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XenOperating system

CVE-2021-28687

MEDIUM · 5.5 CVSS v3.1 Published 2021-06-11
Fix available
A fix is available. Upgrade to after 4.15.0 or later.
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
HVM soft-reset crashes toolstack libxl requires all data structures passed across its public interface to be initialized before use and disposed of afterwards by calling a specific set of functions. Many internal data structures also require this initialize / dispose discipline, but not all of them. When the "soft reset" feature was implemented, the libxl__domain_suspend_state structure didn't require any initialization or disposal. At some point later, an initialization function was introduced for the structure; but the "soft reset" path wasn't refactored to call the initialization function. When a guest nwo initiates a "soft reboot", uninitialized data structure leads to an assert() when later code finds the structure in an unexpected state. The effect of this is to crash the process monitoring the guest. How this affects the system depends on the structure of the toolstack. For xl, this will have no security-relevant effect: every VM has its own independent monitoring process, which contains no state. The domain in question will hang in a crashed state, but can be destroyed by `xl destroy` just like any other non-cooperating domain. For daemon-based toolstacks linked against libxl, such as libvirt, this will crash the toolstack, losing the state of any in-progress operations (localized DoS), and preventing further administrator operations unless the daemon is configured to restart automatically (system-wide DoS). If crashes "leak" resources, then repeated crashes could use up resources, also causing a system-wide DoS.

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 libxl, the soft-reset code path fails to initialize the libxl__domain_suspend_state structure before use. When a guest initiates a soft reboot, this uninitialized structure causes an assert() failure in later code that expects properly initialized data, crashing the monitoring process. For xl, this is isolated; for daemon-based toolstacks like libvirt, it crashes the entire toolstack daemon.

MitigationApply the vendor patch to add the missing initialization call for libxl__domain_suspend_state in the soft reset code path, or upgrade to a patched Xen version.

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:>= 4.12, <= 4.15.0= 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
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 installed Xen hypervisor version
    Run `xl version` or `xm version` or check `/sys/hypervisor/version/` to obtain the Xen version number
    Affected if The version falls within 4.12 through 4.15.0 inclusive
  2. Determine the toolstack in use
    Check if the system runs xl (standalone) or a daemon-based toolstack such as libvirtd. Run `ps aux | grep -E '(xl|libvirtd)'` or check for socket `/var/run/xen/socket/libxl`
    Affected if Using libxl-based toolstack (xl or libvirt daemon) - both are affected, though libvirt daemon crash is more severe
  3. Confirm libxl is the active interface
    Verify that libxl is being used by checking for libxl driver files: `ls /var/run/libxl/`, or for libvirt with libxl driver: `virsh uri` showing `libxl:///`
    Affected if libxl interface is active - this is required for the vulnerable code path to execute
  4. Verify soft reset capability exposure
    Check if any guests are configured that could trigger a soft reset. Guest soft reset is triggered by the guest OS initiating a reboot (not a full power cycle). Inspect guest configuration: `xl list` or `virsh list --all`
    Affected if Any active paravirtualized or HVM guests exist that can perform a soft reboot - the flaw triggers when a guest initiates soft reset

You are affected if running Xen version 4.12 through 4.15.0 with libxl-based toolstack (xl or libvirt) and any guest can trigger a soft reboot operation.

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 a release after 4.15.0
Interim mitigation

Apply the vendor patch to add the missing initialization call for libxl__domain_suspend_state in the soft reset code path, or upgrade to a patched Xen version.

Fix this in Xen Scoped from the published advisory
  • Consultation4.0 h
  • Implementation6.0 h
  • Testing8.0 h
  • Review / QA4.0 h
22.0 hours of engineering $3,800
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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-2021-28687 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
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