CVE-2026-9816: Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21, 11.8.x <= 11.8.3 fail to validate BoardMember.Schem

Routine

HIGH 8.3UPGRADE

CVE-2026-9816 is a high-severity flaw (CVSS 8.3) affecting mattermost server. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21, 11.8.x <= 11.8.3 fail to validate BoardMember.Scheme* fields server-side on insert and archive-import paths which allows a board editor or non-guest team member to grant board admin to arbitrary users via POST /api/v2/boards/{boardID}/members and POST /api/v2/teams/{teamID}/archive/import.. Mattermost Advisory ID: MMSA-2026-00685

Why this priority

High severity

A fixed release or distro patch exists.

Risk facts

CVSS score	8.3 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	Low
User interaction	None
EPSS exploit chance	0.4% (top 72%)
CISA exploited list	No
Weakness	CWE-863
Published	2026-08-17
Last modified	2026-08-19

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

Last updated 2026-08-19 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-08-17 22:17 UTCPublished CVE record published
2026-08-19 13:29 UTCUpdated Record last modified

mattermost mattermost server UPGRADE

Affected: >= 10.11.0 and < 10.11.22 · >= 11.7.0 and < 11.7.7 · >= 11.8.0 and < 11.8.4

Fixed in: 10.11.22, 11.7.7, 11.8.4

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 10.11.22 (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-2026-9816?

mattermost server: upgrade to 10.11.22, 11.7.7, 11.8.4. Exact commands for each OS are on this page.

Is CVE-2026-9816 being exploited?

CVE-2026-9816 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-2026-9816 affect?

CVE-2026-9816 affects mattermost server. Severity is high (CVSS 8.3). Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21, 11.8.x <= 11.8.3 fail to validate BoardMember.Scheme* fields server-side on insert and archive-import paths which allows a board editor or non-guest team member to grant board admin

How urgent is CVE-2026-9816?

Priority 36/100. High severity

Software pages

Mattermost vulnerabilities

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References

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