CVE-2021-1565: Cisco IOS XE CAPWAP DoS Vulnerability

CVE-2021-1565 Overview

Multiple vulnerabilities in the Control and Provisioning of Wireless Access Points (CAPWAP) protocol processing of Cisco IOS XE Software for Cisco Catalyst 9000 Family Wireless Controllers could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition on an affected device. These vulnerabilities are due to insufficient validation of CAPWAP packets. An attacker could exploit the vulnerabilities by sending a malformed CAPWAP packet to an affected device. A successful exploit could allow the attacker to cause the affected device to crash and reload, resulting in a DoS condition.

Critical Impact

Unauthenticated remote attackers can crash and reload Cisco Catalyst 9000 Family Wireless Controllers by sending malformed CAPWAP packets, causing network-wide wireless service disruption.

Affected Products

• Cisco IOS XE (versions 3.15.1xbs, 3.15.2xbs, 17.3.1, 17.3.2, 17.4.1, 17.5.1)
• Cisco Embedded Wireless Controller
• Cisco Catalyst 9300 Series Switches (all variants including 24-port and 48-port models)
• Cisco Catalyst 9300L Series Switches
• Cisco Catalyst 9400 Series Switches and Supervisor Engine-1
• Cisco Catalyst 9500 Series Switches
• Cisco Catalyst 9800 Series Wireless Controllers (9800-40, 9800-80, 9800-CL, 9800-L)

Discovery Timeline

• September 23, 2021 - CVE-2021-1565 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-1565

Vulnerability Analysis

This vulnerability is classified as a Double Free memory corruption issue (CWE-415) in the CAPWAP protocol processing component of Cisco IOS XE Software. The CAPWAP protocol is used to manage wireless access points in enterprise environments, handling communication between wireless LAN controllers and access points.

The flaw exists because the affected software improperly handles memory deallocation when processing specially crafted CAPWAP packets. When malformed packets are received, the vulnerable code path may attempt to free the same memory region twice, leading to memory corruption that triggers a device crash and reload.

Since CAPWAP operates over UDP ports 5246 (control) and 5247 (data), an attacker positioned on the network can send malicious packets without requiring any authentication. The attack is entirely remote and requires no user interaction, making it particularly dangerous for exposed wireless controller infrastructure.

Root Cause

The root cause is insufficient validation of CAPWAP packets before memory operations are performed. Specifically, the double free condition (CWE-415) occurs when the IOS XE software processes malformed CAPWAP protocol messages. The lack of proper bounds checking and packet validation allows an attacker to trigger a code path where memory is deallocated multiple times, corrupting the heap and causing system instability.

Attack Vector

The attack vector is network-based, requiring the attacker to send crafted CAPWAP packets to the wireless controller. The attack flow involves:

1. The attacker identifies a vulnerable Cisco Catalyst 9000 Family Wireless Controller running affected IOS XE versions
2. The attacker crafts a malformed CAPWAP packet designed to trigger the double free condition
3. The malicious packet is sent to UDP port 5246 or 5247 on the target device
4. The wireless controller processes the packet without proper validation
5. The double free condition corrupts memory, causing the device to crash and reload
6. All wireless clients connected through the controller experience service disruption during the reload

The vulnerability can be exploited repeatedly to maintain a persistent denial of service condition against the wireless infrastructure.

Detection Methods for CVE-2021-1565

Indicators of Compromise

• Unexpected wireless controller reboots or crashes without scheduled maintenance
• Syslog entries indicating memory corruption or unexpected system restarts on Catalyst 9000 controllers
• Unusual CAPWAP traffic patterns or malformed packets observed on UDP ports 5246 and 5247
• Multiple consecutive crashinfo files generated by wireless controller hardware

Detection Strategies

• Monitor for anomalous CAPWAP traffic volumes and patterns using network intrusion detection systems
• Configure SNMP traps and alerting for unexpected device reloads on wireless controllers
• Implement deep packet inspection for CAPWAP protocol traffic to identify malformed packets
• Review controller crash logs for patterns consistent with memory corruption or double free conditions

Monitoring Recommendations

• Enable logging for all CAPWAP control and data plane activities on affected devices
• Deploy network flow analysis to baseline normal CAPWAP traffic and detect anomalies
• Configure centralized syslog collection for all wireless infrastructure devices
• Implement real-time alerting for wireless controller availability using network monitoring tools

How to Mitigate CVE-2021-1565

Immediate Actions Required

• Review the Cisco Security Advisory for specific fixed software versions
• Identify all Cisco Catalyst 9000 Family Wireless Controllers in your environment running vulnerable IOS XE versions
• Schedule maintenance windows to apply security updates to affected wireless infrastructure
• Implement network segmentation to limit exposure of CAPWAP management interfaces

Patch Information

Cisco has released software updates that address this vulnerability. Administrators should consult the Cisco Security Advisory cisco-sa-ewlc-capwap-dos-gmNjdKOY for detailed information about fixed software releases. The advisory provides a comprehensive list of affected versions and corresponding fixed releases for Cisco IOS XE Software.

Affected IOS XE versions include 3.15.1xbs, 3.15.2xbs, 17.3.1, 17.3.2, 17.4.1, and 17.5.1. Organizations should upgrade to the recommended fixed releases as specified in the Cisco advisory.

Workarounds

• Restrict access to CAPWAP control ports (UDP 5246 and 5247) using access control lists at network boundaries
• Implement infrastructure access control lists (iACLs) to limit CAPWAP traffic to trusted access point management networks
• Consider deploying dedicated management VLANs for wireless controller traffic to reduce attack surface
• Monitor for exploitation attempts while planning upgrade activities

# Example ACL to restrict CAPWAP access to trusted AP management network
# Apply to interfaces facing untrusted networks
ip access-list extended CAPWAP-RESTRICT
 permit udp 10.10.10.0 0.0.0.255 any eq 5246
 permit udp 10.10.10.0 0.0.0.255 any eq 5247
 deny udp any any eq 5246 log
 deny udp any any eq 5247 log
 permit ip any any