CVE-2026-6339: Mattermost versions 11.5.x <= 11.5.1, 11.4.x <= 11.4.3 fail to validate the X-Requested-With header on the bur

Q: How urgent is CVE-2026-6339?

Priority 15/100. No elevated risk signals yet.

Routine

MEDIUM 4.3UPGRADE

CVE-2026-6339 is a medium-severity flaw (CVSS 4.3) affecting mattermost server. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Mattermost versions 11.5.x <= 11.5.1, 11.4.x <= 11.4.3 fail to validate the X-Requested-With header on the burn-on-read reveal endpoint which allows an authenticated channel member to force the reveal of a burn-on-read message without recipient consent via a crafted Markdown image tag.. Mattermost Advisory ID: MMSA-2026-00636

Why this priority

No elevated risk signals yet

A fixed release or distro patch exists.

Risk facts

CVSS score	4.3 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	Low
User interaction	None
EPSS exploit chance	0.1% (top 99%)
CISA exploited list	No
Weakness	CWE-346
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: >= 11.4.0 and < 11.4.4 · >= 11.5.0 and < 11.5.2

Fixed in: 11.4.4, 11.5.2

Safe to downgrade to: a release before 11.4.0

1. Check installed version

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

2. UPGRADE to 11.4.4 (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 11.4.0 (Debian / Ubuntu)

apt-cache madison mattermost-server sudo apt install mattermost-server=<VERSION_BEFORE_11.4.0> 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_11.4.0> 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-2026-6339?

mattermost server: upgrade to 11.4.4, 11.5.2. Exact commands for each OS are on this page.

Is CVE-2026-6339 being exploited?

CVE-2026-6339 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-2026-6339 affect?

CVE-2026-6339 affects mattermost server. Severity is medium (CVSS 4.3). Mattermost versions 11.5.x <= 11.5.1, 11.4.x <= 11.4.3 fail to validate the X-Requested-With header on the burn-on-read reveal endpoint which allows an authenticated channel member to force the reveal of a burn-on-read message without recip

How urgent is CVE-2026-6339?

Priority 15/100. No elevated risk signals yet.

Software pages

Mattermost vulnerabilities

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References

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