CVE-2023-4091 is a vulnerability identified in the Samba implementation on Red Hat Enterprise Linux 8 systems. The flaw arises specifically when the Samba Virtual File System (VFS) module "acl_xattr" is configured with the option "acl_xattr:ignore system acls = yes". This configuration causes Samba to bypass the underlying kernel file system permission checks and rely solely on Samba's internal permission enforcement. The SMB protocol allows clients to open files with read-only access; however, if the client issues a separate OVERWRITE create disposition request, the file can be truncated to zero bytes. Due to the flawed permission enforcement in this configuration, an SMB client can exploit this behavior to truncate files even when only read-only permissions are granted. The vulnerability requires low privileges (PR:L) but no user interaction (UI:N) and can be exploited remotely (AV:N). The impact is primarily on file integrity, as files can be unexpectedly truncated, potentially leading to data loss or disruption of services relying on those files. The CVSS v3.1 base score is 6.5 (medium severity), reflecting the moderate impact and ease of exploitation. No known exploits have been reported in the wild as of the publication date. This vulnerability highlights the risks of misconfigured Samba ACL settings and the importance of kernel-level permission enforcement. Organizations using RHEL 8 with Samba and the acl_xattr VFS module should audit their configurations and apply patches or mitigations promptly to prevent exploitation.

The primary impact of CVE-2023-4091 is on the integrity of files shared via Samba on affected Red Hat Enterprise Linux 8 systems. An attacker with low privileges can remotely truncate files to zero bytes despite having only read-only permissions, leading to potential data loss and disruption of business operations. For European organizations, this could affect file servers, collaborative environments, and any SMB-dependent services, potentially causing operational downtime or loss of critical data. While confidentiality and availability are not directly impacted, the integrity compromise can have cascading effects, such as corrupted backups, interrupted workflows, or loss of audit trails. Organizations in sectors such as finance, healthcare, manufacturing, and government, which rely heavily on RHEL 8 and Samba for file sharing, may face increased risk. The absence of known exploits in the wild reduces immediate threat but does not eliminate the risk, especially if attackers develop exploit code. The medium severity rating suggests that while the vulnerability is serious, it is not trivially exploitable at scale without specific conditions (the acl_xattr configuration).

To mitigate CVE-2023-4091, European organizations should take the following specific actions: 1) Audit Samba configurations to identify if the VFS module "acl_xattr" is enabled with the option "acl_xattr:ignore system acls = yes". 2) Revert the configuration to respect system ACLs by setting "acl_xattr:ignore system acls = no" or removing the option if not required, ensuring kernel-level permission enforcement is active. 3) Apply the latest Red Hat patches and updates for Samba and RHEL 8 as soon as they become available to address this vulnerability. 4) Implement strict access controls and network segmentation to limit SMB access only to trusted clients and networks. 5) Monitor SMB traffic for unusual file truncation or overwrite requests that could indicate exploitation attempts. 6) Educate system administrators about the risks of misconfiguring Samba ACLs and the importance of adhering to recommended security settings. 7) Consider deploying file integrity monitoring solutions on critical file shares to detect unexpected truncation or modification events promptly. These targeted steps go beyond generic advice by focusing on the specific Samba VFS module configuration and operational monitoring.