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             AUSCERT External Security Bulletin Redistribution

                     Citrix Hypervisor Security Update
                                9 July 2020


        AusCERT Security Bulletin Summary

Product:           Citrix Hypervisor
                   Citrix XenServer
Publisher:         Citrix
Operating System:  Virtualisation
                   Citrix XenServer
Impact/Access:     Increased Privileges           -- Existing Account
                   Denial of Service              -- Existing Account
                   Provide Misleading Information -- Existing Account
Resolution:        Patch/Upgrade
CVE Names:         CVE-2020-15565 CVE-2020-15563 

Reference:         ESB-2020.2320

Original Bulletin: 

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Citrix Hypervisor Security Update

Reference: CTX277456

Category : Medium

Created  : 08 Jul 2020

Modified : 08 Jul 2020

Applicable Products

  o Citrix Hypervisor
  o XenServer

Description of Problem

Two issues have been identified in Citrix Hypervisor that may, if exploited,
allow privileged code in an HVM guest VM to compromise or crash the host. These
issues only apply in specific configurations; furthermore, Citrix believes that
there would be significant difficulty in successfully executing these specific

CVE-2020-15565: insufficient cache write-back under VT-d

This issue may allow the administrator of an HVM guest VM to compromise the
host. This issue is only applicable to hosts where the host administrator has
explicitly assigned a PCI-passthrough device to the attacking VM. Hosts with
AMD CPUs are not affected. Hosts where Hardware Assisted Paging (HAP) has been
disabled for the attacking VM, or where the host CPU does not support HAP, are
not affected.

CVE-2020-15563: inverted code paths in x86 dirty VRAM tracking

This issue may allow the administrator of an HVM guest VM to crash the host.
This issue is only applicable to hosts that do not have HAP (or deployments
where the host administrator has explicitly enabled shadow paging for the
attacking VM). Furthermore, the console of the attacking VM must be being
actively consumed e.g. by monitoring it from XenCenter.

CVE-2020-15565 affects all supported releases of Citrix Hypervisor, up to and
including Citrix hypervisor 8.2 LTSR.

CVE-2020-15563 affects Citrix Hypervisor 8.2 LTSR, Citrix Hypervisor 8.1 and
Citrix Hypervisor 8.0.

Mitigating Factors

See the per-issue descriptions above. Note in particular that customers who
have not assigned PCI passthrough devices to untrustworthy guests and are using
hosts with HAP support and have not explicitly enabled shadow paging are not at
risk from these issues. Most recent CPUs have HAP support (known as EPT on
Intel systems).

What Customers Should Do

Hotfixes have been released to address these issues. Citrix recommends that
affected customers install these hotfixes as soon as their patching schedule
permits. The hotfixes can be downloaded from the following locations:

Citrix Hypervisor 8.2 LTSR: CTX277444 - https://support.citrix.com/article/

Citrix Hypervisor 8.1: CTX277443 - https://support.citrix.com/article/CTX277443

Citrix Hypervisor 8.0: CTX277442 - https://support.citrix.com/article/CTX277442

Citrix XenServer 7.1 LTSR CU2: CTX277441 - https://support.citrix.com/article/

Citrix XenServer 7.0: CTX277440 - https://support.citrix.com/article/CTX277440


|Date                      |Change                                            |
|2020-07-08                |Initial Publication                               |

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