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Openshift Container PlatformApplication · Redhat

CVE-2026-4408

CRITICAL · 9.8 CVSS v3.1 Published 2026-05-28
Fix available
A fix is available. Upgrade to 4.21.0 or later.
See remediation →
100/100
Remediation priority · Urgent
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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 affects non-standard configurations where the "check password script" is used with %u and the samba-dcerpcd service is started as a system service.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

A critical command injection vulnerability in Samba's 'check password script' feature. When configured with the %u substitution parameter, the client-supplied username is passed directly to the script without escaping shell meta-characters, enabling remote command execution. This only affects non-standard configurations where the password script uses %u and samba-dcerpcd runs as a system service.

MitigationReview all Samba configurations for use of 'check password script' with %u substitution; either disable this feature, remove the %u parameter, or implement proper shell escaping of the username variable before passing to the script.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Openshift Container PlatformApplication
Affected:= 4.0
Enterprise LinuxOperating system
Affected:= 6.0= 7.0= 9.0
SambaApplication
Affected:>= 4.1.0, < 4.21.0

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Check installed Samba version
    Run 'smbd --version' or 'rpm -q samba' to get the Samba version number
    Affected if Version is >= 4.1.0 and < 4.21.0
  2. Locate smb.conf configuration file
    Find the smb.conf file, typically at /etc/samba/smb.conf or /etc/smb.conf - run 'testparm -s' to show loaded configuration
    Affected if File exists and is readable
  3. Verify check password script is configured
    Run 'testparm -s 2>/dev/null | grep -i "check password script"' to see if the parameter is set
    Affected if A check password script is explicitly configured in smb.conf
  4. Confirm %u substitution is in use
    Examine the check password script value - look for '%u' variable in the configured script path or arguments
    Affected if The configured check password script contains '%u' which passes username to shell unsanitized
  5. Check if Samba services are running
    Run 'systemctl status smbd' or 'ps aux | grep smbd' to see if Samba daemon is active
    Affected if Samba services are running with vulnerable configuration

You are affected if running Samba version 4.1.0 through 4.20.x with a check password script configured that uses the %u substitution variable.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 4.21.0 or later
Fixed in 4.21.0
Interim mitigation

Review all Samba configurations for use of 'check password script' with %u substitution; either disable this feature, remove the %u parameter, or implement proper shell escaping of the username variable before passing to the script.

Recommended fix High confidence

Samba 4.21.0 or later

  1. 1. Back up the current Samba configuration files (smb.conf) and any custom check password scripts
  2. 2. Upgrade Samba to version 4.21.0 or later using your system's package manager (e.g., yum update samba or dnf update samba)
  3. 3. After upgrading, verify the check password script configuration in smb.conf does not use %u substitution without proper escaping, or remove the %u substitution if possible
  4. 4. Test the configuration in a non-production environment before deploying to production
  5. 5. Restart the smbd, nmbd, and samba-dcerpcd services to apply changes
  6. 6. Monitor logs for any suspicious activity
Caveat Major Samba upgrades may require configuration adjustments; review release notes for changes to authentication, file sharing, or domain controller functionality

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Openshift Container Platform Scoped from the published advisory
  • Consultation4.0 h
  • Implementation3.0 h
  • Testing2.0 h
  • Review / QA1.0 h
10.0 hours of engineering $1,820
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-4408 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data