A vulnerability in the Linux kernel known as Januscape, which is 16 years old, enables attackers to break out of a virtual machine and run arbitrary code on the host system.

Security researcher Hyunwoo Kim identified this guest-to-host escape issue, assigned CVE-2026-53359. It arises from a use-after-free problem in the shadow MMU emulation within KVM/x86, the kernel-based virtual machine for x86 and x86_64 architectures.

The flaw existed in the Linux kernel for about 16 years until it was fixed in June 2026. It served as a zero-day in Google's kvmCTF vulnerability reward program.

Successful exploitation allows attackers with root access inside a guest virtual machine (the default configuration on public cloud instances) to execute code as root on the host and take over all guests running on it or crash the host kernel (knocking every other tenant's virtual machine on the same server offline).

Kim described Januscape as the first guest-to-host exploit that can be triggered on both Intel and AMD processor architectures, rather than being limited to a single platform, and noted that it poses a distinct risk to multi-tenant public cloud environments, such as those offered by Google Cloud and Amazon Web Services.

"With guest-side actions alone, an attacker can compromise the host that runs their VM," Kim explained on Monday. "For example, an attacker who has rented just a single instance on a public cloud could panic the host kernel to take down every other tenant VM on the same physical machine (DoS), or run code with root privilege on the host to take over the host and all the guests on it (RCE)."

On some Linux distros, such as Red Hat Enterprise Linux (RHEL), where /dev/kvm is world-writable, unprivileged attackers can also exploit CVE-2026-53359 to reliably gain root permissions on unpatched devices.

The security researcher published a technical write-up and a proof-of-concept exploit that can trigger a host kernel panic, and said that a full guest-to-host escape exploit will not be released for the foreseeable future.

Administrators running KVM/x86 hosts that accept multi-tenant guests should confirm that patch commit 81ccda30b4e8 has been applied to the host kernel to ensure the hosts are secure against attacks.

In May 2026, Kim also disclosed Dirty Frag, a Linux local privilege escalation flaw that chains xfrm-ESP (CVE-2026-43284) and RxRPC (CVE-2026-43500) page-cache write vulnerabilities to gain root access on major distributions, including Ubuntu, Red Hat Enterprise Linux, CentOS Stream, and Fedora.

Kim noted that attackers without guest root access on a target device could chain the Dirty Frag and Januscape flaws to achieve full compromise.