How severe is CVE-2026-27656?

CVE-2026-27656 has a CVSS 3.x base score of 5.7, rated medium severity. It is exploitable over network with high attack complexity, requires high privileges and user interaction. Impact on confidentiality is high, integrity high, and availability none.

Is CVE-2026-27656 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-2026-27656?

CVE-2026-27656 affects Mattermost Mattermost Server. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-27656?

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

Does CVE-2026-27656 have an EU (EUVD) identifier?

Yes. CVE-2026-27656 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-15805.

When was CVE-2026-27656 published?

CVE-2026-27656 was published on 2026-03-25 and last updated on 2026-06-17.

CVE-2026-27656

CVE-2026-27656 is a medium-severity vulnerability in Mattermost Mattermost Server with a CVSS 3.x base score of 5.7. 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-303.

Key facts

Severity: Medium (CVSS 3.x base score 5.7)
EPSS exploit prediction: 0% (8th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-15805
Weakness: CWE-303
Affected product: Mattermost Mattermost Server
Published: 2026-03-25
Last modified: 2026-06-17

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

Frequently asked questions

What is CVE-2026-27656?: 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
How severe is CVE-2026-27656?: CVE-2026-27656 has a CVSS 3.x base score of 5.7, rated medium severity. It is exploitable over network with high attack complexity, requires high privileges and user interaction. Impact on confidentiality is high, integrity high, and availability none.
Is CVE-2026-27656 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-2026-27656?: CVE-2026-27656 affects Mattermost Mattermost Server. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-27656?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
Does CVE-2026-27656 have an EU (EUVD) identifier?: Yes. CVE-2026-27656 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-15805.
When was CVE-2026-27656 published?: CVE-2026-27656 was published on 2026-03-25 and last updated on 2026-06-17.

References

https://mattermost.com/security-updates

Affected products (1)

cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*

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