CVE-2021-1446: Cisco IOS XE DNS ALG DoS Vulnerability

CVE-2021-1446 Overview

A vulnerability in the DNS application layer gateway (ALG) functionality used by Network Address Translation (NAT) in Cisco IOS XE Software could allow an unauthenticated, remote attacker to cause an affected device to reload. The vulnerability is due to a logic error that occurs when an affected device inspects certain DNS packets. An attacker could exploit this vulnerability by sending crafted DNS packets through an affected device that is performing NAT for DNS packets. A successful exploit could allow an attacker to cause the device to reload, resulting in a denial of service (DoS) condition on an affected device.

Critical Impact

This vulnerability enables remote attackers to cause complete device reloads without authentication, disrupting network services for all users and systems relying on affected Cisco IOS XE routers and switches performing DNS NAT translation.

Affected Products

• Cisco IOS XE Software versions 3.7.x through 3.18.x
• Cisco IOS XE Software versions 16.1.x through 16.12.x
• Cisco IOS XE Software versions 17.1.x through 17.3.x

Discovery Timeline

• March 24, 2021 - CVE-2021-1446 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-1446

Vulnerability Analysis

This vulnerability (CWE-754: Improper Check for Unusual or Exceptional Conditions) exists in the DNS Application Layer Gateway (ALG) component of Cisco IOS XE Software. The DNS ALG is responsible for inspecting and modifying DNS packets as they traverse the device when Network Address Translation is enabled. When certain malformed or specially crafted DNS packets are processed, a logic error in the packet inspection routine fails to properly handle the exceptional condition, leading to a system crash and subsequent device reload.

The vulnerability is exploitable only via IPv4 traffic and cannot be triggered using IPv6 packets. This limitation narrows the attack surface but still represents a significant risk for organizations with IPv4-based infrastructure utilizing NAT with DNS ALG functionality enabled.

Root Cause

The root cause of this vulnerability is a logic error (CWE-754) in the DNS packet inspection code within the NAT ALG functionality. When the affected device processes DNS packets for NAT translation, the inspection routine does not properly validate certain fields or conditions within the DNS packet structure. This improper check for unusual or exceptional conditions causes the system to enter an unrecoverable state, forcing a device reload.

Attack Vector

An attacker can exploit this vulnerability remotely over the network without requiring any authentication or user interaction. The attack requires the attacker to send specially crafted DNS packets through a device that has NAT with DNS ALG enabled. The malicious traffic must traverse the affected device via IPv4—the vulnerability cannot be exploited via IPv6 traffic.

The attack can be initiated from any network position that allows DNS traffic to pass through the vulnerable device. This includes scenarios where the attacker is on the internal network sending requests outbound, or external attackers sending DNS responses inbound through the NAT device.

Detection Methods for CVE-2021-1446

Indicators of Compromise

• Unexpected device reloads with crashinfo logs indicating DNS ALG processing failures
• Unusual patterns of DNS traffic with malformed packets traversing NAT-enabled interfaces
• System log entries showing errors related to DNS packet inspection or NAT ALG operations
• Multiple device crashes occurring in close temporal proximity during DNS-heavy traffic periods

Detection Strategies

• Monitor syslog messages for DNS ALG-related errors and unexpected reload events on Cisco IOS XE devices
• Implement network traffic analysis to detect anomalous DNS packet structures passing through NAT boundaries
• Configure SNMP traps for device reload events and correlate with traffic patterns
• Use SentinelOne Singularity to monitor network device health and detect DoS attack indicators

Monitoring Recommendations

• Enable detailed logging for NAT ALG operations on affected Cisco IOS XE devices
• Configure crashinfo collection to capture forensic data during unexpected reloads
• Establish baseline metrics for DNS traffic volumes and patterns through NAT devices
• Implement alerting for multiple consecutive device reloads within short time windows

How to Mitigate CVE-2021-1446

Immediate Actions Required

• Review the Cisco Security Advisory for the complete list of affected and fixed software versions
• Upgrade Cisco IOS XE Software to a patched version as specified in Cisco's advisory
• Consider disabling DNS ALG functionality if it is not required for network operations
• Implement access control lists to filter potentially malicious DNS traffic at network boundaries

Patch Information

Cisco has released software updates that address this vulnerability. Administrators should consult the Cisco Security Advisory cisco-sa-alg-dos-hbBS7SZE to determine the appropriate fixed software release for their specific IOS XE version. The advisory contains detailed information about affected releases and their corresponding fixed versions across the 3.x, 16.x, and 17.x release trains.

Workarounds

• Disable DNS ALG functionality using no ip nat service alg udp dns if DNS NAT translation is not a business requirement
• Implement access control lists to restrict DNS traffic to known and trusted DNS servers only
• Deploy intrusion prevention systems (IPS) at network boundaries to filter malformed DNS packets
• Consider network segmentation to limit exposure of vulnerable devices to untrusted traffic sources

# Disable DNS ALG functionality (workaround)
configure terminal
no ip nat service alg udp dns
end
write memory

# Verify DNS ALG is disabled
show ip nat statistics | include DNS