Xen 4.0 through 4.3.x, when using AVX or LWP capable CPUs, does not properly clear previous data from registers when using an XSAVE or XRSTOR to extend the state components of a saved or restored vCPU after touching other restored extended registers, which allows local guest OSes to obtain sensitive information by reading the registers.
http://www.securitytracker.com/id/1029090
http://www.openwall.com/lists/oss-security/2013/09/25/2
http://www.debian.org/security/2014/dsa-3006
http://security.gentoo.org/glsa/glsa-201407-03.xml
http://lists.opensuse.org/opensuse-updates/2013-11/msg00009.html
http://lists.opensuse.org/opensuse-security-announce/2014-03/msg00021.html