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
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N
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. …
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Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata
Vulnerability AssessmentAI
| Exploitation | Attacker must hold a low-privileged authenticated account on a Mattermost server (PR:L) and the server must have the image proxy feature disabled, which is the gating configuration explicitly named in the description. … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | Signals are mixed and lean toward moderate, targeted risk rather than mass exploitation. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | A low-privileged user on a Mattermost server whose admin has not enabled the image proxy posts a message containing an inline image referencing https://attacker.example/x.png, where the attacker's server returns an HTTP 401 with WWW-Authenticate: NTLM. When a Windows colleague's Desktop App renders the channel, the OS forwards the user's NTLMv2 response to the attacker, who relays it or cracks it offline to recover the victim's domain credentials. … |
| Remediation | 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). … Detailed patch versions, workarounds, and compensating controls in full report. |
Recommended ActionAI
Within 24 hours: Identify all Mattermost Desktop App installations and document versions in use (prioritize 6.1 and earlier, 5.5.13.0 and earlier). …
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External POC / Exploit Code
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