The memory_exchange function in common/memory.c in Xen 3.2.x through 4.6.x does not properly hand back pages to a domain, which might allow guest OS administrators to cause a denial of service (host crash) via unspecified vectors related to domain teardown.
http://support.citrix.com/article/CTX203451
http://www.debian.org/security/2016/dsa-3519
http://www.securityfocus.com/bid/79038
http://www.securitytracker.com/id/1034391