dbcve.org
FedoraOperating system · Fedoraproject

CVE-2023-4091

MEDIUM · 6.5 CVSS v3.1 Published 2023-11-03
Fix available
A fix is available. Upgrade to 4.17.12 / 4.18.8 or later.
See remediation →
71/100
Remediation priority · Elevated
Remotely reachable 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 vulnerability was discovered in Samba, where the flaw allows SMB clients to truncate files, even with read-only permissions when the Samba VFS module "acl_xattr" is configured with "acl_xattr:ignore system acls = yes". The SMB protocol allows opening files when the client requests read-only access but then implicitly truncates the opened file to 0 bytes if the client specifies a separate OVERWRITE create disposition request. The issue arises in configurations that bypass kernel file system permissions checks, relying solely on Samba's permissions.

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

In Samba with the acl_xattr VFS module configured with 'acl_xattr:ignore system acls = yes', the SMB protocol allows a client to open a file with read-only access but then implicitly truncate it to zero bytes if the client sends an OVERWRITE create disposition request. This bypasses kernel filesystem permission checks and allows read-only files to be truncated, as Samba relies solely on its own permission model when system ACLs are ignored.

MitigationReview the necessity of 'acl_xattr:ignore system acls = yes' configuration; if possible, disable this setting or implement additional share-level access controls to prevent write operations on read-only shares.

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
FedoraOperating system
Affected:= 39
StorageApplication
Affected:= 3.0
Enterprise LinuxOperating system
Affected:= 8.0
Enterprise Linux EusOperating system
Affected:= 9.0
SambaApplication
Affected:< 4.17.12>= 4.18.0, < 4.18.8>= 4.19.0, < 4.19.1

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
Low
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
High
Availability
None

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

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. Identify installed Samba version
    Run 'smbd --version' or 'rpm -q samba' to get the installed Samba version number
    Affected if The version is < 4.17.12, OR >= 4.18.0 and < 4.18.8, OR >= 4.19.0 and < 4.19.1
  2. Locate smb.conf configuration file
    Find the Samba configuration file, typically at /etc/samba/smb.conf
    Affected if File exists and is readable
  3. Check if acl_xattr VFS module is loaded
    Search the smb.conf for lines containing 'vfs objects' or 'vfs module' and verify 'acl_xattr' is listed
    Affected if acl_xattr VFS module is configured in any share or global section
  4. Verify the vulnerable setting is enabled
    Search smb.conf for 'acl_xattr:ignore system acls = yes' configuration directive
    Affected if This exact setting is present and set to 'yes' in the configuration
  5. Confirm share uses the affected configuration
    Review the specific share sections in smb.conf where acl_xattr is active and verify the ignore system acls setting applies to those shares
    Affected if A share has acl_xattr module loaded with 'ignore system acls = yes'

The environment is affected if Samba version is vulnerable AND the acl_xattr VFS module is configured with 'acl_xattr:ignore system acls = yes', allowing read-only files to be truncated via SMB without kernel permission checks.

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 4.17.12 / 4.18.8 / 4.19.1 or later
Fixed in 4.17.124.18.84.19.1
Interim mitigation

Review the necessity of 'acl_xattr:ignore system acls = yes' configuration; if possible, disable this setting or implement additional share-level access controls to prevent write operations on read-only shares.

Recommended fix High confidence

Samba >= 4.17.12, >= 4.18.8, or >= 4.19.1 (depending on your release branch)

  1. 1. Identify the current Samba version installed: rpm -qa | grep samba or smbd --version
  2. 2. For RHEL 8.x systems, update Samba to version 4.17.12 or later using: sudo dnf update samba
  3. 3. For RHEL 9.x systems, update Samba to version 4.18.8 or later using: sudo dnf update samba
  4. 4. For Fedora 39 systems, update Samba to version 4.18.8 or later using: sudo dnf update samba
  5. 5. After updating, restart the Samba services: sudo systemctl restart smb.service nmb.service
  6. 6. Verify the new Samba version is installed: smbd --version
  7. 7. If using the acl_xattr module with 'ignore system acls = yes', review the configuration to ensure it is still required, as this setting bypasses filesystem permission checks
Caveat Major Samba upgrades may introduce configuration changes; review release notes for any backward incompatible changes before applying in production

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

Fix this in Fedora Scoped from the published advisory
  • Consultation4.0 h
  • Implementation3.0 h
  • Testing4.0 h
  • Review / QA2.0 h
13.0 hours of engineering $2,300
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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-2023-4091 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