CVE-2026-31709

A flaw was found in the Linux kernel's Server Message Block (SMB) client, specifically within the cifsacl functionality. A malicious SMB server could provide a malformed Discretionary Access Control List (DACL) that claims to contain more Access Control Entries (ACEs) than are actually present. This insufficient validation allows the kernel to read or copy data beyond the intended memory boundaries, leading to memory corruption. This could potentially result in information disclosure or a denial of service.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

To mitigate this issue, if SMB client functionality is not required, the cifs kernel module can be blacklisted to prevent it from loading. This can be achieved by creating a modprobe configuration file.

1. Create a file named /etc/modprobe.d/disable-cifs.conf with the following content:

   install cifs /bin/true
   blacklist cifs
   

2. Regenerate the initramfs to ensure the change takes effect on boot:

   dracut -f -v
   

   or for systems using mkinitrd:

   mkinitrd -f /boot/initramfs-$(uname -r).img $(uname -r)
   

3. A system reboot is required for the changes to take effect.

Warning: Disabling the cifs module will prevent the system from mounting SMB/CIFS shares. Ensure this functionality is not critical before applying this mitigation.

refer: https://access.redhat.com/solutions/4117281