dbcve.org
FedoraOperating system · Fedoraproject

CVE-2021-28710

HIGH · 8.8 CVSS v3.1 Published 2021-11-21
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
90/100
Remediation priority · Urgent
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
certain VT-d IOMMUs may not work in shared page table mode For efficiency reasons, address translation control structures (page tables) may (and, on suitable hardware, by default will) be shared between CPUs, for second-level translation (EPT), and IOMMUs. These page tables are presently set up to always be 4 levels deep. However, an IOMMU may require the use of just 3 page table levels. In such a configuration the lop level table needs to be stripped before inserting the root table's address into the hardware pagetable base register. When sharing page tables, Xen erroneously skipped this stripping. Consequently, the guest is able to write to leaf page table entries.

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 · high confidence

In Xen hypervisors with VT-d IOMMU support, when sharing page tables between CPUs and IOMMUs, the top-level page table is incorrectly not stripped for configurations requiring only 3 levels instead of 4. This allows a guest VM to write to leaf page table entries, potentially enabling privilege escalation from guest to hypervisor.

MitigationApply the Xen security patch for CVE-2021-28710. If patching is not immediately possible, consider disabling shared page table mode (if supported by hardware and acceptable for performance) as a temporary workaround.

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
FedoraOperating system
Affected:= 35
XenOperating system
Affected:= 4.15.0

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
Changed
Confidentiality
High
Integrity
High
Availability
High

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

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 installation
    Run 'xl info' or 'xm info' to confirm Xen is running, or check for xen-related processes with 'ps aux | grep xen'
    Affected if Xen hypervisor is not present - the CVE only affects Xen environments with VT-d IOMMU support
  2. Check Xen version
    Run 'xl info' or 'xm info' and look for the 'xen_version' field, or check 'rpm -q xen' on Fedora systems
    Affected if Version equals 4.15.0 or matches the affected range (exact version 4.15.0 and Fedora 35)
  3. Verify VT-d IOMMU is enabled
    Run 'xl dmesg | grep -i vt-d' or check for 'iommu=1' in Xen boot parameters via 'cat /proc/cmdline'
    Affected if VT-d IOMMU is enabled - the flaw requires VT-d IOMMU support to be active
  4. Check for shared page table configuration
    Examine Xen configuration files (e.g., /etc/xen/xl.conf or /etc/xen/config) for 'iommu_map' or 'hvm_iommu' settings, or check 'xl dmesg' for shared page table related messages
    Affected if Shared page table mode between CPUs and IOMMU is enabled - the vulnerability requires this sharing configuration

You are affected if you are running Xen 4.15.0 (or Fedora 35 with Xen) with VT-d IOMMU support enabled and shared page table mode configured between CPUs and the IOMMU.

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.

dbcve · scoped
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Apply the Xen security patch for CVE-2021-28710. If patching is not immediately possible, consider disabling shared page table mode (if supported by hardware and acceptable for performance) as a temporary workaround.

Recommended fix High confidence

Xen 4.15.1 or later (or updated distribution-provided Xen package)

  1. 1. Back up all virtual machine configurations and data before updating the hypervisor
  2. 2. For Fedora 35 systems: Run 'dnf update' to install the latest Xen packages which include the security fix
  3. 3. For Gentoo systems: Run 'emerge --sync' followed by 'emerge --update xen' to get patched Xen packages
  4. 4. After updating, verify the Xen version matches a fixed release (4.15.1 or later) using 'xl info' or 'xm info'
  5. 5. Reboot the host system to load the updated hypervisor
  6. 6. Verify that virtual machines start correctly and the hypervisor is running the fixed version
Caveat Updating the hypervisor may require downtime for the host system; ensure VM backups are verified before reboot

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Fedora Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing6.0 h
  • Review / QA2.0 h
14.0 hours of engineering $2,380
Get help mitigating

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,808.

Scan for this in your stack

Free · runs locally
dbcve dependency scanner

Check whether your project pulls in CVE-2021-28710 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.

Get it on GitHub Scan in the browser

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-2021-28710 in production — separate from our analysis above.

No notes yet

Be the first to add a field note for this CVE — a mitigation you’ve verified, a version caveat, or a link to a working fix. Sign in above to contribute.

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
  • No spam, self-promotion, credentials, or personal data