The pygrub boot loader emulator in Xen, when S-expression output format is requested, allows local pygrub-using guest OS administrators to read or delete arbitrary files on the host via string quotes and S-expressions in the bootloader configuration file.
https://support.citrix.com/article/CTX218775
https://security.gentoo.org/glsa/201612-56
http://xenbits.xen.org/xsa/xsa198.patch
http://xenbits.xen.org/xsa/advisory-198.html