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CVE-2026-6517 Mattermost CVE debrief

CVE-2026-6517 is a medium-severity vulnerability in the Mattermost Desktop App. Versions <=6.1 5.5.13.0 fail to restrict the allow list of domains to which NTLM credentials were forwarded. This 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. The vulnerability has a CVSS score of 6.3 and was published on [cvePublishedAt].

Vendor
Mattermost
Product
Unknown
CVSS
MEDIUM 6.3
CISA KEV
Not listed in stored evidence
Original CVE published
2026-06-15
Original CVE updated
2026-06-16
Advisory published
2026-06-15
Advisory updated
2026-06-16

Who should care

Users of Mattermost Desktop App versions <=6.1 5.5.13.0 should update to a patched version to prevent credential interception.

Technical summary

The Mattermost Desktop App did not properly restrict the forwarding of NTLM credentials to external domains. An attacker could exploit this by embedding an image that routes to an external web server, allowing them to intercept credentials from other users.

Defensive priority

medium

Recommended defensive actions

  • Update Mattermost Desktop App to a version greater than 6.1 5.5.13.0
  • Enable the image proxy feature on Mattermost servers

Evidence notes

The CVE was published on [cvePublishedAt] and has a CVSS score of 6.3. The vulnerability was reported by [[email protected]].

Sources and references

Verified primary and authoritative sources

  • CVE-2026-6517 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-6517

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-6517 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-6517

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.