSA enrichment
  4. 2026-06-26 15:44 UTCEG score recompute
  5. 2026-06-26 15:44 UTCGHSA enrichment

Related CVEs(same CWE)

Frequently asked(5)

What is CVE-2026-4339?
CVE-2026-4339 is a medium vulnerability published on June 26, 2026. 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…
When was CVE-2026-4339 disclosed?
CVE-2026-4339 was first published in the National Vulnerability Database on June 26, 2026, with the most recent update on June 29, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2026-4339 actively exploited?
CVE-2026-4339 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 1.2% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.
What is the CVSS score of CVE-2026-4339?
CVE-2026-4339 has a CVSS v3 base score of 6.5 (NVD).
How do I remediate CVE-2026-4339?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2026-4339, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

Dependency Blast Radius

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