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XenOperating system

CVE-2021-28689

MEDIUM · 5.5 CVSS v3.1 Published 2021-06-11
Fix available
A fix is available. Upgrade to 4.12.0 or later.
See remediation →
57/100
Remediation priority · Elevated
Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
x86: Speculative vulnerabilities with bare (non-shim) 32-bit PV guests 32-bit x86 PV guest kernels run in ring 1. At the time when Xen was developed, this area of the i386 architecture was rarely used, which is why Xen was able to use it to implement paravirtualisation, Xen's novel approach to virtualization. In AMD64, Xen had to use a different implementation approach, so Xen does not use ring 1 to support 64-bit guests. With the focus now being on 64-bit systems, and the availability of explicit hardware support for virtualization, fixing speculation issues in ring 1 is not a priority for processor companies. Indirect Branch Restricted Speculation (IBRS) is an architectural x86 extension put together to combat speculative execution sidechannel attacks, including Spectre v2. It was retrofitted in microcode to existing CPUs. For more details on Spectre v2, see: http://xenbits.xen.org/xsa/advisory-254.html However, IBRS does not architecturally protect ring 0 from predictions learnt in ring 1. For more details, see: https://software.intel.com/security-software-guidance/deep-dives/deep-dive-indirect-branch-restricted-speculation Similar situations may exist with other mitigations for other kinds of speculative execution attacks. The situation is quite likely to be similar for speculative execution attacks which have yet to be discovered, disclosed, or mitigated.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · moderate confidence

32-bit x86 PV guest kernels run in ring 1, and IBRS mitigations for Spectre v2 do not architecturally protect ring 0 (hypervisor) from speculative predictions learned in ring 1 (guest). This allows malicious 32-bit PV guests to potentially influence hypervisor speculative execution, exposing sensitive data.

MitigationMigrate away from 32-bit PV guests to 64-bit or use hardware-assisted virtualization (HVM); alternatively, apply hypervisor-level workarounds if available and test thoroughly for functionality impacts.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
XenOperating system
Affected:< 4.12.0

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Local
Complexity
Low
Privileges
Low
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
None
Availability
None

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify Xen hypervisor version
    Run 'xl version' or 'xm version' to get the Xen version number, or check the package manager (e.g., 'dpkg -l | grep xen' on Debian/Ubuntu, 'rpm -q xen' on RHEL/CentOS)
    Affected if The Xen version is below 4.12.0
  2. Identify guest domain type
    For each guest domain, run 'xl list -l <domain_name>' and check the 'builder' field; PV indicates paravirtualized guests
    Affected if Any guest domain shows builder: 'pv' (paravirtualized) rather than 'hvm' (hardware-assisted virtualization)
  3. Identify guest kernel architecture
    Check the guest kernel by examining files inside the guest filesystem image (e.g., /boot/vmlinuz-* or /lib/modules/*) or by running 'file /boot/vmlinuz-*' on the guest kernel image; 32-bit kernels will show 'i386' or 'i686' in the output
    Affected if The guest is running a 32-bit x86 kernel (i386/i686) as a PV (paravirtualized) guest
  4. Confirm PV guest configuration
    Review the guest configuration file in /etc/xen/<guest_name>.cfg (or /var/lib/xen/auto/) and look for 'kernel' directive (indicating PV) rather than 'builder = "hvm"' (which indicates HVM)
    Affected if The guest configuration uses a 'kernel' directive without 'builder = hvm', confirming it is a paravirtualized guest

You are affected if your Xen version is below 4.12.0 AND you are running 32-bit x86 PV (paravirtualized) guests; 64-bit guests or HVM guests are not impacted by this specific flaw.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

From vendor data
Upgrade available Upgrade to 4.12.0 or later
Fixed in 4.12.0
Interim mitigation

Migrate away from 32-bit PV guests to 64-bit or use hardware-assisted virtualization (HVM); alternatively, apply hypervisor-level workarounds if available and test thoroughly for functionality impacts.

Fix this in Xen Scoped from the published advisory
  • Consultation8.0 h
  • Implementation24.0 h
  • Testing16.0 h
  • Review / QA8.0 h
56.0 hours of engineering $9,760
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-28689 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
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