CVE-2026-4274: Mattermost versions 11.2.x <= 11.2.2, 10.11.x <= 10.11.10, 11.4.x <= 11.4.0, 11.3.x <= 11.3.1 fail to restrict

Q: How urgent is CVE-2026-4274?

Priority 20/100. No elevated risk signals yet.

Routine

MEDIUM 5.4UPGRADE

CVE-2026-4274 is a medium-severity flaw (CVSS 5.4) affecting mattermost server. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Mattermost versions 11.2.x <= 11.2.2, 10.11.x <= 10.11.10, 11.4.x <= 11.4.0, 11.3.x <= 11.3.1 fail to restrict team-level access when processing membership sync from a remote cluster, which allows a malicious remote cluster to grant a user access to an entire private team instead of only the shared channel via sending crafted membership sync messages that trigger team membership assignment. Mattermost Advisory ID: MMSA-2026-00574

Why this priority

No elevated risk signals yet

A fixed release or distro patch exists.

Risk facts

CVSS score	5.4 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	Low
User interaction	None
EPSS exploit chance	0.1% (top 96%)
CISA exploited list	No
Weakness	CWE-863
Published	2026-03-26
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

2026-03-26 11:16 UTCPublished CVE record published
2026-06-17 10:56 UTCUpdated Record last modified

mattermost mattermost server UPGRADE

Affected: >= 10.11.0 and < 10.11.11 · >= 11.2.0 and < 11.2.3 · >= 11.3.0 and < 11.3.2 · >= 11.4.0 and < 11.4.1

Fixed in: 10.11.11, 11.2.3, 11.3.2, 11.4.1

Safe to downgrade to: a release before 10.11.0

1. Check installed version

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

2. UPGRADE to 10.11.11 (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.

3. If no update is available yet, DOWNGRADE to a release before 10.11.0 (Debian / Ubuntu)

apt-cache madison mattermost-server sudo apt install mattermost-server=<VERSION_BEFORE_10.11.0> sudo apt-mark hold mattermost-server

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list mattermost-server sudo dnf downgrade mattermost-server-<VERSION_BEFORE_10.11.0> sudo dnf versionlock add mattermost-server

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

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-2026-4274?

mattermost server: upgrade to 10.11.11, 11.2.3, 11.3.2, 11.4.1. Exact commands for each OS are on this page.

Is CVE-2026-4274 being exploited?

CVE-2026-4274 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-2026-4274 affect?

CVE-2026-4274 affects mattermost server. Severity is medium (CVSS 5.4). Mattermost versions 11.2.x <= 11.2.2, 10.11.x <= 10.11.10, 11.4.x <= 11.4.0, 11.3.x <= 11.3.1 fail to restrict team-level access when processing membership sync from a remote cluster, which allows a malicious remote cluster to grant a user

How urgent is CVE-2026-4274?

Priority 20/100. No elevated risk signals yet.

Software pages

Mattermost vulnerabilities

Related CVEs

CVE-2026-19621 MEDIUM 6.5 UPGRADE Mattermost versions 11.10.x <= 11.10.0, 11.7.x <= 11.7.9, 11.9.x <= 11.9.1, 11.8.x <= 11.8.5 fail to strip act
CVE-2026-19600 MEDIUM 4.5 UPGRADE Mattermost versions 11.9.x <= 11.9.1, 11.8.x <= 11.8.5, 11.7.x <= 11.7.10, 11.10.x <= 11.10.1 fail to sanitize
CVE-2026-19076 MEDIUM 4.4 UPGRADE Mattermost versions 11.10.x <= 11.10.0, 11.7.x <= 11.7.9, 11.9.x <= 11.9.1, 11.8.x <= 11.8.5 fail to reject OA
CVE-2026-6346 HIGH 8.7 UPGRADE Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to sanitize sensitive configu
CVE-2026-6347 HIGH 7.6 UPGRADE Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to sanitize sensitive configu

References

← Back to latest CVEs