CVE-2026-21388: Mattermost Plugins versions <=2.3.1 fail to limit the request body size on the {{/lifecycle}} webhook endpoint

Q: How do I fix CVE-2026-21388?

mattermost server: upgrade to newer than 2.3.1. Exact commands for each OS are on this page.

Q: Is CVE-2026-21388 being exploited?

CVE-2026-21388 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-21388 affect?

CVE-2026-21388 affects mattermost server. Severity is low (CVSS 3.7). 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. Ma

Q: How urgent is CVE-2026-21388?

Priority 24/100. Remotely exploitable, no login, no user action

Routine

LOW 3.7REMOTEUPGRADE

CVE-2026-21388 is a low-severity flaw (CVSS 3.7) affecting mattermost server. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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

Why this priority

Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	3.7 (Low)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	0.3% (top 76%)
CISA exploited list	No
Weakness	CWE-770
Published	2026-04-09
Last modified	2026-06-17

Publisher (CNA): Mattermost, Inc. · Vendor: mattermost

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-04-09 11:16 UTCPublished CVE record published
2026-06-17 10:18 UTCUpdated Record last modified

mattermost mattermost server UPGRADE

Affected: <= 2.3.1

Fixed in: newer than 2.3.1

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy mattermost-server || rpm -q mattermost-server || mattermost-server --version

2. UPGRADE to newer than 2.3.1 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade mattermost-server

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade mattermost-server

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-21388?

mattermost server: upgrade to newer than 2.3.1. Exact commands for each OS are on this page.

Is CVE-2026-21388 being exploited?

CVE-2026-21388 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-21388 affect?

CVE-2026-21388 affects mattermost server. Severity is low (CVSS 3.7). 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. Ma

How urgent is CVE-2026-21388?

Priority 24/100. Remotely exploitable, no login, no user action

Software pages

Mattermost vulnerabilities

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References

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