dbcve.org
FreeBSDOperating system

CVE-2021-29631

HIGH · 7.8 CVSS v3.1 Published 2021-08-30
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
80/100
Remediation priority · High
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
In FreeBSD 13.0-STABLE before n246941-20f96f215562, 12.2-STABLE before r370400, 11.4-STABLE before r370399, 13.0-RELEASE before p4, 12.2-RELEASE before p10, and 11.4-RELEASE before p13, certain VirtIO-based device models in bhyve failed to handle errors when fetching I/O descriptors. A malicious guest may cause the device model to operate on uninitialized I/O vectors leading to memory corruption, crashing of the bhyve process, and possibly arbitrary code execution in the bhyve process.

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 FreeBSD's bhyve hypervisor, VirtIO-based device models fail to properly handle errors when fetching I/O descriptors from the guest. This causes the device model to operate on uninitialized I/O vectors, leading to memory corruption. A malicious guest can exploit this to crash the bhyve process (Denial of Service) and potentially achieve arbitrary code execution within the bhyve process context.

MitigationUpgrade FreeBSD to the patched versions: 13.0-STABLE after n246941-20f96f215562, 12.2-STABLE after r370400, 11.4-STABLE after r370399, or 13.0-RELEASE p4+, 12.2-RELEASE p10+, 11.4-RELEASE p13+. If immediate patching is not feasible, limit or monitor VirtIO device usage in bhyve guests.

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
FreeBSDOperating system
Affected:= 11.4= 12.2= 13.0

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
High
Availability
High

CVSS:3.1/AV:L/AC:L/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. Identify FreeBSD version
    Run 'uname -r' or check /etc/os-release to determine the installed FreeBSD version
    Affected if Version matches 11.4, 12.2, or 13.0 (or is within those release branches before patches)
  2. Verify bhyve hypervisor is in use
    Check for running bhyve processes with 'ps aux | grep bhyve' or list active VMs with 'bhyvectl --list'
    Affected if bhyve instances are running on the affected FreeBSD version
  3. Check for VirtIO devices in bhyve configurations
    Examine bhyve VM configurations for VirtIO device specifications such as virtio-blk, virtio-net, or virtio-scsi in the launch arguments or configuration files
    Affected if Any VirtIO-based device model (virtio-blk, virtio-net, virtio-scsi) is attached to a guest VM on the affected version

A system is affected if it runs FreeBSD 11.4, 12.2, or 13.0 with active bhyve virtual machines using VirtIO device models.

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

Upgrade FreeBSD to the patched versions: 13.0-STABLE after n246941-20f96f215562, 12.2-STABLE after r370400, 11.4-STABLE after r370399, or 13.0-RELEASE p4+, 12.2-RELEASE p10+, 11.4-RELEASE p13+. If immediate patching is not feasible, limit or monitor VirtIO device usage in bhyve guests.

Recommended fix High confidence

FreeBSD 13.0-RELEASE-p4+ / 12.2-RELEASE-p10+ / 11.4-RELEASE-p13+, or corresponding STABLE branches after the specified commits

  1. Identify current FreeBSD version with: uname -a
  2. For FreeBSD 13.0: upgrade to 13.0-RELEASE-p4 or later using: freebsd-update upgrade -r 13.0-RELEASE && freebsd-update install, then reboot
  3. For FreeBSD 12.2: upgrade to 12.2-RELEASE-p10 or later using: freebsd-update upgrade -r 12.2-RELEASE && freebsd-update install, then reboot
  4. For FreeBSD 11.4: upgrade to 11.4-RELEASE-p13 or later using: freebsd-update upgrade -r 11.4-RELEASE && freebsd-update install, then reboot
  5. Alternatively, for STABLE branches: update to 13.0-STABLE after commit n246941-20f96f215562, 12.2-STABLE after r370400, or 11.4-STABLE after r370399
  6. After upgrade, restart any bhyve instances to run with the patched device models
Caveat Point releases rarely introduce breaking changes; review FreeBSD release notes for any compatibility notices

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

Fix this in FreeBSD Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing6.0 h
  • Review / QA2.0 h
20.0 hours of engineering $3,500
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Scan for this in your stack

Free · runs locally
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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-2021-29631 in production — separate from our analysis above.

No notes yet

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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