CVE-2021-34737

Cisco IOS XR Software DHCP Version 4 Server Denial of Service Vulnerability

HIGH

7.5

CVSS Score

Published: September 9, 2021 at 05:01 AM UTC

Updated: June 17, 2026 at 03:56 AM UTC

37 products affected across 1 vendorIOS XrAsr 9010Asr 9904

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Severity

HIGH7.5

Exploit prediction

EPSS 1%

SSVC

No known exploit

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.

Affected

Our own answer: ranges merged, affected separated from platform-only, with confidence and review flags.

IOS Xr · Cisco

95%

v < 7.3.27.4.0 ≤ v ≤ 7.4.17.4.0 ≤ v < 7.4.1

Affected range stops below 7.3.2, so it looks safe — our inference, not a vendor statement

Affected range stops below 7.4.1, so it looks safe — our inference, not a vendor statement

cisco-asr-9000v-v2 (environment) · cisco-asr-9001 (environment) · cisco-asr-9006 (environment) · cisco-asr-9010 (environment) · cisco-asr-9901 (environment) · cisco-asr-9902 (environment) · cisco-asr-9903 (environment) · cisco-asr-9904 (environment) · cisco-asr-9906 (environment) · cisco-asr-9910 (environment) · cisco-asr-9912 (environment) · cisco-asr-9922 (environment) · cisco-ncs-5001 (environment) · cisco-ncs-5002 (environment) · cisco-ncs-5011 (environment) · cisco-ncs-5501 (environment) · cisco-ncs-5501-se (environment) · cisco-ncs-5502 (environment) · cisco-ncs-5502-se (environment) · cisco-ncs-5508 (environment) · cisco-ncs-5516 (environment) · cisco-ncs-560-4 (environment) · cisco-ncs-560-7 (environment) · cisco-ncs540-12z20g-sys-a (environment) · cisco-ncs540-12z20g-sys-d (environment) · cisco-ncs540-24z8q2c-m (environment) · cisco-ncs540-24z8q2c-sys (environment) · cisco-ncs540-28z4c-sys-a (environment) · cisco-ncs540-28z4c-sys-d (environment) · cisco-ncs540-acc-sys (environment) · cisco-ncs540x-12z16g-sys-a (environment) · cisco-ncs540x-12z16g-sys-d (environment) · cisco-ncs540x-16z4g8q2c-a (environment) · cisco-ncs540x-16z4g8q2c-d (environment) · cisco-ncs540x-acc-sys (environment) · cisco-ios-xrv-9000 (same device)

Asr 9000V-V2 · Ciscodevice

80%

All versions

Asr 9001 · Ciscodevice

80%

All versions

Asr 9006 · Ciscodevice

80%

All versions

Asr 9010 · Ciscodevice

80%

All versions