CVE-2026-4408: A flaw was found in Samba

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CRITICAL 9.0REMOTEWATCHLISTDOWNGRADEUPGRADEMITIGATE

CVE-2026-4408 is a critical-severity flaw (CVSS 9.0) affecting openshift container platform, samba, enterprise linux. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

A flaw was found in Samba. A remote attacker can exploit a misconfiguration in Samba file servers and classic domain controllers that use the "check password script" feature. If this script is configured with the %u substitution character, the client-controlled username is passed without proper escaping of shell meta-characters. This vulnerability allows an attacker to achieve remote command execution on the affected system. This issue primarily

Why this priority

Critical severity
Remotely exploitable, no login, no user action
On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	9.0 (Critical)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	2.5% (top 15%)
CISA exploited list	No
Weakness	CWE-78
Published	2026-05-28
Last modified	2026-10-26

Publisher (CNA): Red Hat, Inc. · Vendor: redhat · samba

Last updated 2026-10-26 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-05-28 09:16 UTCPublished CVE record published
2026-10-26 11:10 UTCUpdated Record last modified

redhat openshift container platform DOWNGRADE

Affected: 4.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 4.0

1. Check installed version

apt-cache policy openshift-container-platform || rpm -q openshift-container-platform || openshift-container-platform --version

2. No fixed release: DOWNGRADE to a release before 4.0 (Debian / Ubuntu)

apt-cache madison openshift-container-platform sudo apt install openshift-container-platform=<VERSION_BEFORE_4.0> sudo apt-mark hold openshift-container-platform

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list openshift-container-platform sudo dnf downgrade openshift-container-platform-<VERSION_BEFORE_4.0> sudo dnf versionlock add openshift-container-platform

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.

samba samba UPGRADE

Affected: >= 4.1.0 and < 4.21.0

Fixed in: 4.21.0

Safe to downgrade to: a release before 4.1.0

1. Check installed version

apt-cache policy samba || rpm -q samba || samba --version

2. UPGRADE to 4.21.0 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade samba

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade samba

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 4.1.0 (Debian / Ubuntu)

apt-cache madison samba sudo apt install samba=<VERSION_BEFORE_4.1.0> sudo apt-mark hold samba

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

dnf --showduplicates list samba sudo dnf downgrade samba-<VERSION_BEFORE_4.1.0> sudo dnf versionlock add samba

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.

redhat enterprise linux MITIGATE

Affected: 6.0 · 7.0 · 9.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 6.0

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

Exact fixes by distribution

A distro patch exists: prefer upgrading over downgrading.

Fedora 46 · flare-im · FIXED · 0.22.7-1.fc46 · FEDORA-2026-cd5e8062a1 · source

sudo dnf upgrade --advisory=FEDORA-2026-cd5e8062a1

Fedora ELN · libdnf · FIXED · 0.76.0-2.eln159 · FEDORA-2026-dbd263e9d4 · source

sudo dnf upgrade --advisory=FEDORA-2026-dbd263e9d4

Debian 12 · samba · FIXED · 2:4.17.12+dfsg-0+deb12u4

sudo apt update && sudo apt install --only-upgrade samba dpkg --compare-versions "$(dpkg-query -W -f='${Version}' samba)" ge "2:4.17.12+dfsg-0+deb12u4" && echo PATCHED || echo STILL-VULNERABLE

Debian 13 · samba · FIXED · 2:4.22.8+dfsg-0+deb13u2

sudo apt update && sudo apt install --only-upgrade samba dpkg --compare-versions "$(dpkg-query -W -f='${Version}' samba)" ge "2:4.22.8+dfsg-0+deb13u2" && echo PATCHED || echo STILL-VULNERABLE

RHEL 10 · samba · FIXED · samba-4.23.5-109.el10_2

sudo dnf upgrade --advisory=RHSA-2026:22963 rpm -q samba

RHEL 10.0 Extended Update Support · samba · FIXED · samba-4.21.3-114.el10_0.1

sudo dnf upgrade --advisory=RHSA-2026:28055 rpm -q samba

RHEL 7 Extended Lifecycle Support · samba · FIXED · samba-4.10.16-26.el7_9.1

sudo dnf upgrade --advisory=RHSA-2026:28132 rpm -q samba

RHEL 8 · samba · FIXED · samba-4.19.4-16.el8_10

sudo dnf upgrade --advisory=RHSA-2026:22644 rpm -q samba

RHEL 8.4 Advanced Mission Critical Update Support · samba · FIXED · samba-4.13.3-12.el8_4.1

sudo dnf upgrade --advisory=RHSA-2026:28058 rpm -q samba

Alpine 3.23 · samba · FIXED · samba-4.22.10-r0

apk add --upgrade samba apk info -v samba

Alpine 3.24 · samba · FIXED · samba-4.23.8-r0

apk add --upgrade samba apk info -v samba

Amazon Linux 2023 · see advisory · FIXED · ALAS2023-2026-1895 · source

sudo dnf update --advisory=ALAS2023-2026-1895

Amazon Linux 2 · see advisory · FIXED · ALAS2-2026-3493 · source

sudo yum update --advisory=ALAS2-2026-3493

Public exploits and detection

Public code exists for this CVE. Treat unpatched systems as at risk, and only run PoCs in a lab.

GitHub PoC: SafeBreach-Labs/ForgottenButNotGone (10 stars)

Advisories and coverage

Official advisories

AlmaLinux Errata: [ALSA-2026:25049] Critical: samba security update 2026-06-11

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-2026-4408?

openshift container platform: downgrade or apply the listed mitigation; samba: upgrade to 4.21.0; enterprise linux: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2026-4408 being exploited?

CVE-2026-4408 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-2026-4408 affect?

CVE-2026-4408 affects openshift container platform, samba, enterprise linux. Severity is critical (CVSS 9.0). A flaw was found in Samba. A remote attacker can exploit a misconfiguration in Samba file servers and classic domain controllers that use the "check password script" feature. If this script is configured with the %u substitution character,

How urgent is CVE-2026-4408?

Priority 63/100. Critical severity; Remotely exploitable, no login, no user action; On your watchlist

Software pages

Samba vulnerabilities · Red Hat OpenShift vulnerabilities

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References

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