Mattermost: Insufficiently Protected Credentials
CVE-2026-6517 is a high-severity insufficiently protected credentials vulnerability in Mattermost, with a CVSS 3.1 base score of 7.7 out of 10. It can be exploited remotely over the network by an attacker with a low-privileged account, without any action from a user. Affected Mattermost versions: 5.5.13 and earlier. The record does not name a fixed version, so check the vendor advisory. There is no confirmed exploitation so far. EPSS estimates a 0.19% chance of exploitation within 30 days, which is low compared with other CVEs.
High severity with no evidence of exploitation so far.
What to do
- Check which version of Mattermost you run against the affected versions below, and install the vendor's latest release.
- Read the advisory on mattermost.com for workarounds and exact fixed builds.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| Mattermost | Mattermost | 5.5.13 and earlier | Not stated |
| Mattermost | Mattermost Desktop | 5.13.0 and earlier; 6.1.0 through 6.1.5 | Not stated |
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- NetworkCan be exploited remotely over a network, including the internet.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- LowA basic user account is enough.
- User interaction
- NoneNo action by a victim is required.
- Scope
- ChangedThe attack can reach beyond the vulnerable component into other systems.
- Confidentiality
- HighAll data handled by the component can be read.
- Integrity
- NoneNothing can be modified.
- Availability
- NoneAvailability is not affected.
Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N. Scored by NVD.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 3.1 | 6.3 | Medium | CNA (mattermost.com) |
| CVSS 3.1 | 7.7 | High | NVD |
Official 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
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Insufficiently Protected Credentials
The product transmits or stores authentication credentials, but it uses an insecure method that is susceptible to unauthorized interception and/or retrieval.
How developers prevent this class of flaw
- Use an appropriate security mechanism to protect the credentials.
- Make appropriate use of cryptography to protect the credentials.
- Use industry standards to protect the credentials (e.g. LDAP, keystore, etc.).
More Insufficiently Protected Credentials vulnerabilities (CWE-522)
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: none, automatable: no, technical impact: partial).
- Record last changed materially (score, affected products, status or references).
References
Advisories
Other vulnerabilities in Mattermost
- 8.0 High severity
- 7.1 High severity
- 9.3 Critical severity
- 7.1 High severity
Recent Insufficiently Protected Credentials vulnerabilities
- 9.1 Critical severity
- 7.5 High severity
- 7.4 High severity
- 7.5 High severity
- 7.1 High severity
- 8.1 High severity
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