CVE-2024-20402: Cisco ASA Software DoS Vulnerability

CVE-2024-20402 Overview

A vulnerability in the SSL VPN feature for Cisco Adaptive Security Appliance (ASA) Software and Cisco Firepower Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to cause an affected device to reload unexpectedly, resulting in a denial of service (DoS) condition.

This vulnerability is due to a logic error in memory management when the device is handling SSL VPN connections. An attacker could exploit this vulnerability by sending crafted SSL/TLS packets to the SSL VPN server of the affected device. A successful exploit could allow the attacker to cause the device to reload, resulting in a DoS condition.

Critical Impact

Unauthenticated remote attackers can cause network security appliances to reload, disrupting SSL VPN services and potentially impacting business continuity for organizations relying on these devices for secure remote access.

Affected Products

• Cisco Adaptive Security Appliance (ASA) Software versions 9.8.x through 9.19.x
• Cisco Firepower Threat Defense (FTD) Software versions 6.2.3 through 7.3.x
• Multiple maintenance releases across all affected version branches

Discovery Timeline

• October 23, 2024 - CVE-2024-20402 published to NVD
• July 15, 2025 - Last updated in NVD database

Technical Details for CVE-2024-20402

Vulnerability Analysis

This vulnerability affects the SSL VPN (WebVPN) functionality in Cisco ASA and FTD software. The flaw resides in how the devices handle memory management operations during SSL VPN connection processing. When the SSL VPN server processes incoming connections, a logic error in the memory management code path can be triggered, leading to improper memory access patterns that cause the device to reload.

The vulnerability is particularly concerning because it can be exploited without authentication, meaning any remote attacker who can reach the SSL VPN interface can potentially trigger the condition. The attack does not require user interaction and can be executed directly over the network, making it highly accessible to potential threat actors targeting enterprise VPN infrastructure.

Root Cause

The root cause of CVE-2024-20402 is classified as CWE-788: Access of Memory Location After End of Buffer. This weakness occurs when the SSL VPN component attempts to access memory beyond the boundaries of an allocated buffer during connection handling.

Specifically, the logic error in memory management allows the software to improperly track or validate buffer boundaries when processing SSL/TLS handshake packets. When crafted packets are received that trigger this code path, the device attempts to access memory locations that are beyond the legitimate end of the buffer, causing a crash condition that forces the device to reload.

Attack Vector

The attack vector for this vulnerability involves sending specially crafted SSL/TLS packets to the SSL VPN server interface of a vulnerable Cisco ASA or FTD device. The exploitation flow is as follows:

1. The attacker identifies a target device with SSL VPN (WebVPN) enabled and accessible over the network
2. The attacker crafts malicious SSL/TLS packets designed to trigger the memory management logic error
3. These packets are sent to the SSL VPN listening port (typically TCP port 443)
4. The vulnerable code processes the packets, triggering the out-of-bounds memory access
5. The device experiences a crash condition and reloads, causing service disruption

The attack can be repeated to maintain a persistent denial of service condition, preventing legitimate users from establishing SSL VPN connections.

Detection Methods for CVE-2024-20402

Indicators of Compromise

• Unexpected device reloads or restarts of Cisco ASA or FTD appliances with SSL VPN enabled
• Crash logs or core dumps indicating memory access violations in SSL VPN related processes
• Increased frequency of failover events in high-availability configurations
• User reports of SSL VPN connection failures or service unavailability

Detection Strategies

• Monitor device logs for unexpected reload events and analyze crashinfo files for SSL VPN related crashes
• Implement network intrusion detection rules to identify anomalous SSL/TLS traffic patterns targeting VPN endpoints
• Configure SNMP traps or syslog alerts for device reload events to enable rapid incident response
• Review Cisco ASA/FTD system logs for messages indicating memory-related errors in the WebVPN subsystem

Monitoring Recommendations

• Enable logging of all SSL VPN connection attempts and failures for forensic analysis
• Deploy network traffic analysis tools to baseline normal SSL VPN traffic and alert on deviations
• Configure high-availability monitoring to detect and alert on failover events that may indicate exploitation attempts
• Implement rate limiting on SSL VPN connections to reduce the impact of potential DoS attacks

How to Mitigate CVE-2024-20402

Immediate Actions Required

• Review the Cisco Security Advisory to determine if your software version is affected
• Plan and schedule upgrades to patched software versions as specified in the Cisco advisory
• Implement access control lists to restrict SSL VPN access to known, trusted IP ranges where feasible
• Ensure high-availability configurations are properly functioning to minimize service disruption during potential attacks

Patch Information

Cisco has released software updates that address this vulnerability. Customers should upgrade to a fixed software release as documented in the Cisco Security Advisory for CVE-2024-20402.

For Cisco ASA Software, affected versions span from 9.8.1 through 9.19.1.31. For Cisco FTD Software, affected versions range from 6.2.3 through 7.3.1.2. Administrators should consult the advisory for the specific fixed release applicable to their current software branch.

Organizations using Cisco Smart Net Total Care can use the Cisco Software Checker to identify fixed releases. The advisory provides detailed version-specific guidance for upgrade paths.

Workarounds

• There are no direct workarounds that fully address this vulnerability; software updates are required
• Restricting access to the SSL VPN interface using ACLs can reduce exposure but does not eliminate the risk
• Consider temporarily disabling SSL VPN functionality if it is not critical to operations until patching can be completed
• Implementing network-based DoS protection in front of affected devices may help mitigate attack impact

# Example: Restrict SSL VPN access to trusted networks on Cisco ASA
# This reduces exposure but does not fully mitigate the vulnerability
access-list outside_access_in extended permit tcp 10.0.0.0 255.0.0.0 host 192.168.1.1 eq 443
access-list outside_access_in extended deny tcp any host 192.168.1.1 eq 443
access-group outside_access_in in interface outside