Nine-Year-Old RefluXFS Linux Flaw Gives Local Users Root on Default RHEL Installs

A nine-year-old vulnerability (CVE-2026-64600) in the Linux kernel’s XFS filesystem allows unprivileged local users to gain root access on default Red Hat Enterprise Linux, CentOS Stream, and Amazon Linux installations. The flaw stems from a race condition during XFS reflink operations, where a cloned file can overwrite root-owned files. Qualys discovered the issue, aided by an AI model, and Red Hat has released patches for affected versions. The vulnerability is particularly concerning because there are no practical mitigation options beyond applying vendor updates and rebooting.

Qualys found the vulnerability after an AI model, Claude Mythos Preview from Anthropic, was prompted to find a vulnerability similar to Dirty COW. The model successfully identified the race condition, wrote a working root exploit, and drafted the advisory. Red Hat’s bug tracker logged the flaw under the title "kernel: XFS data corruption using reflink." The issue was initially described as possible data corruption from reflinking a file.

The vulnerability requires three conditions to be met: the system runs Linux 4.11 or later without the RefluXFS fix, the XFS filesystem was created with reflink=1, and a readable target and an attacker-writable directory share the same XFS filesystem. Exploitation involves cloning a root-owned file into a scratch file using `FICLONE`, then racing concurrent O_DIRECT writes against the clone. XFS reflinks use copy-on-write, so both files initially reference the same physical disk blocks. A second writer can complete the copy-on-write operation during that gap and remap the cloned file to a new block. When the first writer reacquires the lock, it refreshes the copy-on-write fork but continues using the old data-fork mapping.

Red Hat has issued Important-rated kernel advisories across affected RHEL 8, 9, and 10 streams. The errata began landing on July 14, eight days before the coordinated disclosure: RHSA-2026:39179 and RHSA-2026:39180 for RHEL 8 and RHSA-2026:39494 for RHEL 10, with extended-support and SAP streams following through July 17. Coverage is stream-specific, so confirm an advisory exists for your exact release. Anyone who applied those errata on schedule was covered before RefluXFS had a name. Check your patch dates before assuming exposure.

Debian, Ubuntu, SLES, and openSUSE do not generally use XFS for the root filesystem by default, but they are exposed if an administrator chose XFS with reflink enabled at install time. There is no mount option or sysctl that disables XFS reflinks after a filesystem has been created, and Qualys said no practical mitigation or temporary configuration change is available. SELinux in Enforcing mode, seccomp, kernel lockdown, and container boundaries all failed to stop it in the company’s testing. Memory protections like KASLR and SMEP never applied: this is a block-layer write, not memory corruption.

Red Hat’s bug tracker logged a public proof-of-concept on July 22, pointing at the advisory posted to the oss-security list, which sets out the race and the exploitation steps in full. None of the vendors tracking the flaw had reported exploitation in the wild at the time of writing. The Hacker News has reached out to Red Hat for comment on its assessment of the flaw’s impact and to Qualys for further detail on the finding.