CVE-2026-12985: Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7 Mattermost failed to validate Dynamic Client Registration redirect URIs by URL…

PriorityP354critical9.3CVSS 3.1

AVNACLPRNUIRSCCHIHAN

EPSS

0.27%

17.9th percentile

Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7 Mattermost failed to validate Dynamic Client Registration redirect URIs by URL component (matching glob patterns against the raw URI string instead) which allows a remote unauthenticated attacker to register an OAuth client with an attacker-controlled callback host that bypasses the configured redirect URI allowlist via a crafted redirect URI that places an allowlisted host/path suffix inside the query string.. Mattermost Advisory ID: MMSA-2026-00700

Affected

6 ranges

Vendor	Product	Version range	Fixed in
mattermost	mattermost	11.7.0 – 11.7.7	—
mattermost	mattermost	11.8.0 – 11.8.4	—
mattermost	mattermost	11.9.0 – 11.9.0	—
mattermost	mattermost_server	>= 11.7.0 < 11.7.8	11.7.8
mattermost	mattermost_server	>= 11.8.0 < 11.8.5	11.8.5
mattermost	mattermost_server	>= 11.9.0 < 11.9.1	11.9.1

Stop checking back — get the weekly exploitation signal.

Every Monday: what got weaponized or added to CISA KEV in the last seven days — each CVE cross-linked to its PoC, Nuclei template, and detection rule. Free, one email a week, unsubscribe in one click.