CVE-2023-4091 Samba: smb clients can truncate files with read-only permissions

Medium 6.5 Published 3 Nov 2023 Updated 20 Nov 2025

SMB clients can truncate read-only files when Samba is configured to ignore system ACLs, allowing unauthorized data modification.

Exploited in the wild: Not known
Login needed: Any signed-in user
Attackable: Over the internet
Victim action needed: Not needed
Fix: Available

What is CVE-2023-4091?

CVE-2023-4091 is a medium-severity missing authorization vulnerability in Fedoraproject Fedora, published on 3 Nov 2023. Any signed-in user can exploit it over the network, with no action from the victim.

Official description

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.

Weakness: CWE-276

Which versions are affected by CVE-2023-4091?

It affects 5 products.

Linux distribution packages (4)

`samba` SUSE:Linux Enterprise High Availability Extension 12 SP5	All versions	`4.15.13+git.625.ac658f2f12-3.88.1`
`samba` SUSE:Linux Enterprise Server 12 SP5	All versions	`4.15.13+git.625.ac658f2f12-3.88.1`
`samba` SUSE:Linux Enterprise Server for SAP Applications 12 SP5	All versions	`4.15.13+git.625.ac658f2f12-3.88.1`
`samba` SUSE:Linux Enterprise Software Development Kit 12 SP5	All versions	`4.15.13+git.625.ac658f2f12-3.88.1`

Products include Fedoraproject Fedora, Redhat Enterprise Linux, Redhat Enterprise Linux Eus, Redhat Storage, Samba Samba.

How do I fix CVE-2023-4091?

Apply the vendor's patch.

Upgrade, and check permissions on every request on the server, not only in the interface. More about Missing authorization.

Is CVE-2023-4091 being exploited?

There is no known exploitation of CVE-2023-4091 in the wild and no public exploit code.

All Fedoraproject Fedora vulnerabilities

Sources

Also known as ALPINE-CVE-2023-4091, AZL-31900, AZL-37025, CLSA-2025-1738833923, CLSA-2026-1779132536, CLSA-2026-1779224622 and 5 more.