Is CVE-2021-20166 actively exploited?

Yes. KEV Intelligence tracks CVE-2021-20166 as a known exploited vulnerability. Confidence is high.

What exploitation activity has KEV Intelligence observed?

KEV Intelligence has not yet recorded sensor-observed exploitation attempts for CVE-2021-20166. Our sensors continue to monitor for activity targeting this CVE.

Which versions are affected?

CVE-2021-20166 affects Netgear RAX43. Reported affected version: 1.0.3.96.

No. CVE-2021-20166 is not currently listed in CISA KEV.

Vulnerability report

Exploited in the wild High confidence Not in CISA KEV

CVE-2021-20166

RAX43

NETGEAR / RAX43 · 1.0.3.96

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Severity: CVSS 8.8 · High
Confidence: High
Exploit status: Exploited in the wild
EPSS: 2.2%
First observed: —
Last observed: —

Decision summary

What security teams need to know first

Direct answers before the deeper technical record.

What it is

CVE-2021-20166 is an unauthenticated vulnerability affecting Netgear RAX43. Netgear RAX43 version 1.0.3.96 contains a buffer overrun vulnerability. The URL parsing functionality in the cgi-bin endpoint of the router...

Is it exploited?

Yes. KEV Intelligence tracks this CVE as a known exploited vulnerability. Confidence is high.

Who is affected?

Netgear / RAX43 1.0.3.96.

What should we do?

Prioritize remediation. Validate affected assets and apply vendor fixes on an accelerated timeline.

Overview

Netgear RAX43 version 1.0.3.96 contains a buffer overrun vulnerability

Netgear RAX43 version 1.0.3.96 contains a buffer overrun vulnerability.

The URL parsing functionality in the cgi-bin endpoint of the router containers a buffer overrun issue that can redirection control flow of the applicaiton.

Vendor Product Affected Status

n/a Netgear RAX43 1.0.3.96 Affected

Published: 30 Dec 2021
Exploitation Reported: 07 Jun 2026
Attack vector: Adjacent
Complexity: Low
Privileges: None
User interaction: None

CVE References

CVE Record CVE.org · CVE Record https://www.cve.org/CVERecord?id=CVE-2021-20166
tenable.com/security/research/tra-2021-55 tenable.com · CVE Record https://www.tenable.com/security/research/tra-2021-55

Exploitation evidence

Why KEV Intelligence marks this CVE as exploited

Third-party attestation and first-party sensor observation are shown separately so teams can judge the evidence chain.

Exploited in the wild

The Shadowserver

Recorded 07 Jun 2026

Independent exploitation attestation added to the KEV Intelligence record.

Known exploited vulnerability sources

Per-source evidence links for KEV attestations are available through the KEV Intelligence Pro API.

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Source	Added
The Shadowserver First	2026-06-07 00:00 UTC

Detection

Operational artifacts and observed signals

Make the evidence actionable in scanner, SOC, and edge-control workflows.

Observed signals

Request targets: 0
User-Agents: 0

Raw values available in Pro and Enterprise.

Scanner coverage

No scanner integrations recorded yet.

Virtual patch status

No KEV Intelligence virtual patch is currently available. Future rules ship for ModSecurity, Cloudflare, and AWS WAF.

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No detection artifacts or sensor request patterns are available for this CVE yet.

Check back as sensor telemetry and scanner integrations are updated.

Risk and context

Severity, weaknesses, and research context

CVSS v3.1

8.8 High

CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

EPSS

2.2%

Weaknesses

Buffer Copy without Checking Size of Input ('Classic Buffer Overflow') CWE-120

All CVSS Scores

CVSS v3.1 8.8 High

CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

CVSS v2.0 5.8 Medium

AV:A/AC:L/Au:N/C:P/I:P/A:P

Timeline

From disclosure to observed exploitation

07 Jun 2026
Added to KEV Intelligence KEV Feed

High-confidence, third-party attested exploitation
30 Dec 2021
CVE published

Vulnerability disclosed publicly
17 Dec 2020
CVE ID reserved

Identifier reserved by the CNA

Pro API

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Evidence confidence and provenance
First-party sensor telemetry
PoC and scanner references
Affected versions and enrichment
Automation-ready JSON delivery

GET /api/v2/pro/kevs/CVE-2021-20166

Free JSON includes basic KEV fields

{
  "cve_id": "CVE-2021-20166",
  "confidence": "High",
  "cvss_score": 8.8,
  "cvss_estimated": false,
  "epss_score": 0.02195,
  "exploit_status": {
    "exploited_in_the_wild": true,
    "active_exploitation_observed": false
  },
  "sensor_telemetry": { "attempts": 0, "sensors": 0 }
}

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