RefluXFS (CVE-2026-64600): Nine-Year-Old Linux Kernel XFS Race Condition Lets Local Users Gain Root

On July 22, the Qualys Threat Research Unit disclosed CVE-2026-64600, a nine-year-old race condition in the Linux kernel’s XFS filesystem that lets an unprivileged local user overwrite root-owned files and gain persistent root access on default installations of Red Hat Enterprise Linux and its derivatives, Fedora Server, and Amazon Linux.

The vulnerability sits in the XFS copy-on-write path, the code that manages how file data blocks are shared and duplicated when multiple inodes reference the same physical blocks on disk. It was introduced in Linux kernel version 4.11, released in February 2017, and a fix was committed on July 16, 2026 — meaning the flaw remained undetected for over nine years across the entire Linux kernel tree.

An attacker with an ordinary unprivileged local account can exploit the race to overwrite files they can read but not normally write, including root-owned system files. The vulnerability was discovered by Qualys using Anthropic’s Claude Mythos Preview AI model, making RefluXFS one of the highest-profile examples yet of AI-assisted vulnerability research producing a fully weaponized local privilege escalation to root.

The Qualys Threat Research Unit estimates the vulnerability could affect over 16.4 million systems worldwide, given the nine-year exposure window and the broad deployment of XFS in enterprise environments. Any system running Linux kernel 4.11 or later with XFS reflink enabled — the default configuration in Red Hat Enterprise Linux, Fedora Server, and Amazon Linux — is potentially exploitable.

For enterprise security teams, RefluXFS represents a difficult class of threat: a local privilege escalation that requires no special permissions, no unusual kernel configuration, and no physical access beyond a standard user account. The race itself targets the copy-on-write operation that XFS performs when a process writes to a file whose disk blocks are shared via reflink with another file. By carefully timing a write against the filesystem’s block-sharing logic, an attacker can redirect the kernel’s write to a root-owned file they have read access to but cannot normally modify — turning a read-only permission into a full root compromise.

The disclosure also marks a milestone in security research methodology. Qualys identified the vulnerability through a collaboration with Anthropic that integrated Claude Mythos Preview into their research pipeline, demonstrating that AI models are now capable of finding subtle race conditions in mature, widely deployed kernel code that human auditors had missed for years. The fix, committed on July 16, closes the race window in the copy-on-write path, and administrators on affected distributions should prioritize patching.

Sources

• Nine-Year-Old RefluXFS Linux Flaw Gives Local Users Root on Default RHEL Installs — The Hacker News
• New RefluXFS Linux flaw lets attackers gain root privileges — BleepingComputer
• RefluXFS: LPE in the Linux kernel via XFS reflink race (CVE-2026-64600) — Openwall oss-security mailing list
• RefluXFS: A Linux Kernel Local Privilege Escalation to Root in XFS (CVE-2026-64600) — Qualys Blog