Medium severity · published Jun 26, 2026
CVE-2026-4339
SSRF via unvalidated attachment URLs in Mattermost Agents plugin MCP server
6.5
CVSS
0.1%
EPSS
No
On KEV
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
CVSS vector
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N
Attack vector
Local
Complexity
Low
Privileges
Low
User interaction
None
Confidentiality
High
Integrity
None
Availability
None
Scope
Changed
Exploitation
Is it being used?
Exploit prediction (EPSS)0.1% · higher than 3% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
MA
Mattermost · Mattermost
10.11.0 through 10.11.18; 11.6.0 through 11.6.3; 11.5.0 through 11.5.6 · source: vendor
MS
Mattermost Server · Mattermost
10.11.0 before 10.11.19; 11.5.0 before 11.5.7; 11.6.0 before 11.6.4 · source: NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-4339
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-4339
Vendor Advisory
https://mattermost.com/security-updates
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