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XenOperating system

CVE-2021-28703

HIGH · 7.0 CVSS v3.1 Published 2021-12-07
Fix available
A fix is available. Upgrade to 14.4 or later.
See remediation →
72/100
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
grant table v2 status pages may remain accessible after de-allocation (take two) Guest get permitted access to certain Xen-owned pages of memory. The majority of such pages remain allocated / associated with a guest for its entire lifetime. Grant table v2 status pages, however, get de-allocated when a guest switched (back) from v2 to v1. The freeing of such pages requires that the hypervisor know where in the guest these pages were mapped. The hypervisor tracks only one use within guest space, but racing requests from the guest to insert mappings of these pages may result in any of them to become mapped in multiple locations. Upon switching back from v2 to v1, the guest would then retain access to a page that was freed and perhaps re-used for other purposes. This bug was fortuitously fixed by code cleanup in Xen 4.14, and backported to security-supported Xen branches as a prerequisite of the fix for XSA-378.

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

This is a use-after-free vulnerability in Xen hypervisor's grant table v2 implementation. When a guest switches from grant table v2 to v1, status pages are de-allocated, but due to a race condition where the hypervisor only tracks one mapped location, multiple mappings can exist. This allows a malicious guest to retain access to freed pages that may have been re-allocated for other purposes.

MitigationApply the Xen security patch (fixed in Xen 4.14 and backported to security-supported branches). This is a hypervisor-level fix requiring a Xen upgrade or patch deployment.

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:< 14.4

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
High
Privileges
Low
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/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. Check Xen hypervisor version
    Run 'xl version' or 'xm version' to display the Xen hypervisor version, or read /sys/hypervisor/version
    Affected if The version displayed is earlier than 4.14 (e.g., 4.13.x, 4.12.x, etc.)
  2. Identify guests using grant table v2
    For each running guest, inspect the grant table configuration. With 'xl list', identify active guests, then check guest configuration files in /etc/xen/ for 'gnttab' or 'grant_tables' settings, or query the guest via 'xl debug-keys g' to dump grant table state
    Affected if Any guest is configured with grant table v2 (often indicated by 'grant_tables_version=2' or similar configuration)
  3. Verify if guests can switch between grant table versions
    Review guest configuration files for dynamic grant table version switching settings, or monitor guest runtime behavior for version transitions using 'xl top' or xenwatch logging
    Affected if Guest configurations allow or perform switching between grant table v2 and v1 at runtime

The environment is affected if the Xen hypervisor version is below 4.14 AND any guest is actively using grant table v2 with the ability to switch versions, triggering the race condition during de-allocation of status pages.

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
Upgrade available Upgrade to 14.4 or later
Fixed in 14.4
Interim mitigation

Apply the Xen security patch (fixed in Xen 4.14 and backported to security-supported branches). This is a hypervisor-level fix requiring a Xen upgrade or patch deployment.

Recommended fix High confidence

Xen 4.14 or later stable release (also available as backported patches in security-supported Xen branches such as 4.13, 4.12, etc.)

  1. 1. Identify the current Xen hypervisor version running on the host using `xl version` or `xm version`
  2. 2. If running a version prior to Xen 4.14, plan for an upgrade to Xen 4.14 or later stable release
  3. 3. For production environments, test the upgrade in a staging environment first
  4. 4. Ensure all VMs are properly shut down or migrated off the host before upgrading
  5. 5. Upgrade Xen hypervisor to version 4.14 or later using the distribution's package manager or from xenproject.org
  6. 6. After upgrade, verify the new version is installed: `xl version`
  7. 7. Restart the hypervisor (or reboot the host) as required by the upgrade
  8. 8. Confirm VMs start correctly and grant table functionality is normal
Caveat Standard Xen upgrade considerations apply - ensure VM compatibility and test thoroughly before production deployment

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

Fix this in Xen Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing6.0 h
  • Review / QA4.0 h
22.0 hours of engineering $3,860
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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-2021-28703 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
  • No spam, self-promotion, credentials, or personal data