CVE-2026-6517Network attack vector

Credential interception via unrestricted NTLM forwarding

Published Jun 15, 2026 · Updated Jun 15, 2026

Credential exposure in Mattermost Desktop App 6.x before 6.2.0 and 5.13.x before 5.13.6 allows remote authenticated users to intercept NTLM credentials. The Electron session permits NTLM or Negotiate authentication to arbitrary hosts instead of limiting credential forwarding to configured Mattermost server hostnames. Exploitation requires server access, a server with the image proxy disabled, and a crafted image whose external URL causes another desktop client to authenticate.

CVSS severity6.3
Medium
EPSS probability0.19%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Credential exposure in Mattermost Desktop App 6.x before 6.2.0 and 5.13.x before 5.13.6 allows remote authenticated users to intercept NTLM credentials. The Electron session permits NTLM or Negotiate authentication to arbitrary hosts instead of limiting credential forwarding to configured Mattermost server hostnames. Exploitation requires server access, a server with the image proxy disabled, and a crafted image whose external URL causes another desktop client to authenticate.

The record

CVE
CVE-2026-6517
Published
Jun 15, 2026
Updated
Jun 15, 2026
Vendor
Mattermost
Product
Mattermost
Classifications
CWE-522, T1187
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Jun 15, 2026CVE publishedPublication date reported by the CVE source.
  2. Jun 15, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=0 <=5.5.13

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Mattermost · Mattermostversion=0 <=5.5.13

Published CVSS scores

7.7NIST NVDCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N
6.3Mattermost, Inc.CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:N/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-522Insufficiently Protected Credentials
T1187Forced Authentication

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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