CVE-2026-4339SSRF via unvalidated attachment URLs in Mattermost Agents plugin MCP server
- 6.5MediumCVSS base score
- 0.14%Exploitation in 30 daysEPSS · top 97% of all CVEs
- NoKnown exploitednot in CISA's KEV catalog
- NonePublic exploitsnone known yet
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
Timeline
ID reserved
By Mattermost
- 3 months later
· 3 months later
Published
Changes after publication (a new score, a public exploit, EPSS crossing 1, 10 or 50 %, leaving KEV) are kept since November 2026; earlier ones are not known.
Scores
| Scored by | CVSS | Score | Vector |
|---|---|---|---|
| MattermostCNA (assigner) | 3.1 | Medium6.5 | CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N |
| NVDNVD · Analyzed | 3.1 | Medium6.5 | CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N |
- EPSS (exploitation in 30 days)
- 0.14% · top 97% of all CVEs (12 Nov 2026)
- CISA SSVC
- Exploitation: none · Automatable: no · Technical impact: partial
The list uses the assigner's score when there is one, then CISA's, then NVD's: Medium 6.5.
Affected products
- affected 10.11.0 – ≤ 10.11.18
- affected 11.6.0 – ≤ 11.6.3
- affected 11.5.0 – ≤ 11.5.6
- unaffected 11.7.0
- unaffected 10.11.19
- unaffected 11.6.4
- unaffected 11.5.7
What to do
Solution: Update Mattermost to versions 11.7.0, 10.11.19, 11.6.4, 11.5.7 or higher.
References
References1
- MMSA-2026-00635 (opens in a new tab)vendor-advisory
Sources
The same CVE in the primary databases and aggregators, to check against.
- CVE.org record (opens in a new tab)
- CVE List JSON (GitHub) (opens in a new tab)
- NVD (opens in a new tab)
- FIRST EPSS (opens in a new tab)
- EUVD EUVD-2026-39779 (opens in a new tab)
- OSV.dev (opens in a new tab)
- GitHub Advisories (opens in a new tab)
- CIRCL Vulnerability-Lookup (opens in a new tab)
Badge for a README or a ticket
[](https://cveledger.app/cve/CVE-2026-4339)Record last updated .