The (1) libxl_set_memory_target function in tools/libxl/libxl.c and (2) libxl__build_post function in tools/libxl/libxl_dom.c in Xen 3.4.x through 4.6.x do not properly calculate the balloon size when using the populate-on-demand (PoD) system, which allows local HVM guest users to cause a denial of service (guest crash) via unspecified vectors related to "heavy memory pressure."
https://security.gentoo.org/glsa/201604-03
http://xenbits.xen.org/xsa/advisory-153.html
http://www.securitytracker.com/id/1034036
http://www.securityfocus.com/bid/77365
http://www.debian.org/security/2015/dsa-3414
http://support.citrix.com/article/CTX202404
http://lists.opensuse.org/opensuse-updates/2015-11/msg00063.html
http://lists.fedoraproject.org/pipermail/package-announce/2015-November/171249.html
http://lists.fedoraproject.org/pipermail/package-announce/2015-November/171185.html
http://lists.fedoraproject.org/pipermail/package-announce/2015-November/171082.html