CVE-2021-34768 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, 17.3.2a, 17.4.1, 17.5.1)
- Cisco Catalyst 9800 Wireless Controller
- Cisco Catalyst 9800-40 Wireless Controller
- Cisco Catalyst 9800-80 Wireless Controller
- Cisco Catalyst 9800-CL Cloud Wireless Controller
- Cisco Catalyst 9800-L Wireless Controller
- Cisco Catalyst 9800-L-C Wireless Controller
- Cisco Catalyst 9800-L-F Wireless Controller
- Cisco Catalyst 9800 Embedded Wireless Controller
Discovery Timeline
- September 23, 2021 - CVE-2021-34768 published to NVD
- November 21, 2024 - Last updated in NVD database
Technical Details for CVE-2021-34768
Vulnerability Analysis
This vulnerability is classified as CWE-415 (Double Free), which occurs when a program calls free() twice on the same memory address. In the context of Cisco IOS XE's CAPWAP implementation, the insufficient validation of incoming CAPWAP packets allows an attacker to trigger memory corruption conditions that lead to a device crash.
The CAPWAP protocol is used for communication between wireless access points and wireless LAN controllers. When the affected wireless controller receives a specially crafted malformed CAPWAP packet, the improper handling of memory during packet processing causes the device to enter an unstable state. This results in a complete device crash and automatic reload, disrupting wireless services for all connected access points and clients.
The vulnerability requires no authentication and can be exploited remotely over the network, making it particularly dangerous in enterprise environments where wireless controllers manage large numbers of access points.
Root Cause
The root cause of this vulnerability is insufficient validation of CAPWAP packets in Cisco IOS XE Software. The CAPWAP protocol processing code fails to properly validate packet contents before processing, leading to a double-free memory condition (CWE-415). When malformed packets are received, the software attempts to free memory that has already been deallocated, causing memory corruption and subsequent device instability.
Attack Vector
The attack vector is network-based, requiring no privileges or user interaction. An attacker can exploit this vulnerability by:
- Identifying target Cisco Catalyst 9800 series wireless controllers on the network
- Crafting malformed CAPWAP protocol packets designed to trigger the double-free condition
- Sending the malicious packets to the target device's CAPWAP service (typically UDP ports 5246 and 5247)
- Causing the wireless controller to crash and reload
The attack can be repeated to create a sustained denial of service condition. Since CAPWAP is essential for wireless controller operations, this vulnerability affects the entire wireless infrastructure managed by the affected controller.
Detection Methods for CVE-2021-34768
Indicators of Compromise
- Unexpected wireless controller crashes and automatic reloads with no apparent hardware cause
- Crash logs showing memory corruption or double-free errors in CAPWAP processing modules
- Unusual CAPWAP traffic patterns with malformed or anomalous packet structures on UDP ports 5246/5247
- Multiple access points simultaneously losing connectivity to the wireless controller
Detection Strategies
- Monitor syslog messages for crash events related to CAPWAP packet processing or memory allocation errors
- Deploy network intrusion detection systems (IDS) with signatures for malformed CAPWAP packets
- Implement SNMP traps to alert on unexpected device reloads
- Analyze CAPWAP traffic using packet capture tools to identify anomalous packet structures
Monitoring Recommendations
- Enable crash dump collection on wireless controllers to assist in forensic analysis
- Configure centralized logging with real-time alerting for device crash events
- Monitor controller uptime metrics and set alerts for unexpected reboots
- Track CAPWAP session statistics for anomalies in packet processing errors
How to Mitigate CVE-2021-34768
Immediate Actions Required
- Review the Cisco Security Advisory for affected version details and recommended upgrade paths
- Identify all Cisco Catalyst 9800 series wireless controllers in your environment running affected IOS XE versions
- Prioritize patching for internet-facing or externally accessible wireless controllers
- Implement network segmentation to limit CAPWAP traffic exposure to trusted network segments only
Patch Information
Cisco has released software updates to address this vulnerability. Administrators should consult the Cisco Security Advisory for specific fixed software releases and upgrade instructions. The affected versions include Cisco IOS XE 3.15.1xbs, 3.15.2xbs, 17.3.1, 17.3.2, 17.3.2a, 17.4.1, and 17.5.1. Organizations should upgrade to a non-vulnerable release as documented in the advisory.
Workarounds
- Restrict CAPWAP traffic (UDP 5246/5247) to only trusted access point IP addresses using access control lists
- Implement infrastructure ACLs to limit access to the wireless controller management interfaces
- Consider deploying wireless controllers behind firewalls that can filter malformed CAPWAP packets
- Monitor for anomalous traffic patterns while planning the upgrade to patched software versions
# Example ACL to restrict CAPWAP access to trusted AP subnets
access-list 100 permit udp 10.1.0.0 0.0.255.255 host 10.0.0.1 eq 5246
access-list 100 permit udp 10.1.0.0 0.0.255.255 host 10.0.0.1 eq 5247
access-list 100 deny udp any host 10.0.0.1 eq 5246 log
access-list 100 deny udp any host 10.0.0.1 eq 5247 log
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
