dbcve.org
FedoraOperating system · Fedoraproject

CVE-2021-28699

MEDIUM · 5.5 CVSS v3.1 Published 2021-08-27
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
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
inadequate grant-v2 status frames array bounds check The v2 grant table interface separates grant attributes from grant status. That is, when operating in this mode, a guest has two tables. As a result, guests also need to be able to retrieve the addresses that the new status tracking table can be accessed through. For 32-bit guests on x86, translation of requests has to occur because the interface structure layouts commonly differ between 32- and 64-bit. The translation of the request to obtain the frame numbers of the grant status table involves translating the resulting array of frame numbers. Since the space used to carry out the translation is limited, the translation layer tells the core function the capacity of the array within translation space. Unfortunately the core function then only enforces array bounds to be below 8 times the specified value, and would write past the available space if enough frame numbers needed storing.

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

This is a vulnerability in the Xen hypervisor's v2 grant table interface. When 32-bit guests on x86 request the frame numbers of the grant status table, translation of the request must occur due to differing interface structure layouts. The bounds check in the core function only enforces array bounds below 8 times the specified capacity value, allowing writes past the available translation space.

MitigationThis is a hypervisor-level vulnerability requiring a Xen security patch. Hosts should restrict untrusted guests and apply vendor patches once available.

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:= 33= 34= 35
Debian LinuxOperating system
Affected:= 11.0
XenOperating system
Affected:>= 4.10.0

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
None
Integrity
None
Availability
High

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/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 the Xen hypervisor version
    Run 'xl version' or 'xm version' or check the xen-utils-common package version with 'rpm -q xen' (Fedora) or 'dpkg -l xen' (Debian)
    Affected if The installed Xen version is 4.10.0 or higher
  2. Confirm the host is running Xen hypervisor
    Check for the presence of Xen kernel or hypervisor with 'uname -a' (look for 'xen' in the kernel name) or check for xen daemon processes with 'ps aux | grep xen'
    Affected if The system is not running Xen hypervisor (this CVE does not apply)
  3. Identify active guest domains
    List running domains with 'xl list' or 'xm list' and note the bit-width of each guest
    Affected if There are 32-bit (i386/i686) guest domains running on this Xen host
  4. Determine if grant table v2 interface is in use
    Inspect guest configuration files in /etc/xen/ for the grant_table parameter, or check Xen logs for gnttab-related entries with 'xl dmesg | grep -i grant'
    Affected if Guests are configured to use grant table v2 (grant_table='v2' in config) or v2 is the default for 32-bit guests on this Xen version

You are likely affected if this is a Xen host version 4.10.0 or higher running 32-bit guests with the v2 grant table interface active.

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

This is a hypervisor-level vulnerability requiring a Xen security patch. Hosts should restrict untrusted guests and apply vendor patches once available.

Recommended fix Moderate confidence

Xen 4.14.2 or later (or latest available security patch for your distribution)

  1. 1. Identify the current Xen hypervisor version running on the system using 'xl version' or 'xm version'
  2. 2. Check for available Xen security updates from your distribution (Fedora, Debian, or Xen upstream)
  3. 3. For Fedora systems: run 'dnf update xen' to apply available security patches
  4. 4. For Debian systems: run 'apt-get update && apt-get upgrade xen*' to apply available security patches
  5. 5. For upstream Xen: upgrade to Xen 4.14.2 or later which contains the fix for this vulnerability
  6. 6. After applying updates, reboot the host to load the updated hypervisor
  7. 7. Verify the new Xen version is running post-reboot with 'xl version'
Caveat Upgrading the Xen hypervisor may require rebooting all running VMs; ensure proper VM shutdown and migration procedures are in place before applying the update

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

Fix this in Fedora Scoped from the published advisory
  • Consultation4.0 h
  • Implementation12.0 h
  • Testing16.0 h
  • Review / QA8.0 h
40.0 hours of engineering $6,800
Get help mitigating

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $10,880.

Scan for this in your stack

Free · runs locally
dbcve dependency scanner

Check whether your project pulls in CVE-2021-28699 — 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-2021-28699 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