Is CVE-2026-4915 in the CISA KEV catalog?

No. CVE-2026-4915 is not currently on the CISA Known Exploited Vulnerabilities list.

What is the CVSS severity of CVE-2026-4915?

CVE-2026-4915 has a CVSS v3 base score of 6.5 (MEDIUM).

What is the EPSS score for CVE-2026-4915?

CVE-2026-4915 has an EPSS score of 0.28% (19th percentile), estimating the probability of exploitation in the next 30 days.

How do I fix CVE-2026-4915?

Upgrade mattermost server to 11.6.1 or later.

CVE & CISA-KEV Catalog

CVE-2026-4915

MEDIUM

6.5

CVSS v3

NVD

Description

Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to filter nil elements from outgoing webhook attachment payloads before processing, which allows an authenticated user to cause a denial of service (server process termination) via a crafted webhook callback response containing a null attachment entry.. Mattermost Advisory ID: MMSA-2026-00641

CWE-754

How to fix

Remediation Available

mattermost serverNVD

Affected:>= 11.6.0, < 11.6.1Fixed in:11.6.1CVE-2026-4915derived from NVD

Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.

CVSS v3 Vector

Exploitability

Attack VectorNetwork

Attack ComplexityLow

Privileges RequiredLow

User InteractionNone

ScopeUnchanged

Impact

ConfidentialityNone

IntegrityNone

AvailabilityHigh

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Exploit Intelligence

0.28%probability of exploitation in 30 days

19thpercentile

Low risk: more likely to be exploited than 19% of all known CVEs.

References

Vendor Advisory1

mattermost.com

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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-06-01.