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CVE Record

CVE-2021-47111: xen-netback: take a reference to the RX task thread

In the Linux kernel, the following vulnerability has been resolved: xen-netback: take a reference to the RX task thread Do this in order to prevent the task from being freed if the thread returns (which can be triggered by the frontend) before the call to kthread_stop done as part of the backend tear down. Not taking the reference will lead to a use-after-free in that scenario. Such reference was taken before but dropped as part of the rework done in 2ac061ce97f4. Reintroduce the reference taking and add a comment this time explaining why it's needed. This is XSA-374 / CVE-2021-28691.

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysisunknown

Security readout for executives and security teams

Plain-English summary

This is a Linux kernel Xen networking bug. A Xen guest frontend can trigger a timing condition where the backend RX thread is freed before teardown completes, causing a use-after-free. The provided sources do not state active exploitation, CVSS severity, or full business impact.

Executive priority

Treat this as a targeted virtualization-host risk, not a general internet-facing application issue. Prioritize patch confirmation on Xen infrastructure, especially where guests are operated by less-trusted tenants or customers.

Technical view

CVE-2021-47111 fixes xen-netback by taking a reference to the RX task thread before backend teardown. Without that reference, a frontend-triggered thread return before kthread_stop can leave teardown using freed task memory. The record identifies this as XSA-374 / CVE-2021-28691.

Likely exposure

Exposure appears limited to Linux systems using Xen netback networking, especially virtualization hosts serving Xen guest frontends. The provided affected data references Linux 5.5 and fixes around 5.10.43, 5.12.10, and 5.13, but the version range details are incomplete in the bundle.

Exploitation context

The source says the condition can be triggered by the frontend, implying guest-to-backend interaction in Xen networking. No cited source in the bundle reports public exploitation, KEV listing, weaponized exploit availability, or a confirmed attacker outcome beyond use-after-free.

Researcher notes

The strongest evidence is the kernel commit description: a frontend-triggered RX thread return can race backend teardown and cause use-after-free. The bundle lacks CVSS, CWE, exploitability details, and distribution-specific fix status, so validation should rely on kernel commit/backport checks.

Mitigation direction

  • Identify Xen hosts using Linux xen-netback networking.
  • Prioritize kernel updates to versions containing the referenced stable fixes.
  • Check Linux distribution advisories for backported kernel fixes.
  • Review Xen security advisory XSA-374 / CVE-2021-28691 mapping.
  • Limit untrusted guest access where kernel updates are pending.

Validation and detection

  • Inventory kernels on Xen virtualization hosts.
  • Confirm whether xen-netback is loaded or in use.
  • Compare installed kernel packages with vendor fixed versions.
  • Verify the referenced stable commits are present or backported.
  • Check vulnerability scanners for CVE-2021-47111 and CVE-2021-28691 coverage.
Prepared
Confidence
medium
Sources
5

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

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CVE-2021-47111 mapping review

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Vulnerability profileCVE Program record
Severity
Unknown
CVSS
Not scored
Known Exploited
No
Published
Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

0CVSS vectors
3Timeline events
2ADP providers
4Source links

SSVC decision data

CISA-ADPCISA Coordinator
Timestamp
Version
2.0.3
Exploitation: noneAutomatable: noTechnical Impact: partial

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  2. CVE publishedCVE Program

    The CVE record was published.

  3. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

ADP provider summaries

CISA-ADPCISA ADP Vulnrichment
other:ssvc
CVECVE Program Container

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
LinuxLinux2ac061ce97f413bfbbdd768f7d2e0fda2e8170df, 2ac061ce97f413bfbbdd768f7d2e0fda2e8170df, 2ac061ce97f413bfbbdd768f7d2e0fda2e8170dfunaffected
LinuxLinux5.5, 0, 5.10.43, 5.12.10, 5.13affected
Weakness

CWE details

No CWE listed

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.