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AUSCERT External Security Bulletin Redistribution
1 November 2019
AusCERT Security Bulletin Summary
Operating System: Xen
Impact/Access: Denial of Service -- Existing Account
CVE Names: CVE-2019-18420
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Xen Security Advisory CVE-2019-18420 / XSA-296
UPDATES IN VERSION 4
hypercall_create_continuation() is a variadic function which uses a
printf-like format string to interpret its parameters. Error handling
for a bad format character was done using BUG(), which crashes Xen.
One path, via the VCPUOP_initialise hypercall, has a bad format
character. The BUG() can be hit if VCPUOP_initialise executes for a
sufficiently long period of time for a continuation to be created.
Malicious guests may cause a hypervisor crash, resulting in a Denial of
Xen versions 4.6 and newer are vulnerable. Xen versions 4.5 and earlier
are not vulnerable.
Only x86 PV guests can exploit the vulnerability. HVM and PVH guests,
and guests on ARM systems, cannot exploit the vulnerability.
There are no mitigations.
This issue was discovered by Andrew Cooper of Citrix.
Applying the appropriate attached patch resolves this issue.
xsa296.patch Xen 4.9 ... unstable
xsa296-4.8.patch Xen 4.7 ... 4.8
$ sha256sum xsa296*
DEPLOYMENT DURING EMBARGO
Deployment of the patches and/or mitigations described above (or
others which are substantially similar) is permitted during the
embargo, even on public-facing systems with untrusted guest users and
But: Distribution of updated software is prohibited (except to other
members of the predisclosure list).
Predisclosure list members who wish to deploy significantly different
patches and/or mitigations, please contact the Xen Project Security
(Note: this during-embargo deployment notice is retained in
post-embargo publicly released Xen Project advisories, even though it
is then no longer applicable. This is to enable the community to have
oversight of the Xen Project Security Team's decisionmaking.)
For more information about permissible uses of embargoed information,
consult the Xen Project community's agreed Security Policy:
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