How severe is CVE-2026-6517?

CVE-2026-6517 has a CVSS 3.x base score of 6.3, rated medium severity. It is exploitable over network with high attack complexity, requires low privileges and no user interaction. Impact on confidentiality is high, integrity none, and availability none.

Is CVE-2026-6517 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (8th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-6517?

CVE-2026-6517 primarily affects Mattermost Mattermost Desktop. In total, 2 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.

How do I fix CVE-2026-6517?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.

Does CVE-2026-6517 have an EU (EUVD) identifier?

Yes. CVE-2026-6517 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-36725.

When was CVE-2026-6517 published?

CVE-2026-6517 was published on 2026-06-15 and last updated on 2026-06-17.

CVE-2026-6517

CVE-2026-6517 is a medium-severity vulnerability in Mattermost Mattermost Desktop with a CVSS 3.x base score of 6.3. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-522.

Key facts

Severity: Medium (CVSS 3.x base score 6.3)
EPSS exploit prediction: 0% (8th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-36725
Weakness: CWE-522
Affected product: Mattermost Mattermost Desktop
Published: 2026-06-15
Last modified: 2026-06-17

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

Frequently asked questions

What is CVE-2026-6517?: 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
How severe is CVE-2026-6517?: CVE-2026-6517 has a CVSS 3.x base score of 6.3, rated medium severity. It is exploitable over network with high attack complexity, requires low privileges and no user interaction. Impact on confidentiality is high, integrity none, and availability none.
Is CVE-2026-6517 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (8th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-6517?: CVE-2026-6517 primarily affects Mattermost Mattermost Desktop. In total, 2 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.
How do I fix CVE-2026-6517?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
Does CVE-2026-6517 have an EU (EUVD) identifier?: Yes. CVE-2026-6517 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-36725.
When was CVE-2026-6517 published?: CVE-2026-6517 was published on 2026-06-15 and last updated on 2026-06-17.

References

https://mattermost.com/security-updates

Affected products (2)

cpe:2.3:a:mattermost:mattermost_desktop:*:-:*:*:*:*:*:*
cpe:2.3:a:mattermost:mattermost_desktop:*:*:*:*:*:*:*:*

More vulnerabilities in Mattermost Mattermost Desktop

CVE-2016-11064 — Critical (CVSS 9.8): An issue was discovered in Mattermost Desktop App before 3.4.0. Strings could be executed as code via injection.
CVE-2019-20856 — Critical (CVSS 9.8): An issue was discovered in Mattermost Desktop App before 4.3.0 on macOS. It allows dylib injection.
CVE-2019-20861 — High (CVSS 8.8): An issue was discovered in Mattermost Desktop App before 4.2.2. It allows attackers to execute arbitrary code via a…
CVE-2026-1046 — High (CVSS 7.6): Mattermost Desktop App versions <=6.0 6.2.0 5.2.13.0 fail to validate help links which allows a malicious Mattermost…
CVE-2020-14456 — High (CVSS 7.3): An issue was discovered in Mattermost Desktop App before 4.4.0. The Same Origin Policy is mishandled during…
CVE-2026-8683 — Medium (CVSS 6.5): Mattermost Desktop App versions <=6.1 5.5.13.0 fail to account for attempting to open extremely long URLs in the…

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