Researchers at Qualys’ Threat Research Unit (TRU) have disclosed RefluXFS, a race condition vulnerability in the Linux kernel’s XFS filesystem tracked as CVE-2026-64600. The flaw allows a local unprivileged user to overwrite protected root-owned files, including SUID-root binaries, and escalate to root privileges.

The bug affects systems running Linux kernel 4.11 or later with XFS and reflink enabled, a default configuration on major enterprise Linux distributions. Exploitation requires a directory writable by an unprivileged user and a high-value target such as /etc/passwd or a SUID-root binary like /usr/bin/su.

How the attack works

According to Qualys, an attacker reflink-clones a target root-owned file into a scratch file they control, then races concurrent O_DIRECT writes against it. A lock-drop window in the kernel’s copy-on-write allocation path allows one of those writes to land in the physical block still backing the original file rather than the attacker’s own storage. Because the write occurs directly at the filesystem allocation layer, the inode of the target file is never touched, meaning a modified SUID-root binary retains its SUID bit.

Standard Linux hardening mechanisms, including SELinux, kernel lockdown, container isolation, KASLR, SMEP, and SMAP, do not block the technique because it operates below the layer where those protections apply. Qualys says exploitation is highly reliable, generates no kernel log output, and the malicious on-disk modification survives a reboot.

Scope and origins

RefluXFS has existed since February 2017, introduced via commit 3c68d44a2b49 in kernel 4.11, and has remained present in every mainline and stable kernel since. It was patched on July 16 via commit 2f4acd0. Affected distributions include Red Hat Enterprise Linux, Oracle Linux, Amazon Linux, Fedora, CentOS Stream, Rocky Linux, AlmaLinux, and CloudLinux. Qualys estimates more than 16.4 million systems are potentially exposed, based on its Cybersecurity Asset Management data.

Notably, Qualys said the flaw was surfaced through a research collaboration with Anthropic, in which an AI model (Claude Mythos Preview) was integrated into the manual audit workflow and tasked with hunting for Dirty COW-style race conditions. After iterative refinement, the model identified the XFS flaw and produced a functional proof-of-concept, which Qualys researchers then independently reviewed, reproduced, and verified before coordinating disclosure with kernel maintainers.

Remediation

Qualys’ head of TRU, Saeed Abbasi, urges immediate patching, noting that exploitation succeeds consistently under standard hardening and that no reliable mitigations or configuration workarounds currently exist. Vendor-fixed kernels are being backported to enterprise distributions; organizations should prioritize multi-tenant and exposed systems and confirm a reboot to verify the update took effect.