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Linux KernelOperating system · Linux

CVE-2021-28691

HIGH · 7.8 CVSS v3.1 Published 2021-06-29
Fix available
A fix is available. Upgrade to 5.12.2 or later.
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
Guest triggered use-after-free in Linux xen-netback A malicious or buggy network PV frontend can force Linux netback to disable the interface and terminate the receive kernel thread associated with queue 0 in response to the frontend sending a malformed packet. Such kernel thread termination will lead to a use-after-free in Linux netback when the backend is destroyed, as the kernel thread associated with queue 0 will have already exited and thus the call to kthread_stop will be performed against a stale pointer.

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

A use-after-free vulnerability exists in Linux xen-netback where a malicious or buggy paravirtualized network frontend can trigger termination of the receive kernel thread for queue 0 by sending malformed packets. When the backend is subsequently destroyed, kthread_stop is called against a stale pointer because the kernel thread has already exited, leading to use-after-free.

MitigationApply the upstream Linux kernel patch or update to a patched kernel version that addresses the xen-netback use-after-free. In production Xen environments, ensure network frontend inputs are validated and consider network isolation as a compensating control until patching can be completed.

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
Linux KernelOperating system
Affected:>= 5.5.0, < 5.12.2
Cloud BackupApplication
Affected:all versions
H410c FirmwareOperating system
Affected:all versions
H300s FirmwareOperating system
Affected:all versions
H500s FirmwareOperating system
Affected:all versions
H700s FirmwareOperating system
Affected:all versions
H300e FirmwareOperating system
Affected:all versions
H500e FirmwareOperating system
Affected:all versions

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 the Linux kernel version
    Run 'uname -r' or check '/proc/version' to get the kernel version string
    Affected if The kernel version is >= 5.5.0 and < 5.12.2, placing it within the vulnerable range
  2. Confirm Xen hypervisor environment
    Check if the system is running under Xen by examining 'xl info' output, 'xen-detect', or looking for Xen-specific entries in '/var/log/dmesg' or '/var/log/messages'
    Affected if The system is a Xen host or guest using paravirtualized network backends (xen-netback)
  3. Verify xen-netback module status
    Run 'lsmod | grep xen_netback' to see if the module is loaded, or check '/sys/module/xen_netback' if the module exists in the kernel
    Affected if The xen_netback module is loaded or built into the running kernel on a Xen host (dom0 or driver domain)
  4. Check kernel configuration for Xen support
    Inspect '/boot/config-$(uname -r)' or '/proc/config.gz' for 'CONFIG_XEN_NETBACK=y' or 'CONFIG_XEN_NETBACK=m'
    Affected if Xen netback driver support is enabled in the kernel configuration

You are affected if running a Linux kernel between 5.5.0 and 5.12.2 on a Xen host or driver domain with xen-netback enabled, where a malicious or buggy paravirtualized network frontend can send malformed packets to trigger the use-after-free.

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 5.12.2 or later
Fixed in 5.12.2
Interim mitigation

Apply the upstream Linux kernel patch or update to a patched kernel version that addresses the xen-netback use-after-free. In production Xen environments, ensure network frontend inputs are validated and consider network isolation as a compensating control until patching can be completed.

Fix this in Linux Kernel Scoped from the published advisory
  • Consultation3.0 h
  • Implementation6.0 h
  • Testing4.0 h
  • Review / QA2.0 h
15.0 hours of engineering $2,640
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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-2021-28691 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