CVE-2021-34737: Cisco IOS XR Software DHCP Version 4 Server Denial of Service Vulnerability

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2021-09-09 · updated 2026-06-17
Weakness: CWE-476

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 products

Cisco IOS XR Software: before 7.3.2; from 7.4.0 through 7.4.1; from 7.4.0 before 7.4.1 — fixed in 7.3.2, 7.4.1

How to fix

Upgrade to a fixed version: Cisco IOS XR Software 7.3.2, 7.4.1. Check the vendor advisory for mitigations.

References

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