SSRF via unvalidated attachment URLs in Mattermost Agents plugin MCP server

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Summary

CVECVE-2026-4339
StatePUBLISHED
AssignerMattermost
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-06-26 15:16:39 UTC
Updated2026-06-29 19:26:34 UTC
DescriptionMattermost 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

Risk And Classification

Primary CVSS: v3.1 6.5 MEDIUM from [email protected]

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

EPSS: 0.001040000 probability, percentile 0.012320000 (date 2026-06-29)

Problem Types: CWE-918 | CWE-918 CWE-918: Server-Side Request Forgery (SSRF)

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary6.5MEDIUMCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N
3.1CNACVSS6.5MEDIUMCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N

CVSS v3.1 Breakdown

Attack Vector
Local
Attack Complexity
Low
Privileges Required
Low
User Interaction
None
Scope
Changed
Confidentiality
High
Integrity
None
Availability
None

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

NVD Known Affected Configurations (CPE 2.3)

TypeVendorProductVersionUpdateEditionLanguage
Application Mattermost Mattermost Server All All All All

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Mattermost Mattermost affected 10.11.0 10.11.18 semver Not specified
CNA Mattermost Mattermost affected 11.6.0 11.6.3 semver Not specified
CNA Mattermost Mattermost affected 11.5.0 11.5.6 semver Not specified
CNA Mattermost Mattermost unaffected 11.7.0 Not specified
CNA Mattermost Mattermost unaffected 10.11.19 Not specified
CNA Mattermost Mattermost unaffected 11.6.4 Not specified
CNA Mattermost Mattermost unaffected 11.5.7 Not specified

References

ReferenceSourceLinkTags
mattermost.com/security-updates [email protected] mattermost.com Vendor Advisory
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

Vendor Comments And Credit

Discovery Credit

CNA: s00me00ne (en)

Additional Advisory Data

Solutions

CNA: Update Mattermost to versions 11.7.0, 10.11.19, 11.6.4, 11.5.7 or higher.

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