CVE-2026-12985: Mattermost DCR redirect URI allowlist bypass via improper URL component validation

Severity: critical (CVSS 9.3 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: 2026-09-14 · updated 2026-10-07
Weakness: CWE-601

Description

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 products

Mattermost: from 11.7.0 before 11.7.8; from 11.8.0 before 11.8.5; from 11.9.0 before 11.9.1 — fixed in 11.7.8, 11.8.5, 11.9.1

How to fix

Upgrade to a fixed version: Mattermost 11.7.8, 11.8.5, 11.9.1. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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