Is CVE-2026-27656 in the CISA KEV catalog?

No. CVE-2026-27656 is not currently on the CISA Known Exploited Vulnerabilities list.

What is the CVSS severity of CVE-2026-27656?

CVE-2026-27656 has a CVSS v3 base score of 5.7 (MEDIUM).

What is the EPSS score for CVE-2026-27656?

CVE-2026-27656 has an EPSS score of 0.18% (8th percentile), estimating the probability of exploitation in the next 30 days.

How do I fix CVE-2026-27656?

Upgrade mattermost server to 11.4.1 or later.

CVE & CISA-KEV Catalog

CVE-2026-27656

MEDIUM

5.7

CVSS v3

NVD

Description

Mattermost versions 11.4.x <= 11.4.0, 11.3.x <= 11.3.1, 11.2.x <= 11.2.3, 10.11.x <= 10.11.11 fail to properly validate user identity in the OpenID {{IsSameUser()}} comparison logic, which allows an attacker to take over arbitrary user accounts via an overly permissive substring matching flaw in the user discovery flow.. Mattermost Advisory ID: MMSA-2026-00590

CWE-303

How to fix

Remediation Available

mattermost serverNVD

Affected:>= 11.4.0, < 11.4.1Fixed in:11.4.1CVE-2026-27656derived 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 ComplexityHigh

Privileges RequiredHigh

User InteractionRequired

ScopeUnchanged

Impact

ConfidentialityHigh

IntegrityHigh

AvailabilityNone

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

Exploit Intelligence

0.18%probability of exploitation in 30 days

8thpercentile

Low risk: more likely to be exploited than 8% 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 2026-03-26.