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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

PUBLISHED Reserved 2026-03-17 | Published 2026-06-26 | Updated 2026-06-26 | Assigner Mattermost




MEDIUM: 6.5CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N

Problem types

CWE-918: Server-Side Request Forgery (SSRF)

Product status

Default status
unaffected

10.11.0 (semver)
affected

11.6.0 (semver)
affected

11.5.0 (semver)
affected

11.7.0
unaffected

10.11.19
unaffected

11.6.4
unaffected

11.5.7
unaffected

Credits

s00me00ne finder

References

mattermost.com/security-updates (MMSA-2026-00635) vendor-advisory

cve.org (CVE-2026-4339)

nvd.nist.gov (CVE-2026-4339)

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