CVE-2026-96259: Mattermost server-side request forgery via OAuth endpoints configurable by a System Administrator

Severity: medium (CVSS 5.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-09-22 · updated 2026-10-07
Weakness: CWE-918

Description

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

Affected products

Mattermost: from 11.7.0 before 11.7.11; from 11.8.0 before 11.8.6; from 11.9.0 before 11.9.2; from 11.10.0 before 11.10.2 — fixed in 11.10.2, 11.7.11, 11.8.6, 11.9.2

How to fix

Upgrade to a fixed version: Mattermost 11.10.2, 11.7.11, 11.8.6, 11.9.2. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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