CVE-2020-26070: Cisco IOS XR Software DoS Vulnerability

CVE-2020-26070 Overview

A vulnerability in the ingress packet processing function of Cisco IOS XR Software for Cisco ASR 9000 Series Aggregation Services Routers could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition on an affected device. The vulnerability is due to improper resource allocation when an affected device processes network traffic in software switching mode (punted). An attacker could exploit this vulnerability by sending specific streams of Layer 2 or Layer 3 protocol data units (PDUs) to an affected device.

Critical Impact

A successful exploit could cause the affected device to run out of buffer resources, making the device unable to process or forward traffic, resulting in a complete denial of service condition requiring a device restart to regain functionality.

Affected Products

• Cisco IOS XR Software
• Cisco ASR 9000V (v2)
• Cisco ASR 9001
• Cisco ASR 9006
• Cisco ASR 9010
• Cisco ASR 9901
• Cisco ASR 9903
• Cisco ASR 9904
• Cisco ASR 9906
• Cisco ASR 9910
• Cisco ASR 9912
• Cisco ASR 9922

Discovery Timeline

• 2020-11-12 - CVE CVE-2020-26070 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-26070

Vulnerability Analysis

This vulnerability affects Cisco ASR 9000 Series routers running vulnerable versions of Cisco IOS XR Software. The core issue resides in how the device handles ingress packet processing when operating in software switching mode, also known as "punted" traffic. Under normal operations, high-performance routers like the ASR 9000 series process the majority of traffic in hardware (fast path). However, certain packets require software processing—these packets are "punted" to the CPU for handling.

The vulnerability stems from improper resource allocation (CWE-404: Improper Resource Shutdown or Release) during this software switching process. When specially crafted Layer 2 or Layer 3 PDUs are received, the system fails to properly manage buffer resources, leading to buffer exhaustion. This resource exhaustion vulnerability does not require authentication, making it particularly dangerous in network edge deployments where the ASR 9000 series is commonly positioned.

Root Cause

The root cause is classified under CWE-404 (Improper Resource Shutdown or Release). The vulnerability exists because the ingress packet processing function does not properly release or manage buffer resources when handling certain types of network traffic in software switching mode. This improper resource management allows an attacker to systematically exhaust the device's available buffer pool through sustained transmission of malicious PDUs.

Attack Vector

The attack can be executed remotely over the network without requiring any authentication or user interaction. An attacker needs only network connectivity to the vulnerable device to launch the attack.

The exploitation process involves:

1. The attacker identifies a Cisco ASR 9000 Series router running vulnerable Cisco IOS XR Software
2. The attacker crafts and transmits specific Layer 2 or Layer 3 protocol data units designed to trigger software switching mode processing
3. These PDUs cause the device to improperly allocate buffer resources without proper cleanup
4. As the attack continues, buffer resources become exhausted
5. The device becomes unable to process or forward legitimate traffic
6. A complete denial of service condition occurs, requiring a device restart to restore functionality

Detection Methods for CVE-2020-26070

Indicators of Compromise

• Unusual spikes in CPU utilization on ASR 9000 series routers, particularly related to packet processing functions
• Rapid depletion of buffer pool resources on affected devices
• Increased rate of packets being punted to software switching mode
• Interface or system logging messages indicating buffer allocation failures
• Network connectivity degradation or complete traffic forwarding failure

Detection Strategies

• Monitor Cisco IOS XR system logs for buffer allocation errors or resource exhaustion warnings
• Implement SNMP monitoring for buffer utilization metrics on ASR 9000 series devices
• Configure threshold-based alerts for CPU utilization anomalies on control plane processors
• Analyze NetFlow or traffic flow data for unusual PDU patterns targeting management interfaces

Monitoring Recommendations

• Enable detailed logging for the ingress packet processing subsystem on affected devices
• Implement continuous monitoring of show memory summary and show controllers np commands for buffer utilization trends
• Deploy network-based intrusion detection systems (IDS) to identify anomalous PDU patterns
• Establish baseline buffer utilization metrics and alert on significant deviations

How to Mitigate CVE-2020-26070

Immediate Actions Required

• Review the Cisco Security Advisory for affected version details and available patches
• Identify all Cisco ASR 9000 series routers in your environment running Cisco IOS XR Software
• Prioritize patching for internet-facing or edge-deployed ASR 9000 series devices
• Implement network access controls to limit exposure of vulnerable devices during the patching process

Patch Information

Cisco has released security updates to address this vulnerability. Administrators should consult the Cisco Security Advisory (cisco-sa-xr-cp-dos-ej8VB9QY) for specific fixed software versions and upgrade instructions. Organizations should follow their standard change management procedures when applying updates to critical network infrastructure.

Workarounds

• Implement Control Plane Policing (CoPP) to rate-limit traffic destined for software processing
• Configure infrastructure access control lists (ACLs) to restrict access to router management interfaces
• Enable NetFlow or similar traffic analysis to detect and respond to unusual traffic patterns
• Consider segmenting network traffic to reduce the attack surface on vulnerable routers

# Example CoPP configuration to rate-limit punted traffic
control-plane
 management-plane
  inband
   interface all
    allow SSH
    allow SNMP
    !
   !
  !
 !
!