CVE-2022-20795 Overview
A vulnerability in the implementation of the Datagram TLS (DTLS) protocol in Cisco Adaptive Security Appliance (ASA) Software and Cisco Firepower Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to cause high CPU utilization, resulting in a denial of service (DoS) condition. This vulnerability is due to suboptimal processing that occurs when establishing a DTLS tunnel as part of an AnyConnect SSL VPN connection.
An attacker could exploit this vulnerability by sending a steady stream of crafted DTLS traffic to an affected device. A successful exploit could allow the attacker to exhaust resources on the affected VPN headend device, causing existing DTLS tunnels to stop passing traffic and preventing new DTLS tunnels from establishing.
Critical Impact
Unauthenticated remote attackers can cause denial of service on VPN headend devices by exhausting CPU resources through crafted DTLS traffic, disrupting both existing and new VPN connections for enterprise networks.
Affected Products
- Cisco Adaptive Security Appliance (ASA) Software
- Cisco Firepower Threat Defense (FTD) Software
- Cisco ASA 5500-X Series (5505, 5512-x, 5515-x, 5525-x, 5545-x, 5555-x, 5580, 5585-x)
- Cisco ASA for Nexus 1000V
- Cisco Firepower 1000 Series (1010, 1120, 1140, 1150)
- Cisco Firepower 2100 Series (2110, 2120, 2130, 2140)
- Cisco Firepower 4100 Series (4110, 4112, 4115, 4120, 4125, 4140, 4145, 4150)
- Cisco Firepower 9300
Discovery Timeline
- April 21, 2022 - CVE-2022-20795 published to NVD
- November 21, 2024 - Last updated in NVD database
Technical Details for CVE-2022-20795
Vulnerability Analysis
This vulnerability exists in the DTLS protocol implementation used by Cisco ASA and FTD software for AnyConnect SSL VPN connections. The flaw stems from insufficient data authenticity verification (CWE-345) during DTLS tunnel establishment, allowing attackers to send malformed traffic that consumes excessive CPU resources on the target device.
The attack requires no authentication and can be launched remotely over the network. The vulnerability specifically affects the DTLS processing code path, where suboptimal handling of incoming connection requests leads to resource exhaustion. While the vulnerability does not allow data exfiltration or unauthorized access, it can completely disrupt VPN services for legitimate users.
Notably, the device recovers gracefully once attack traffic stops, indicating that the resource exhaustion does not cause permanent damage or require manual intervention to restore service.
Root Cause
The root cause of CVE-2022-20795 is classified under CWE-345 (Insufficient Verification of Data Authenticity). The DTLS implementation fails to properly validate incoming traffic during the tunnel establishment phase, allowing malformed or crafted packets to be processed without adequate verification. This suboptimal processing allows attackers to trigger computationally expensive operations without proper authentication, leading to CPU exhaustion on the VPN headend device.
Attack Vector
The attack is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by:
- Identifying a target Cisco ASA or FTD device running AnyConnect SSL VPN with DTLS enabled
- Sending a continuous stream of specially crafted DTLS packets to the device
- The malformed packets trigger suboptimal processing during DTLS tunnel establishment
- CPU resources become exhausted as the device attempts to process the malicious traffic
- Legitimate VPN connections are disrupted and new connections cannot be established
The vulnerability affects the DTLS protocol processing during AnyConnect SSL VPN tunnel establishment. Unlike a crash-based DoS, this attack causes resource exhaustion that persists only while attack traffic is being sent to the target device.
Detection Methods for CVE-2022-20795
Indicators of Compromise
- Sustained high CPU utilization on Cisco ASA or FTD devices, particularly on processes handling DTLS/VPN connections
- Significant increase in DTLS connection attempts from unusual or single source IP addresses
- User reports of AnyConnect VPN connectivity issues or dropped connections
- Log entries indicating DTLS tunnel establishment failures or timeouts
Detection Strategies
- Monitor CPU utilization metrics on ASA/FTD devices and alert on sustained spikes above normal thresholds
- Implement rate limiting and anomaly detection for incoming DTLS traffic on port 443/UDP
- Deploy network intrusion detection systems (NIDS) with rules to identify abnormal DTLS handshake patterns
- Review VPN session logs for unusual connection establishment patterns or high failure rates
Monitoring Recommendations
- Configure SNMP traps or syslog alerts for CPU utilization thresholds on affected devices
- Enable detailed logging for VPN and DTLS-related events on ASA/FTD platforms
- Implement NetFlow or similar traffic analysis to baseline normal DTLS traffic patterns
- Set up automated alerting for deviations from established VPN connection baselines
How to Mitigate CVE-2022-20795
Immediate Actions Required
- Review the Cisco Security Advisory for specific guidance and affected version information
- Evaluate whether DTLS is required for your AnyConnect VPN deployment; if not, consider disabling it
- Implement rate limiting for DTLS traffic at network perimeter devices
- Monitor affected devices for signs of exploitation while planning remediation
Patch Information
Cisco has released security updates to address this vulnerability. Administrators should consult the Cisco Security Advisory cisco-sa-vpndtls-dos-TunzLEV for detailed information on fixed software versions and upgrade guidance specific to their deployment.
Organizations should prioritize patching VPN headend devices that are exposed to the internet and provide critical remote access services.
Workarounds
- Disable DTLS on the VPN headend if TLS-only mode is acceptable for your environment (note: this may impact performance for some users)
- Implement access control lists (ACLs) to restrict DTLS traffic to known, trusted IP ranges where feasible
- Deploy upstream rate limiting or traffic scrubbing services to filter high-volume attack traffic before it reaches the VPN headend
# Example: Disable DTLS on Cisco ASA (forces TLS-only mode)
# Note: This may impact VPN performance for mobile users
ciscoasa(config)# group-policy DfltGrpPolicy attributes
ciscoasa(config-group-policy)# webvpn
ciscoasa(config-group-webvpn)# anyconnect ssl dtls none
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
