A critical Linux privilege escalation flaw, CVE-2023-0386, has been added to the CISA Known Exploited Vulnerabilities (KEV) Catalog following reports of exploitation. The flaw lies in the OverlayFS subsystem of the Linux kernel and allows local users to gain root privileges, with high risk in multi-user systems, containers, and cloud, it is reported. Since the CVSS score is 7.8, the bug occurs because of improper file ownership and capabilities management during copy-up across many mounts. The issue arises when the kernel fails to remove setuid/setgid bits while copying user-editable files from a nosuid mount to a writable OverlayFS mount. This enables an attacker to retain higher privilege over files copied to a new mount, enabling them to execute code with root access. Security researcher Xkaneiki demonstrated a proof-of-concept (PoC) exploit in Ubuntu 22.04, which instantly promoted a regular user to root. The attack relies on a FUSE filesystem with a setuid binary which everyone has write access to. The attacker employs user and mount namespaces to contain the operation, creates an OverlayFS mount, and induces a copy-up by modifying the file. Due to the faulty logic, the setuid bits are preserved, and the attacker is able to execute the binary from the upper directory as root. Any system operating a vulnerable version of the Linux kernel with OverlayFS enabled and unprivileged namespace access—such as desktop Linux systems, containerized workloads, or shared hosting environments—is impacted. The United States Cybersecurity and Infrastructure Security Agency (CISA) has instructed all Federal Civilian Executive Branch (FCEB) agencies to install patches and mitigate this vulnerability no later than July 8, 2025. Fixing it as quickly as possible is recommended to protect affected systems.
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