CVE-2026-6517
Received Received - Intake
Mattermost Desktop App NTLM Credential Forwarding Flaw

Publication date: 2026-06-15

Last updated on: 2026-06-15

Assigner: Mattermost, Inc.

Description
Mattermost Desktop App versions <=6.1 5.5.13.0 fail to restrict the allow list of domains to which NTLM credentials were forwarded to in the Mattermost Desktop App which allows any user on a server without the image proxy enabled to intercept other users credentials via embedding an image that routes to an external web server. Mattermost Advisory ID: MMSA-2026-00651
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Meta Information
Published
2026-06-15
Last Modified
2026-06-15
Generated
2026-06-15
AI Q&A
2026-06-15
EPSS Evaluated
N/A
NVD
CVE-2026-6517
EUVD
EUVD-2026-36725
Affected Vendors & Products
Showing 1 associated CPE
Vendor Product Version / Range
mattermost desktop_app to 6.1 (inc)
Helpful Resources
Exploitability
CWE
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KEV
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CWE ID Description
CWE-522 The product transmits or stores authentication credentials, but it uses an insecure method that is susceptible to unauthorized interception and/or retrieval.
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Executive Summary

This vulnerability exists in Mattermost Desktop App versions up to 6.1 5.5.13.0, where the application fails to properly restrict the allow list of domains to which NTLM credentials are forwarded.

Because of this, any user on a server that does not have the image proxy enabled can intercept other users' credentials by embedding an image that routes to an external web server.

Impact Analysis

This vulnerability can lead to credential interception, where an attacker can capture NTLM credentials of other users on the same server.

Such interception can compromise user accounts and potentially allow unauthorized access to sensitive information or systems.

Mitigation Strategies

To mitigate this vulnerability, you should upgrade the Mattermost Desktop App to a version later than 6.1 5.5.13.0 where the issue with unrestricted NTLM credential forwarding has been fixed.

Additionally, ensure that the image proxy feature is enabled on your Mattermost server to prevent users from intercepting credentials via embedded images routing to external servers.

Compliance Impact

The vulnerability in Mattermost Desktop App versions <=6.1 5.5.13.0 allows any user on a server without the image proxy enabled to intercept other users' NTLM credentials by embedding an image that routes to an external web server.

This unauthorized interception of credentials could lead to exposure of sensitive user information, which may impact compliance with data protection regulations such as GDPR and HIPAA that require safeguarding personal and sensitive data.

However, the provided context and resources do not explicitly discuss the direct impact of this vulnerability on compliance with these or other common standards and regulations.

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