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CVE Record

CVE-2021-47109: neighbour: allow NUD_NOARP entries to be forced GCed

In the Linux kernel, the following vulnerability has been resolved: neighbour: allow NUD_NOARP entries to be forced GCed IFF_POINTOPOINT interfaces use NUD_NOARP entries for IPv6. It's possible to fill up the neighbour table with enough entries that it will overflow for valid connections after that. This behaviour is more prevalent after commit 58956317c8de ("neighbor: Improve garbage collection") is applied, as it prevents removal from entries that are not NUD_FAILED, unless they are more than 5s old.

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

This Linux kernel issue can let neighbor table entries accumulate on point-to-point IPv6 interfaces until legitimate connections are affected. The business impact is primarily availability: systems using affected kernel versions and relevant network configurations may experience connection failures when the table overflows.

Executive priority

Treat as a targeted availability risk. Prioritize patch validation for network-facing Linux systems and infrastructure using point-to-point IPv6, but do not escalate as actively exploited based on the supplied evidence.

Technical view

The flaw involves Linux neighbor garbage collection for NUD_NOARP entries used by IPv6 on IFF_POINTOPOINT interfaces. After a neighbor garbage-collection change, entries not marked NUD_FAILED were harder to remove, allowing table pressure that could block valid neighbor entries.

Likely exposure

Exposure is most likely on Linux systems running affected kernel ranges with IPv6 over point-to-point interfaces. The source data lists Linux kernel affected ranges and stable kernel commits, but does not provide distribution-specific package status.

Exploitation context

The provided sources do not show CISA KEV listing, public exploitation, exploit maturity, or attacker requirements. The described outcome is resource exhaustion leading to valid connection failures, not code execution.

Researcher notes

The evidence supports a kernel neighbor table garbage-collection flaw affecting NUD_NOARP entries. Version mapping should be validated against vendor backports because distribution kernels may include fixes without matching upstream version numbers.

Mitigation direction

  • Check vendor or distribution kernel advisories for CVE-2021-47109 status.
  • Update to a kernel containing the referenced stable fix commits.
  • Prioritize systems using IPv6 on point-to-point interfaces.
  • Monitor neighbor table pressure and connection failure symptoms until patched.

Validation and detection

  • Inventory Linux kernel versions across exposed network infrastructure.
  • Identify hosts using IPv6 on point-to-point interfaces.
  • Confirm whether vendor kernels include the referenced stable fixes.
  • Review monitoring for neighbor table exhaustion or unexplained connection failures.
Prepared
Confidence
medium
Sources
6

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

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CVE-2021-47109 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
5Source 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
LinuxLinux58956317c8de52009d1a38a721474c24aef74fe7, 58956317c8de52009d1a38a721474c24aef74fe7, 58956317c8de52009d1a38a721474c24aef74fe7, 58956317c8de52009d1a38a721474c24aef74fe7unaffected
LinuxLinux5.0, 0, 5.4.125, 5.10.43, 5.12.10, 5.13affected
Weakness

CWE details

No CWE listed

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