CVE-2021-34737Network attack vector

DHCP service interruption via malformed DHCPv4 messages

Published Sep 9, 2021 · Updated Nov 7, 2024

Denial of service in Cisco IOS XR Software allows remote attackers to crash dhcpd by sending a malformed DHCPv4 message. The DHCPv4 message handler insufficiently validates certain malformed messages and dereferences a NULL pointer during processing. Exposure requires the DHCPv4 server or proxy feature to be enabled and bound to an interface; exploitation interrupts DHCPv4 service for up to about two minutes while dhcpd restarts, without reloading the router.

CVSS severity5.8
Medium
EPSS probability1.33%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Denial of service in Cisco IOS XR Software allows remote attackers to crash dhcpd by sending a malformed DHCPv4 message. The DHCPv4 message handler insufficiently validates certain malformed messages and dereferences a NULL pointer during processing. Exposure requires the DHCPv4 server or proxy feature to be enabled and bound to an interface; exploitation interrupts DHCPv4 service for up to about two minutes while dhcpd restarts, without reloading the router.

The record

CVE
CVE-2021-34737
Published
Sep 9, 2021
Updated
Nov 7, 2024
Vendor
ClamAV
Product
Cisco IOS XR Software
Classifications
CWE-476, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Sep 9, 2021CVE publishedPublication date reported by the CVE source.
  2. Nov 7, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

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  1. Affected versionversion=n/a

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

ClamAV · Cisco IOS XR Softwareversion=n/a

Published CVSS scores

7.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
5.8CiscoCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:L/E:X/RL:X/RC:X

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-476NULL Pointer Dereference
T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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