CVE-2026-12985

Received Received - Intake

OAuth Client Registration Bypass in Mattermost

Publication date: 2026-09-14

Last updated on: 2026-09-15

Assigner: Mattermost, Inc.

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

CVSS Scores

EPSS Scores

Probability:
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Meta Information

Published

2026-09-14

Last Modified

2026-09-15

Generated

2026-09-15

AI Q&A

2026-09-14

EPSS Evaluated

N/A

NVD

CVE-2026-12985

EUVD

EUVD-2026-77520

Affected Vendors & Products

Vendor	Product	Version / Range
mattermost	mattermost	to 11.9.0 (inc)
mattermost	mattermost	to 11.8.4 (inc)
mattermost	mattermost	to 11.7.7 (inc)

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-601	The web application accepts a user-controlled input that specifies a link to an external site, and uses that link in a redirect.

Attack-Flow Graph

Executive Summary

Mattermost versions between 11.7.7 and 11.9.0 failed to properly validate OAuth client redirect URIs. Instead of matching against the full URL, it checked glob patterns against the raw URI string. This allowed attackers to bypass redirect URI allowlists by crafting a URI that placed an allowlisted host or path inside the query string.

Detection Guidance

The provided CVE data does not include specific detection methods or commands for identifying this vulnerability on a network or system. Review Mattermost logs for unusual OAuth client registrations or callback URI patterns. Check for redirects to unexpected hosts in web server logs.

Impact Analysis

An unauthenticated remote attacker could exploit this to register a malicious OAuth client with a callback host under their control. This could lead to unauthorized access to user accounts, data theft, or further attacks if the attacker can intercept OAuth tokens or redirect users to phishing sites.

Compliance Impact

This vulnerability could lead to unauthorized data access or exfiltration, violating GDPR's data protection principles and HIPAA's security requirements for safeguarding protected health information. Organizations using affected Mattermost versions may face compliance violations and potential regulatory penalties.

Mitigation Strategies

Update Mattermost to a patched version (11.9.1 or later, 11.8.5 or later, 11.7.8 or later) to address the OAuth client registration redirect URI validation flaw.

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