SSRF in Mattermost

CVE-2026-96259

Mattermost versions 11.9.x <= 11.9.1, 11.8.x <= 11.8.5, 11.7.x <= 11.7.10, 11.10.x <= 11.10.1 fail to apply the internal-connection filter to OAuth endpoint requests, which allows a System Administrator to make the server issue requests to internal network addresses and read the responses via the configured OAuth token and userinfo endpoints.. Mattermost Advisory ID: MMSA-2026-00776

Vulnerability class: SSRF (Server-Side Request Forgery)

Published .

CVSS v3 metric

CVSS v3 base score 5.5 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:C/C:L/I:L/A:N.

EPSS exploit prediction

No EPSS score from FIRST.org yet. New CVEs usually get one within a few days; no score means no signal yet, not low risk. How EPSS works.

Affected products

Weakness classification (CWE)

References

Frequently asked questions

What is CVE-2026-96259?
CVE-2026-96259 is a medium-severity vulnerability in Mattermost, classified under Server-Side Request Forgery (SSRF). CVSS score: 5.5/10. Published 2026-09-22.
How severe is CVE-2026-96259?
Medium severity. CVSS v3 base score is 5.5 out of 10.