CVE-2026-96259

Received Received - Intake

Server-Side Request Forgery in Mattermost OAuth Endpoints

Publication date: 2026-09-22

Last updated on: 2026-09-22

Assigner: Mattermost, Inc.

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

CVSS Scores

EPSS Scores

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Meta Information

Published

2026-09-22

Last Modified

2026-09-22

Generated

2026-09-23

AI Q&A

2026-09-23

EPSS Evaluated

N/A

NVD

CVE-2026-96259

Affected Vendors & Products

Vendor	Product	Version / Range
mattermost	mattermost	to 11.9.1 (inc)
mattermost	mattermost	to 11.8.5 (inc)
mattermost	mattermost	to 11.7.10 (inc)
mattermost	mattermost	to 11.10.1 (inc)

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-918	The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination.

Attack-Flow Graph

Executive Summary

Mattermost versions between 11.7.0 and 11.10.1 fail to enforce the internal-connection filter on OAuth endpoint requests. This allows a System Administrator to bypass network restrictions and make the server send requests to internal network addresses. The responses can then be read using the configured OAuth token and userinfo endpoints.

Detection Guidance

This vulnerability involves OAuth endpoint requests not being filtered for internal network access. To detect it, check Mattermost server logs for unusual OAuth token or userinfo endpoint requests to internal IP addresses. Review network traffic logs for outbound connections from the Mattermost server to internal network ranges. Ensure the internal-connection filter is properly configured in the Mattermost server settings.

Impact Analysis

An attacker with System Administrator privileges could exploit this to access internal network resources, potentially stealing sensitive data or performing unauthorized actions. This could lead to data breaches or further network compromise.

Compliance Impact

This vulnerability could lead to unauthorized access to sensitive data, violating GDPR's data protection requirements and HIPAA's security rules. Organizations may face compliance penalties, legal consequences, and reputational damage.

Mitigation Strategies

Upgrade Mattermost to a version that is not affected by this vulnerability. Specifically, upgrade to versions 11.9.2, 11.8.6, 11.7.11, or 11.10.2 or later to resolve the issue.

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