Several security issues have been discovered in Citrix Hypervisor that, collectively, may allow privileged code in a guest VM to compromise or crash the host.
These issues have the following identifiers:
 

CVE-ID	Description	Pre-requisites
CVE-2021-28694	Host denial of service	Malicious privileged code execution in a guest VM running on a host with firmware that declares ACPI tables which includes memory regions that are identity mapped for devices that have been explicitly assigned, by the host administrator, to that guest VM in PCI passthrough mode.
CVE-2021-28697	Host compromise	Malicious privileged code execution in a guest VM that has two or more vCPUs allocated to it
CVE-2021-28698	Host denial of service	Malicious privileged code execution in a guest VM
CVE-2021-28699	Host compromise	Malicious privileged code execution in a guest VM on a host where the host administrator has modified the guest or host grant table limits
CVE-2021-28701	Host compromise	Malicious privileged code execution in a guest VM that has two or more vCPUs allocated to it

All currently supported versions of Citrix Hypervisor are affected by all of these issues with the exception of CVE-2021-28699 which only affects Citrix Hypervisor 8.2 LTSR.

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Instructions

Citrix has released hotfixes to address these issues. Citrix recommends that affected customers install these hotfixes as their patching schedule allows.  The hotfixes can be downloaded from the following locations:
Citrix Hypervisor 8.2 LTSR: CTX324257 – https://support.citrix.com/article/CTX324257
Citrix Hypervisor 7.1 LTSR CU2: CTX324256 – https://support.citrix.com/article/CTX324256