Because of a logical error in XSA-407 (Branch Type Confusion), the mitigation is not applied properly when it is intended to be used. XSA-434 (Speculative Return Stack Overflow) uses the same infrastructure, so is equally impacted. For more details, see: https://xenbits.xen.org/xsa/advisory-407.html https://xenbits.xen.org/xsa/advisory-434.html