CVE-2026-6517: Mattermost Desktop App versions <=6.1 5.5.13.0 fail to restrict the allow list of domains to which NTLM creden

Q: How urgent is CVE-2026-6517?

Priority 24/100. No elevated risk signals yet.

Routine

MEDIUM 6.3UPGRADE

CVE-2026-6517 is a medium-severity flaw (CVSS 6.3) affecting mattermost desktop. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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

Why this priority

No elevated risk signals yet

A fixed release or distro patch exists.

Risk facts

CVSS score	6.3 (Medium)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	Low
User interaction	None
EPSS exploit chance	0.2% (top 92%)
CISA exploited list	No
Weakness	CWE-522
Published	2026-06-15
Last modified	2026-06-17

Publisher (CNA): Mattermost, Inc. · Vendor: mattermost

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-06-15 14:16 UTCPublished CVE record published
2026-06-17 11:00 UTCUpdated Record last modified

mattermost mattermost desktop UPGRADE

Affected: <= 5.13.0 · >= 6.1.0 and <= 6.1.5

Fixed in: newer than 6.1.5

Safe to downgrade to: a release before 6.1.0

1. Check installed version

apt-cache policy mattermost-desktop || rpm -q mattermost-desktop || mattermost-desktop --version

2. UPGRADE to newer than 6.1.5 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade mattermost-desktop

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade mattermost-desktop

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 6.1.0 (Debian / Ubuntu)

apt-cache madison mattermost-desktop sudo apt install mattermost-desktop=<VERSION_BEFORE_6.1.0> sudo apt-mark hold mattermost-desktop

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list mattermost-desktop sudo dnf downgrade mattermost-desktop-<VERSION_BEFORE_6.1.0> sudo dnf versionlock add mattermost-desktop

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-6517?

mattermost desktop: upgrade to newer than 6.1.5. Exact commands for each OS are on this page.

Is CVE-2026-6517 being exploited?

CVE-2026-6517 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-6517 affect?

CVE-2026-6517 affects mattermost desktop. Severity is medium (CVSS 6.3). 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 interce

How urgent is CVE-2026-6517?

Priority 24/100. No elevated risk signals yet.

Software pages

Mattermost vulnerabilities

Related CVEs

CVE-2026-9602 MEDIUM 6.5 UPGRADE Mattermost Desktop App versions <=6.2 6.0.2 5.6.13.0 fail to validate payloads sent from the Mattermost Web Ap
CVE-2026-8075 MEDIUM 6.5 UPGRADE Mattermost Desktop App versions <=6.2 5.5.13 6.0.2.0 fail to properly null check when checking for headers in
CVE-2026-75025 MEDIUM 4.7 UPGRADE Mattermost Desktop App versions <=6.2 6.2.2.0 Fixed an issue where Mattermost Desktop did not sufficiently res
CVE-2026-75587 LOW 3.6 UPGRADE Mattermost Desktop App versions <=6.2 6.2.2.0 fail to redact the pre-auth secret when generating a diagnostics
CVE-2026-8683 MEDIUM 6.5 UPGRADE Mattermost Desktop App versions <=6.1 5.5.13.0 fail to account for attempting to open extremely long URLs in t

References

← Back to latest CVEs