How severe is CVE-2026-21388?

CVE-2026-21388 has a CVSS 3.x base score of 3.7, rated low severity. It is exploitable over network with high attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability low.

Is CVE-2026-21388 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (23rd percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-21388?

CVE-2026-21388 affects Mattermost Mattermost Server. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-21388?

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

Does CVE-2026-21388 have an EU (EUVD) identifier?

Yes. CVE-2026-21388 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-20880.

When was CVE-2026-21388 published?

CVE-2026-21388 was published on 2026-04-09 and last updated on 2026-06-17.

CVE-2026-21388

CVE-2026-21388 is a low-severity vulnerability in Mattermost Mattermost Server with a CVSS 3.x base score of 3.7. 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-770.

Key facts

Severity: Low (CVSS 3.x base score 3.7)
EPSS exploit prediction: 0% (23rd percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-20880
Weakness: CWE-770
Affected product: Mattermost Mattermost Server
Published: 2026-04-09
Last modified: 2026-06-17

Description

Mattermost Plugins versions <=2.3.1 fail to limit the request body size on the {{/lifecycle}} webhook endpoint which allows an authenticated attacker to cause memory exhaustion and denial of service via sending an oversized JSON payload. Mattermost Advisory ID: MMSA-2026-00610

Frequently asked questions

What is CVE-2026-21388?: Mattermost Plugins versions <=2.3.1 fail to limit the request body size on the {{/lifecycle}} webhook endpoint which allows an authenticated attacker to cause memory exhaustion and denial of service via sending an oversized JSON payload. Mattermost Advisory ID: MMSA-2026-00610
How severe is CVE-2026-21388?: CVE-2026-21388 has a CVSS 3.x base score of 3.7, rated low severity. It is exploitable over network with high attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability low.
Is CVE-2026-21388 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (23rd percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-21388?: CVE-2026-21388 affects Mattermost Mattermost Server. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-21388?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
Does CVE-2026-21388 have an EU (EUVD) identifier?: Yes. CVE-2026-21388 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-20880.
When was CVE-2026-21388 published?: CVE-2026-21388 was published on 2026-04-09 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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