Eric Blake from Red Hat notes:

The LXC driver will open paths under /proc/$PID/root for some operations it performs on running guests. For the virDomainShutdown and virDomainReboot APIs it will use this to access the /dev/initctl path in the container. For the virDomainDeviceAttach / virDomainDeviceDettach APIs it will use this to create device nodes in the container's /dev filesystem. If any of the path components under control of the container are symlinks the container can cause the libvirtd daemon to access the incorrect files.

Impact
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A container can cause the administrator to shutdown or reboot the host OS if /dev/initctl in the container is made to be an absolute symlink back to itself or /run/initctl. A container can cause the host administrator to mknod in an arbitrary host directory when invoking the virDomainDeviceAttach API by replacing '/dev' with an
absolute symlink. A container can cause the host administrator to delete host device when invoking the virDomainDeviceDettach API by replacing '/dev' with an absolute symlink.

Workaround
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Do not use the virDomainShutdown or virDomainReboot APIs without also passing the VIR_DOMAIN_SHUTDOWN_SIGNAL or VIR_DOMAIN_REBOOT_SIGNAL flags respectively. These will cause the LXC driver to send a SIGTERM or SIGHUP signal respectively, to the init process instead of using /dev/initct. Do not use the virDomainDeviceAttach or virDomainDeviceDetach APIs at all unless the guest OS is trusted.

This issue affects the versions of libvirt in Fedora 19 and later. Red Hat Enterprise Linux 5 and 6 are not affected.