What is CVE-2026-2455?

CVE-2026-2455 is a medium-severity vulnerability in Mattermost, classified under Server-Side Request Forgery (SSRF). CVSS score: 4.3/10. Published 2026-03-16.

How severe is CVE-2026-2455?

Medium severity. CVSS v3 base score is 4.3 out of 10.

SSRF in Mattermost

CVE-2026-2455

Mattermost versions 11.3.x <= 11.3.0, 11.2.x <= 11.2.2, 10.11.x <= 10.11.10 fail to canonicalize IPv4-mapped IPv6 addresses before reserved IP validation which allows an attacker to perform SSRF attacks against internal services via IPv4-mapped IPv6 literals (e.g., [::ffff:127.0.0.1]).. Mattermost Advisory ID: MMSA-2026-00585

Vulnerability class: SSRF (Server-Side Request Forgery)

EPSS: 0.002 (6.1th percentile) — read the EPSS interpretation.

CVSS v3 metric

CVSS v3 base score 4.3 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N.

Affected products

Mattermost — versions 11.3.0, 11.2.0, 10.11.0
Mattermost Mattermost_server

Weakness classification (CWE)

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

References

responsibledisclosure@mattermost.com (vendor-advisory, Vendor Advisory)

Frequently asked questions

What is CVE-2026-2455?: CVE-2026-2455 is a medium-severity vulnerability in Mattermost, classified under Server-Side Request Forgery (SSRF). CVSS score: 4.3/10. Published 2026-03-16.
How severe is CVE-2026-2455?: Medium severity. CVSS v3 base score is 4.3 out of 10.