CVE-2026-6333: Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13 fail to validate the Host header when constructing r

Q: How urgent is CVE-2026-6333?

Priority 13/100. No elevated risk signals yet.

Routine

LOW 3.5UPGRADE

CVE-2026-6333 is a low-severity flaw (CVSS 3.5) affecting mattermost server. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13 fail to validate the Host header when constructing response URLs for custom slash commands which allows an authenticated attacker to redirect slash command responses to an attacker-controlled server via a spoofed Host header.. Mattermost Advisory ID: MMSA-2026-00582

Why this priority

No elevated risk signals yet

A fixed release or distro patch exists.

Risk facts

CVSS score	3.5 (Low)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	Low
User interaction	None
EPSS exploit chance	0.1% (top 97%)
CISA exploited list	No
Weakness	CWE-918
Published	2026-05-18
Last modified	2026-10-14

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

Last updated 2026-10-14 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-05-18 09:16 UTCPublished CVE record published
2026-10-14 20:10 UTCUpdated Record last modified

mattermost mattermost server UPGRADE

Affected: >= 10.11.0 and < 10.11.14 · >= 11.5.0 and < 11.5.2

Fixed in: 10.11.14, 11.5.2

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 10.11.14 (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-2026-6333?

mattermost server: upgrade to 10.11.14, 11.5.2. Exact commands for each OS are on this page.

Is CVE-2026-6333 being exploited?

CVE-2026-6333 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-2026-6333 affect?

CVE-2026-6333 affects mattermost server. Severity is low (CVSS 3.5). Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13 fail to validate the Host header when constructing response URLs for custom slash commands which allows an authenticated attacker to redirect slash command responses to an attacker-c

How urgent is CVE-2026-6333?

Priority 13/100. No elevated risk signals yet.

Software pages

Mattermost vulnerabilities

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References

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