CVE-2025-21993

In the Linux kernel, the following vulnerability has been resolved:

iscsi_ibft: Fix UBSAN shift-out-of-bounds warning in ibft_attr_show_nic()

When performing an iSCSI boot using IPv6, iscsistart still reads the /sys/firmware/ibft/ethernetX/subnet-mask entry. Since the IPv6 prefix length is 64, this causes the shift exponent to become negative, triggering a UBSAN warning. As the concept of a subnet mask does not apply to IPv6, the value is set to ~0 to suppress the warning message.

This vulnerability is rated as Moderate severity because exploitation is restricted to local access and requires a specialized deployment configuration. The flaw only affects hosts booted via iSCSI over an IPv6 network where the BIOS or firmware supplies an iSCSI Boot Firmware Table (iBFT). Under these conditions, querying the network interface subnet mask via sysfs triggers an invalid bitwise shift that may lead to system instability or denial of service, but cannot be reached remotely or on systems using conventional storage.

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.