CVE-2021-34720: Cisco IOS XR DoS Vulnerability

CVE-2021-34720 Overview

A denial of service vulnerability exists in the IP Service Level Agreements (IP SLA) responder and Two-Way Active Measurement Protocol (TWAMP) features of Cisco IOS XR Software. This vulnerability allows an unauthenticated, remote attacker to cause device packet memory exhaustion or crash the IP SLA process, resulting in a denial of service (DoS) condition that can impact critical network infrastructure.

Critical Impact

Successful exploitation can exhaust packet memory on affected devices, impacting routing protocols and other critical processes, or crash the IP SLA process entirely, leading to network measurement and monitoring failures.

Affected Products

• Cisco IOS XR Software
• Cisco ASR 9000 Series (9000v-v2, 9001, 9006, 9010, 9901, 9902, 9903, 9904, 9906, 9910, 9912, 9922)
• Cisco NCS Series (520, 540, 560, 1001, 1002, 1004, 4009, 4016, 5001, 5002, 5011, 5501, 5502, 5508, 5516, 6000, 6008)
• Cisco IOS XRv and IOS XRv 9000
• Cisco 8000 Series Routers (8101, 8102, 8201, 8202, 8804, 8808, 8812, 8818)

Discovery Timeline

• September 9, 2021 - CVE-2021-34720 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-34720

Vulnerability Analysis

The vulnerability stems from improper handling of socket creation failures during the IP SLA and TWAMP processes within Cisco IOS XR Software. When socket creation fails, the affected processes do not properly manage the failure condition, leading to resource mismanagement that can be exploited by attackers sending specially crafted packets.

The IP SLA responder is designed to respond to measurement packets for network performance monitoring, while TWAMP provides standardized two-way active measurement capabilities. Both features share underlying socket handling code that contains the vulnerability.

This vulnerability is classified under CWE-771 (Missing Reference to Active Allocated Resource), indicating that the software fails to properly track and release allocated resources when socket creation failures occur. This improper resource management allows an attacker to trigger conditions that exhaust the device's packet memory.

Root Cause

The root cause of CVE-2021-34720 is the mishandling of socket creation failures in the IP SLA and TWAMP processing code. When the software attempts to create a socket and fails, the failure condition is not properly handled, leading to:

1. Incomplete cleanup of allocated resources
2. Accumulation of orphaned memory allocations
3. Progressive depletion of available packet memory

This design flaw allows an attacker to repeatedly trigger socket creation failures through crafted packets, systematically exhausting device memory resources.

Attack Vector

An unauthenticated, remote attacker can exploit this vulnerability by sending specifically crafted IP SLA or TWAMP packets to an affected device. The attack requires:

1. Network access to the target device running vulnerable Cisco IOS XR Software
2. The IP SLA responder or TWAMP feature enabled on the target device
3. Ability to send UDP packets to the IP SLA or TWAMP service ports

The attack does not require authentication or user interaction. By sending malicious packets that trigger repeated socket creation failures, the attacker can progressively exhaust packet memory, eventually impacting routing protocols and other critical processes, or crash the IP SLA process entirely.

Detection Methods for CVE-2021-34720

Indicators of Compromise

• Unusual increases in memory utilization on affected Cisco IOS XR devices, particularly in packet memory pools
• IP SLA process crashes or unexpected restarts visible in system logs
• Degraded performance or failures in routing protocols due to memory exhaustion
• High volume of IP SLA or TWAMP packets from unexpected sources

Detection Strategies

• Monitor device memory utilization using SNMP or CLI commands such as show memory summary and show processes memory
• Implement syslog alerting for IP SLA process crash events and memory allocation failures
• Deploy network intrusion detection rules to identify anomalous IP SLA or TWAMP traffic patterns
• Use NetFlow or similar flow monitoring to detect high-volume traffic targeting IP SLA responder ports

Monitoring Recommendations

• Configure SNMP traps for memory threshold alerts on affected Cisco IOS XR devices
• Enable detailed logging for IP SLA and TWAMP processes to capture failure events
• Establish baseline memory utilization patterns to detect abnormal consumption
• Implement centralized log collection for correlation of potential exploitation attempts across the network

How to Mitigate CVE-2021-34720

Immediate Actions Required

• Review Cisco Security Advisory cisco-sa-ipsla-ZA3SRrpP for specific patch information and fixed software versions
• Disable the IP SLA responder feature if not required using no ipsla responder configuration command
• Disable TWAMP responder functionality if not operationally necessary
• Implement access control lists (ACLs) to restrict IP SLA and TWAMP traffic to trusted management sources only

Patch Information

Cisco has released software updates addressing CVE-2021-34720. Administrators should consult the Cisco Security Advisory for the complete list of fixed software releases applicable to their specific hardware platforms. The advisory provides detailed version information for all affected IOS XR releases and corresponding fixed versions.

Given the broad impact on Cisco's routing portfolio including ASR 9000, NCS, and 8000 series devices, organizations should prioritize patching based on the criticality of affected infrastructure.

Workarounds

• Disable the IP SLA responder feature using the command no ipsla responder under global configuration mode if the feature is not required
• Disable TWAMP responder functionality using no twamp reflector if two-way measurements are not needed
• Apply infrastructure ACLs to limit access to IP SLA and TWAMP services to authorized management stations only
• Implement Control Plane Policing (CoPP) to rate-limit traffic destined to the router's control plane

# Example: Disable IP SLA responder on Cisco IOS XR
configure terminal
no ipsla responder
commit
end

# Example: ACL to restrict IP SLA access to trusted sources
ipv4 access-list IP-SLA-RESTRICT
 10 permit udp host 10.1.1.100 any eq 1967
 20 deny udp any any eq 1967
 30 permit ipv4 any any