A critical vulnerability in the widely used Xen hypervisor allows attackers to break out of a guest operating system running inside a virtual machine and access the host system's entire memory.

This is a serious violation of the security barrier enforced by the hypervisor and poses a particular threat to multi-tenant data centers where the customers' virtualized servers share the same underlying hardware.

The open-source Xen hypervisor is used by cloud computing providers and virtual private server hosting companies, as well as by security-oriented operating systems like Qubes OS.

The new vulnerability affects Xen 4.8.x, 4.7.x, 4.6.x, 4.5.x, and 4.4.x and has existed in the Xen code base for over four years. It was unintentionally introduced in December 2012 as part of a fix for a different issue.

The Xen project released a patch Tuesday that can be applied manually to vulnerable deployments. The good news is that the vulnerability can only be exploited from 64-bit paravirtualized guest operating systems.

Xen supports two types of virtual machines: Hardware Virtual Machines (HVMs), which use hardware-assisted virtualization, and paravirtualized (PV) VMs that use software-based virtualization. Based on whether they use PV VMs, Xen users might be affected or not.

For example, Amazon Web Services said in an advisory that its customers' data and instances were not affected by this vulnerability and no customer action is required. Meanwhile, virtual private server provider Linode had to reboot some of its legacy Xen servers in order to apply the fix.

Qubes OS, an operating system that uses Xen to isolate applications inside virtual machines, also put out an advisory warning that an attacker who exploits another vulnerability, for example inside a browser, can exploit this Xen issue to compromise the whole Qubes system.

The Qubes developers have released a patched Xen package for Qubes 3.1 and 3.2 and reiterated their intention to stop using paravirtualization altogether in the upcoming Qubes 4.0.

Vulnerabilities that allow breaking the isolation layer of virtual machines can be very valuable for attackers. The recent Pwn2Own hacking contest offered a $100,000 reward for virtual machine escapes in VMware Workstation or Microsoft Hyper-V. Exploit acquisition firm Zerodium offers up to $50,000 for such an exploit.