High severity · published Jun 15, 2026
CVE-2026-6517
Mattermost Desktop App fails to restrict the allow list of domains which NTLM credentials are passed
7.7
CVSS
0.2%
EPSS
No
On KEV
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
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N
Attack vector
Network
Complexity
Low
Privileges
Low
User interaction
None
Confidentiality
High
Integrity
None
Availability
None
Scope
Changed
Exploitation
Is it being used?
Exploit prediction (EPSS)0.2% · higher than 8% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
MA
Mattermost · Mattermost
through 5.5.13 · source: vendor
MD
Mattermost Desktop · Mattermost
through 5.13.0; 6.1.0 through 6.1.5 · source: NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-6517
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-6517
Vendor Advisory
https://mattermost.com/security-updates
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