CVE-2020-3220: Cisco IOS XE DoS Vulnerability

CVE-2020-3220 Overview

A vulnerability in the hardware crypto driver of Cisco IOS XE Software for Cisco 4300 Series Integrated Services Routers and Cisco Catalyst 9800-L Wireless Controllers could allow an unauthenticated, remote attacker to disconnect legitimate IPsec VPN sessions to an affected device. The vulnerability is due to insufficient verification of authenticity of received Encapsulating Security Payload (ESP) packets. An attacker could exploit this vulnerability by tampering with ESP cleartext values as a man-in-the-middle.

Critical Impact

Successful exploitation allows remote attackers to disrupt IPsec VPN connectivity, potentially causing denial of service for legitimate VPN users and disrupting secure communications across enterprise networks.

Affected Products

• Cisco IOS XE Software versions 16.4.x through 16.12.x
• Cisco 4300 Series Integrated Services Routers
• Cisco Catalyst 9800-L Wireless Controllers

Discovery Timeline

• June 3, 2020 - CVE-2020-3220 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-3220

Vulnerability Analysis

This vulnerability affects the hardware crypto driver implementation in Cisco IOS XE Software, specifically how it processes Encapsulating Security Payload (ESP) packets within IPsec VPN tunnels. The flaw stems from CWE-345: Insufficient Verification of Data Authenticity, where the affected devices fail to adequately validate the integrity and authenticity of incoming ESP packets before processing them.

The attack requires the adversary to position themselves in a man-in-the-middle (MITM) position between the VPN client and the affected Cisco device. While this positioning requirement increases attack complexity, once achieved, the attacker can manipulate ESP cleartext values to trigger the denial of service condition. The scope of this vulnerability extends beyond the vulnerable component, meaning successful exploitation can impact resources beyond the immediate security context.

Root Cause

The root cause lies in insufficient verification of data authenticity (CWE-345) within the hardware crypto driver. The driver does not properly validate ESP packet contents before passing them to the cryptographic processing pipeline. This allows an attacker who can intercept and modify network traffic to inject malformed or tampered ESP packets that cause the device to terminate legitimate VPN sessions.

The hardware crypto acceleration feature, while providing performance benefits for cryptographic operations, introduces this vulnerability when processing specially crafted packets that bypass normal authentication checks.

Attack Vector

The attack is network-based and requires the attacker to establish a man-in-the-middle position between the VPN endpoint and the affected Cisco device. From this position, the attacker intercepts legitimate ESP packets and modifies cleartext values within the packet structure before forwarding them to the target device.

When the affected router or wireless controller processes the tampered ESP packet, the hardware crypto driver fails to detect the manipulation, leading to improper handling that results in disconnection of the associated VPN session. This can be repeated to continuously disrupt VPN connectivity for targeted users or the entire site.

The attack does not require authentication or user interaction, making it exploitable by any attacker with network positioning capabilities. However, the high attack complexity stemming from the MITM requirement somewhat limits the ease of exploitation.

Detection Methods for CVE-2020-3220

Indicators of Compromise

• Unexpected IPsec VPN session disconnections or frequent tunnel re-establishment
• Anomalous network traffic patterns between VPN endpoints suggesting packet interception
• Increased error logs related to ESP packet processing on affected Cisco devices
• Reports from users experiencing intermittent VPN connectivity issues

Detection Strategies

• Monitor Cisco IOS XE system logs for ESP-related errors and unexpected VPN session terminations
• Implement network-based intrusion detection to identify potential man-in-the-middle attack patterns
• Deploy traffic analysis tools to detect anomalies in IPsec tunnel traffic flows
• Configure SNMP traps for IPsec tunnel state changes to enable rapid alerting

Monitoring Recommendations

• Enable detailed logging for IPsec VPN services on affected Cisco devices
• Implement continuous monitoring of VPN tunnel uptime and stability metrics
• Deploy network segmentation to limit attacker positioning opportunities for MITM attacks
• Review and audit network infrastructure for potential interception points

How to Mitigate CVE-2020-3220

Immediate Actions Required

• Identify all Cisco 4300 Series ISRs and Catalyst 9800-L Wireless Controllers in your environment
• Review the Cisco Security Advisory for detailed remediation guidance
• Prioritize patching for devices handling critical IPsec VPN traffic
• Implement network segmentation to reduce MITM attack surface where possible

Patch Information

Cisco has released software updates that address this vulnerability. Administrators should consult the Cisco Security Advisory for specific fixed software versions and upgrade paths. The affected versions span Cisco IOS XE 16.4.1 through 16.12.1y, requiring upgrades to patched releases as specified by Cisco.

Organizations should use the Cisco Software Checker tool to determine exposure and identify the appropriate fixed software release for their specific hardware platform and current software version.

Workarounds

• Implement additional network-level protections to prevent man-in-the-middle positioning
• Deploy IPsec anti-replay mechanisms and ensure they are properly configured
• Consider using software-based crypto processing as a temporary measure if hardware offloading can be disabled
• Implement strict access controls on network segments carrying VPN traffic to limit attacker access

# Verify current IOS XE version on affected devices
show version

# Check IPsec VPN session status and monitor for unexpected disconnections
show crypto ipsec sa

# Review crypto engine status
show crypto engine brief

# Monitor for ESP-related errors in logs
show logging | include ESP|IPsec|crypto