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Debian LinuxOperating system · Debian

CVE-2020-25601

MEDIUM · 5.5 CVSS v3.1 Published 2020-09-23
Fix available
A fix is available. Upgrade to after 4.14.0 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
An issue was discovered in Xen through 4.14.x. There is a lack of preemption in evtchn_reset() / evtchn_destroy(). In particular, the FIFO event channel model allows guests to have a large number of event channels active at a time. Closing all of these (when resetting all event channels or when cleaning up after the guest) may take extended periods of time. So far, there was no arrangement for preemption at suitable intervals, allowing a CPU to spend an almost unbounded amount of time in the processing of these operations. Malicious or buggy guest kernels can mount a Denial of Service (DoS) attack affecting the entire system. All Xen versions are vulnerable in principle. Whether versions 4.3 and older are vulnerable depends on underlying hardware characteristics.

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 lack of preemption in Xen hypervisor's evtchn_reset() and evtchn_destroy() functions allows malicious guest kernels to cause CPU starvation. The FIFO event channel model permits guests to open many event channels; closing all of them during reset or cleanup can consume unbounded CPU time without yielding, enabling a guest to mount a system-wide DoS.

MitigationApply Xen security patches that add preemption points to evtchn_reset() and evtchn_destroy(). Until patched, monitor for guests generating abnormal numbers of event channel operations.

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
Debian LinuxOperating system
Affected:= 10.0
FedoraOperating system
Affected:= 31= 32= 33
XenOperating system
Affected:<= 4.14.0
LeapOperating system
Affected:= 15.2

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. Verify Xen hypervisor installation and version
    Run 'xl version' or 'xm version' to display the Xen hypervisor version, or check the installed xen package via package manager (e.g., 'rpm -q xen' or 'dpkg -l xen')
    Affected if The installed Xen version is 4.14.0 or lower
  2. Confirm FIFO event channel model is in use
    Check the Xen hypervisor configuration file (typically /etc/xen/xl.conf) or inspect guest configuration files for 'evtchn_fifo' setting; also verify via 'xl list' or by examining hypervisor boot parameters
    Affected if The FIFO event channel model is enabled (this is the specific model where the vulnerability exists)
  3. Check for CPU saturation from guest domains
    Run 'xl top' or 'xm top' to view per-guest CPU usage; look for a single guest consuming excessive CPU time without yielding, particularly during reset or shutdown operations
    Affected if A guest domain shows abnormally high CPU consumption that does not fluctuate, suggesting the unbounded loop without preemption is occurring
  4. Inspect hypervisor event channel usage
    Use 'xl debug-keys i' to dump internal event channel state, then examine the output for large numbers of active event channels associated with a single guest; also check domain logs for repeated evtchn_reset or evtchn_destroy operations
    Affected if A guest has opened an unusually large number of event channels (hundreds to thousands) and is performing reset or cleanup operations

A system is affected if it runs a Xen hypervisor version 4.14.0 or lower with the FIFO event channel model enabled, and exhibits symptoms of CPU starvation from a single guest performing event channel operations.

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 4.14.0
Interim mitigation

Apply Xen security patches that add preemption points to evtchn_reset() and evtchn_destroy(). Until patched, monitor for guests generating abnormal numbers of event channel operations.

Fix this in Debian Linux Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing12.0 h
  • Review / QA4.0 h
28.0 hours of engineering $4,760
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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-2020-25601 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