A significant race condition vulnerability, now tracked as **CVE-2026-64600**, has been identified within the **Linux kernel**'s XFS filesystem. This flaw, dubbed **RefluXFS** by the **Qualys Threat Research Unit (TRU)**, has existed for nine years and enables local attackers to overwrite protected system files, leading to root privilege escalation. ### Widespread Impact and Evasion of Defenses **RefluXFS** affects systems utilizing an XFS filesystem with reflink enabled—a default configuration on major enterprise Linux distributions. Systems running **Linux kernel** v4.11 or later, with a directory writable by an unprivileged local user and a high-value target (such as a root-owned configuration file or an SUID-root binary), are vulnerable. Notably, this vulnerability circumvents standard security measures, including **Security-Enhanced Linux (SELinux)**, kernel lockdown, container isolation mechanisms, and memory-protection features like **KASLR**, **SMEP**, and **SMAP**. This is because **RefluXFS** operates at the filesystem allocation layer, beneath where these protections typically apply. ### How RefluXFS Works According to **Qualys**, exploitation is highly reliable, leaves no kernel log output, and the on-disk modification persists even after a system reboot. "The attacker reflink-clones a target file (for example `/etc/passwd`, or a SUID-root binary such as `/usr/bin/su`) into a scratch file they own, then races concurrent O_DIRECT writes on that scratch file," the **Qualys TRU** team explained in their technical write-up. They further detailed: "A lock-drop window in the kernel's copy-on-write allocation path lets one of those writes land, not in the attacker's own storage, but in the physical block that still backs the original file. The change is made directly on disk, persists across reboot, produces no kernel log output, and does not touch the target file's inode — so a modified SUID-root binary keeps its SUID bit." ### Origin and Discovery The vulnerability was introduced in February 2017 with **kernel version 4.11** via commit `3c68d44a2b49`. It has been present in every mainline and stable kernel since then until its recent patch on July 16, following the merge of commit `2f4acd0` into the **Linux kernel** source tree. Impacted **Linux** distributions include **Red Hat Enterprise Linux (RHEL)**, **Oracle Linux**, **Amazon Linux**, **Fedora**, **CentOS Stream**, **Rocky Linux**, **AlmaLinux**, and **CloudLinux**. **Qualys** estimates that over 16.4 million systems could be affected based on their analysis. ### AI-Assisted Discovery **Saeed Abbasi**, head of **Qualys' Threat Research Unit**, revealed that the discovery of **RefluXFS** was aided by an AI model. In a research collaboration with **Anthropic**, the **Claude Mythos Preview** AI model was integrated into their manual audit workflow. The AI was tasked with identifying race conditions similar to the infamous "**Dirty COW**" vulnerability class, ultimately pinpointing the flaw in XFS and generating a functional proof-of-concept. **Qualys** researchers then verified the exploit and coordinated disclosure. ### Immediate Patching Recommended "Immediate kernel patching is recommended to neutralize this vulnerability. Exploitation succeeds consistently under standard hardening settings, and the on-disk modification survives a system reboot," stated **Abbasi**. He added, "Vendor-fixed kernels are now available and being backported to enterprise distributions. Organizations should prioritize patching exposed and multi-tenant systems and ensure a reboot to verify the update. As of now, there are no reliable or practical mitigations or temporary configuration changes available." **RefluXFS** joins a growing list of **Linux** privilege escalation vulnerabilities disclosed this year, including **CIFSwitch**, **PinTheft**, **Copy Fail**, **Dirty Frag**, **Fragnesia**, **Pack2TheRoot**, and **DirtyDecrypt/DirtyCBC**.