Qualys has discovered a vulnerability in the Linux kernel XFS file system that allows a local user to gain root privileges. The vulnerability, identified as CVE-2026-64600, was named RefluXFS and was found during an experiment using the Claude Mythos Preview AI model. This issue was uncovered in response to a request to find a Dirty COW vulnerability in the Linux kernel due to a race condition in the file system code. The AI model detected a similar vulnerability in the XFS code and created a prototype exploit that enabled root access in Fedora Server 44.
The vulnerability enables the overwriting of any file in the XFS file system created with reflink support, which has been set by default in mkfs.xfs since 2019 and backported to RHEL 8. The reflink operation allows the creation of file copies by cloning the metadata of a file and creating a link to existing data without physically copying it, sharing data only after changes are made through the copy-on-write mechanism.
The attack involves creating a clone of a user-owned file, such as /etc/passwd or /usr/bin/su, via reflink, and initiating a race condition by sending simultaneous requests to the clone in O_DIRECT mode. Due to an incorrectly set lock during copy-on-write operation, there is a brief time window where the block will be copied, but the reference counter has not been updated, resulting in a write to the physical block assigned to the source file. This modification is saved directly in physical blocks on the disk without altering the inode state and without being reflected in the page cache.
The vulnerability exists in RHEL 8-10, CentOS, Rocky, AlmaLinux, Oracle Linux, CloudLinux 8-10, Fedora Server 31+, Amazon Linux 2023, and Amazon Linux 2 AMI. The RHEL 7 branch is not affected, but the RHEL 8 branch is vulnerable only when installed from scratch, as reflink is not enabled during an upgrade from RHEL 7. On Debian, Ubuntu, Fedora Workstation, SLES, openSUSE, and Arch Linux, the susceptibility to the issue depends on the choice of XFS during installation.
The problem has been present since the release of kernel version 4.11 in February 2017 and has been fixed in the Linux kernel as of June 14. The fix is included in kernel maintenance releases such as 7.1.4 and 6.18.39.