CVE-2022-20715: Cisco Firepower Threat Defense DoS Flaw

CVE-2022-20715 Overview

A vulnerability in the remote access SSL VPN features of Cisco Adaptive Security Appliance (ASA) Software and Cisco Firepower Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition on an affected device. This vulnerability is due to improper validation of errors that are logged as a result of client connections that are made using remote access VPN. An attacker could exploit this vulnerability by sending crafted requests to an affected system. A successful exploit could allow the attacker to cause the affected device to restart, resulting in a DoS condition.

Critical Impact

Unauthenticated remote attackers can cause affected Cisco ASA and FTD devices to restart, disrupting VPN services and network connectivity for all users relying on the affected security appliance.

Affected Products

• Cisco Adaptive Security Appliance (ASA) Software
• Cisco Firepower Threat Defense (FTD) Software
• Cisco Firepower Threat Defense 7.1.0

Discovery Timeline

• May 3, 2022 - CVE-2022-20715 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2022-20715

Vulnerability Analysis

This vulnerability represents a Denial of Service condition affecting Cisco's enterprise-grade security appliances. The flaw exists within the remote access SSL VPN functionality, which is commonly deployed in enterprise environments to provide secure remote connectivity. The improper validation of errors logged during client VPN connections creates an exploitable condition that can be triggered remotely without authentication.

The vulnerability is classified under CWE-20 (Improper Input Validation) and CWE-399 (Resource Management Errors), indicating that the core issue stems from insufficient validation of input data combined with improper handling of system resources. When malformed requests are processed by the SSL VPN module, the resulting error logging mechanism fails to properly handle certain error conditions, leading to a system crash.

The attack can be executed across the network without requiring any privileges or user interaction, and the scope of impact extends beyond the vulnerable component itself, potentially affecting connected services and dependent systems.

Root Cause

The root cause of this vulnerability lies in improper input validation within the error logging subsystem of the remote access SSL VPN feature. When client connections generate specific error conditions, the logging mechanism fails to properly validate and handle these error states. This results in a resource management failure that causes the device to restart unexpectedly.

The combination of CWE-20 (Improper Input Validation) and CWE-399 (Resource Management Errors) indicates that crafted inputs can trigger resource exhaustion or corruption in the error handling pathway, ultimately destabilizing the system.

Attack Vector

The attack vector is network-based, requiring only that the attacker can reach the SSL VPN interface of the target device. The attacker sends specially crafted requests to the remote access VPN service endpoint. These requests are designed to trigger specific error conditions during the connection process.

When the VPN service attempts to log these malformed connection errors, the improper validation causes the logging subsystem to mishandle the data. This cascades into a critical failure state that forces the entire device to restart, effectively causing a denial of service for all services running on the affected appliance.

The attack requires no authentication and no user interaction, making it particularly dangerous for internet-facing VPN concentrators. Organizations with exposed SSL VPN endpoints are at risk of service disruption from remote attackers.

Detection Methods for CVE-2022-20715

Indicators of Compromise

• Unexpected device reboots or restarts of Cisco ASA or FTD appliances
• Anomalous connection attempts to SSL VPN endpoints from suspicious IP addresses
• Increase in failed VPN connection attempts preceding device restarts
• Error log entries related to VPN client connections immediately before system crashes

Detection Strategies

• Monitor Cisco ASA and FTD system logs for unexpected restart events and correlate with VPN connection logs
• Implement network-based intrusion detection to identify malformed SSL VPN connection attempts
• Deploy anomaly detection on VPN endpoint traffic patterns to identify potential exploitation attempts
• Review syslog entries for repeated error conditions in the VPN subsystem

Monitoring Recommendations

• Enable comprehensive logging on all remote access VPN features and forward logs to a centralized SIEM
• Configure alerting for device restart events and correlate with preceding VPN activity
• Monitor SSL VPN connection metrics for unusual patterns such as high failure rates or connection floods
• Implement real-time monitoring of device availability to detect DoS conditions quickly

How to Mitigate CVE-2022-20715

Immediate Actions Required

• Review the Cisco Security Advisory to determine if your software version is affected
• Apply the appropriate software update from Cisco to remediate the vulnerability
• Restrict access to SSL VPN endpoints to known IP ranges where possible
• Ensure high availability configurations are in place to minimize service disruption during potential attacks

Patch Information

Cisco has released software updates that address this vulnerability. Administrators should consult the Cisco Security Advisory to identify the appropriate fixed software release for their deployment. The advisory provides detailed information about affected versions and the corresponding patched releases.

Organizations should prioritize patching based on the exposure of their SSL VPN endpoints and the criticality of the affected devices to their network infrastructure.

Workarounds

• Implement access control lists (ACLs) to limit which source IP addresses can connect to SSL VPN services
• Consider disabling remote access VPN features if not actively required until patches can be applied
• Deploy rate limiting on VPN connection attempts to reduce the impact of potential exploitation
• Enable high availability failover configurations to maintain service continuity during potential attacks

# Example: Restrict VPN access to specific networks on Cisco ASA
# Configure an access-list to limit SSL VPN connections
access-list outside_access_in extended permit tcp host 192.168.1.0 255.255.255.0 host 10.0.0.1 eq 443
access-list outside_access_in extended deny tcp any host 10.0.0.1 eq 443
access-group outside_access_in in interface outside