CVE-2026-4339: Mattermost versions 10.11.x <= 10.11.18, 11.6.x <= 11.6.3, 11.5.x <= 11.5.6 fail to validate attachment URLs a

Q: How urgent is CVE-2026-4339?

Priority 24/100. No elevated risk signals yet.

Routine

MEDIUM 6.5UPGRADE

CVE-2026-4339 is a medium-severity flaw (CVSS 6.5) affecting mattermost server. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Mattermost versions 10.11.x <= 10.11.18, 11.6.x <= 11.6.3, 11.5.x <= 11.5.6 fail to validate attachment URLs against internal or private IP ranges in the Mattermost Agents plugin MCP server which allows an attacker with access to the MCP server in stdio mode to perform server-side request forgery (SSRF) and exfiltrate data from internal network services via supplying internal URLs as file attachments in post creation requests.. Mattermost Advisor

Why this priority

No elevated risk signals yet

A fixed release or distro patch exists.

Risk facts

CVSS score	6.5 (Medium)
Attack vector	Local
Attack complexity	Low
Privileges needed	Low
User interaction	None
EPSS exploit chance	0.1% (top 97%)
CISA exploited list	No
Weakness	CWE-918
Published	2026-06-26
Last modified	2026-06-29

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

Last updated 2026-06-29 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-06-26 15:16 UTCPublished CVE record published
2026-06-29 19:26 UTCUpdated Record last modified

mattermost mattermost server UPGRADE

Affected: >= 10.11.0 and < 10.11.19 · >= 11.5.0 and < 11.5.7 · >= 11.6.0 and < 11.6.4

Fixed in: 10.11.19, 11.5.7, 11.6.4

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 10.11.19 (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-2026-4339?

mattermost server: upgrade to 10.11.19, 11.5.7, 11.6.4. Exact commands for each OS are on this page.

Is CVE-2026-4339 being exploited?

CVE-2026-4339 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-2026-4339 affect?

CVE-2026-4339 affects mattermost server. Severity is medium (CVSS 6.5). Mattermost versions 10.11.x <= 10.11.18, 11.6.x <= 11.6.3, 11.5.x <= 11.5.6 fail to validate attachment URLs against internal or private IP ranges in the Mattermost Agents plugin MCP server which allows an attacker with access to the MCP se

How urgent is CVE-2026-4339?

Priority 24/100. No elevated risk signals yet.

Software pages

Mattermost vulnerabilities

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References

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