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             AUSCERT External Security Bulletin Redistribution

                Citrix XenServer Multiple Security Updates
                              5 January 2018


        AusCERT Security Bulletin Summary

Product:           Citrix XenServer
Publisher:         Citrix
Operating System:  Xen
Impact/Access:     Access Privileged Data -- Existing Account
                   Denial of Service      -- Existing Account
Resolution:        Patch/Upgrade
CVE Names:         CVE-2017-5754 CVE-2017-5753 CVE-2017-5715

Reference:         ASB-2018.0002.3

Original Bulletin: 

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Citrix XenServer Multiple Security Updates

Security Bulletin | High | Created: 04 Jan 2018 | Modified: 04 Jan 2018

Applicable Products

    XenServer 7.3
    XenServer 7.2
    XenServer 7.1 LTSR Cumulative Update 1
    XenServer 7.0
    XenServer 6.5
    XenServer 6.2.0
    XenServer 6.0.2

Description of Problem

This hotfix provides mitigations for certain recently disclosed vulnerabilities
in the speculative execution functionality of multiple vendors' CPUs: 

    CVE-2017-5753, also known as ‘Variant 1: bounds check bypass’
    CVE-2017-5715, also known as ‘Variant 2: branch target injection’
    CVE-2017-5754, also known as ‘Variant 3: rogue data cache load’ 

For Variant 1, Citrix is not currently aware of any exploit vectors in Citrix 

For Variant 2, an attacker running code in a guest VM may be able to read 
in-memory data from other VMs on the same host. This is independent of the CPU

For Variant 3, an attacker running code in a 64 bit PV guest VM running on an
Intel CPU may be able to read in-memory data from other VMs on the same host.

As these are issues in the underlying hardware, all versions of Citrix
XenServer are affected. 

In addition to the mitigations for these CPU speculative execution issues,
this hotfix also addresses a number of vulnerabilities that have been
identified in Citrix XenServer:

    CVE-2017-TBD - x86 PV guests may gain access to internally used pages
    CVE-2017-TBD - broken x86 shadow mode refcount overflow check
    CVE-2017-TBD - improper x86 shadow mode refcount error handling
    CVE-2017-TBD - improper bug check in x86 log-dirty handling

Collectively, these four issues could allow a malicious guest administrator to
crash the host.

What Customers Should Do

The CPU speculative execution mitigations require system firmware/BIOS upgrades
to be applied before becoming fully effective. Citrix strongly recommends that
customers contact their hardware vendors for further information on these
firmware upgrades.

As these issues are in optimisation features of the underlying physical CPU,
mitigating them will necessarily cause a reduction of CPU performance. This
performance impact will depend on a number of factors, including workload and
CPU model. Customers are recommended to monitor their system loads after
installing these hotfixes.

After applying the relevant firmware/BIOS upgrades and XenServer hotfixes,
guest VMs will need to be fully shut down and started at least once after the
application of relevant guest operating system updates. This will allow any
corresponding security updates for the guest operating system to become fully

Citrix has released hotfixes that contain mitigations for Variant 2. These
hotfixes can be found on the Citrix website at the following locations:

Citrix XenServer 7.3: Citrix is actively working on a hotfix for this version.
This document will be updated when a hotfix is available.

Citrix XenServer 7.2: Citrix is actively working on a hotfix for this version.
This document will be updated when a hotfix is available.

Citrix XenServer 7.1 LTSR CU1: CTX230788 – 

Citrix XenServer 7.0: Citrix is actively working on a hotfix for this version.
This document will be updated when a hotfix is available.

Note that these updates are not Livepatchable.

Customers using End of Maintenance versions of Citrix XenServer, i.e. Citrix
XenServer version 6.0.2 Common Criteria, 6.2 SP1 and 6.5 SP1 are strongly 
recommended to upgrade to a more recent version. 

Citrix is actively working on additional mitigations for Variant 3, but
strongly recommends that customers that have deployed untrusted PV guests
on Intel CPUs consider transitioning to HVM-based guests.

What Citrix Is Doing

Citrix is notifying customers and channel partners about this potential
security issue. This article is also available from the Citrix Knowledge 
Center at  http://support.citrix.com/.

Obtaining Support on This Issue

If you require technical assistance with this issue, please contact Citrix
Technical Support. Contact details for Citrix Technical Support are available
at https://www.citrix.com/support/open-a-support-case.html. 

Reporting Security Vulnerabilities

Citrix welcomes input regarding the security of its products and considers
any and all potential vulnerabilities seriously. For guidance on how to report
security-related issues to Citrix, please see the following document: CTX081743
– Reporting Security Issues to Citrix


Date  			Change
4th January 2018 	Initial publishing

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