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No exploitation reportedFix available

CVE-2021-34737

No exploitation reported by the sources we read.

  • Published2021-09-09
  • Fixed in7.4.1 · 7.3.2
  • WeaknessCWE-476

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

Description

A vulnerability in the DHCP version 4 (DHCPv4) server feature of Cisco IOS XR Software could allow an unauthenticated, remote attacker to trigger a crash of the dhcpd process, resulting in a denial of service (DoS) condition. This vulnerability exists because certain DHCPv4 messages are improperly validated when they are processed by an affected device. An attacker could exploit this vulnerability by sending a malformed DHCPv4 message to an affected device. A successful exploit could allow the attacker to cause a NULL pointer dereference, resulting in a crash of the dhcpd process. While the dhcpd process is restarting, which may take up to approximately two minutes, DHCPv4 server services are unavailable on the affected device. This could temporarily prevent network access to clients that join the network during that time period. Note: Only the dhcpd process crashes and eventually restarts automatically. The router does not reload.

Timeline

dates in UTC
  1. 2021-09-0905:15CVE publishedNVD

Observed activity

isMalicious corpus · updated 2026-10-23 21:37 UTC
Indicators tagged with this CVE, per day0 indicators over 7 days
Block these indicators
  • Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2021-34737?format=txt
  • CSV export: https://api.ismalicious.com/edl/cve/CVE-2021-34737?format=csv
  • TAXII collection of exploited CVEs
  • The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD
Cisco ios xrversion to verify
BranchAffectedFixed in
7.4≥ 7.4.0, < 7.4.17.4.1
7.3< 7.3.27.3.2
7.4≥ 7.4.0, ≤ 7.4.1no fix published

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Fix

  1. Update Cisco ios xr to 7.4.1 · 7.3.2

Advisories and patches

Detection

Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Proof-of-concept repositories
none listed
PoC-in-GitHub

Sources

  • CISA-ADPSSVC decisionnone2024-11-07
  • OpenCVESSVC decisionnone—
  • CNACVSS 3.15.8read 2026-10-23
  • NVDCVSS 3.17.5read 2026-10-23
  • NVDCVSS 2.05read 2026-10-23
  • CNAweakness (CWE)CWE-476read 2026-10-23
  • NVDweakness (CWE)CWE-476read 2026-10-23
  • FIRST EPSSEPSS score1.3 %2026-10-23

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Published
2021-09-09
Last Modified
2026-06-17
NVD status
Modified

Frequently asked questions

What is CVE-2021-34737?

A vulnerability in the DHCP version 4 (DHCPv4) server feature of Cisco IOS XR Software could allow an unauthenticated, remote attacker to trigger a crash of the dhcpd process, resulting in a denial of service (DoS) condition. This vulnerability exists because certain DHCPv4 messages are improperly validated when they are processed by an affected device. An attacker could exploit this vulnerability by sending a malformed DHCPv4 message to an affected device. A successful exploit could allow the attacker to cause a NULL pointer dereference, resulting in a crash of the dhcpd process. While the dhcpd process is restarting, which may take up to approximately two minutes, DHCPv4 server services are unavailable on the affected device. This could temporarily prevent network access to clients that join the network during that time period. Note: Only the dhcpd process crashes and eventually restarts automatically. The router does not reload.

Is CVE-2021-34737 actively exploited?

Active exploitation of CVE-2021-34737 has not been confirmed. Its EPSS score was 1.3% on 2026-10-23, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2021-34737?

CVE-2021-34737 has a CVSS v3 base score of 7.5 (HIGH severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.

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