EUVD-2026-36725
GHSA-2wmq-3m94-g4jr
Severity by source
AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N
Network-reachable via posted image, low complexity, requires a low-privileged server account (PR:L), no user interaction, credentials of another user are disclosed so scope changes and C:H with no I/A impact.
Primary rating from Vendor (Mattermost).
CVSS VectorNVD
Lifecycle Timeline
6DescriptionNVD
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
AnalysisAI
Credential leakage in Mattermost Desktop App versions up to 6.1 and 5.5.13.0 allows authenticated server users to harvest NTLM credentials from other users by embedding images that resolve to attacker-controlled external web servers. The client fails to enforce a strict domain allow list before forwarding Windows NTLM authentication challenges, so any post in a server without the image proxy enabled becomes a credential capture vector. No public exploit identified at time of analysis and EPSS is low (0.18%), but the scope-change CVSS of 7.7 reflects cross-user impact.
Technical ContextAI
The Mattermost Desktop App is an Electron/Chromium-based wrapper that renders chat content including remote images. On Windows, when the underlying network stack encounters an HTTP 401 challenge with the NTLM scheme from a remote host, it can transparently forward the logged-in user's NT hash / NTLMv2 response if the host is considered trusted. This issue is a CWE-522 (Insufficiently Protected Credentials) caused by an overly permissive allow list of domains permitted to receive NTLM authentication. The Mattermost server's image proxy, when enabled, normally rewrites embedded image URLs so the desktop client only ever connects back to the trusted Mattermost host; with the proxy disabled, the client fetches embedded images directly and the missing domain restriction lets NTLM travel to arbitrary external endpoints. Affected CPE is cpe:2.3:a:mattermost:mattermost:*:*:*:*:*:*:*:* per NVD.
RemediationAI
Upgrade the Mattermost Desktop App to a fixed release as listed in MMSA-2026-00651 on https://mattermost.com/security-updates (patch available per vendor advisory; an exact fix version was not provided in the input, so confirm against the advisory before rollout). As an immediate compensating control on the server side, enable the Mattermost image proxy (System Console → Site Configuration → Image Proxy) so embedded images are fetched server-side and never cause the desktop client to connect to untrusted external hosts - note this adds latency and bandwidth on the server and may break some external image referrers. On Windows endpoints, you can further reduce exposure by hardening LAN Manager authentication (Group Policy: Network security: Restrict NTLM: Outgoing NTLM traffic to remote servers = Deny all or Audit, plus enforce NTLMv2-only) to prevent silent credential forwarding to arbitrary internet hosts, accepting that this may break legitimate integrations that rely on NTLM SSO. Monitor outbound HTTP from desktop clients for unexpected 401/NTLM exchanges to non-corporate domains.
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Same weakness CWE-522 – Insufficiently Protected Credentials
View allSame technique Information Disclosure
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External POC / Exploit Code
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