Cisco ASA and FTD Remote Access VPN (RAVPN) Resource Exhaustion Denial of Service Vulnerability (CVE-2024-20481) Security Advisory
Executive & SecOps Summary
SECOPS ASSESSMENTCisco Adaptive Security Appliance (ASA) Software and Cisco Firepower Threat Defense (FTD) Software with Remote Access VPN (RAVPN) service enabled
Unauthenticated, remote attackers can exhaust system resources by flooding VPN authentication requests, leading to a Denial of Service (DoS) of the RAVPN service and potentially requiring a device reload
Apply official Cisco security patches immediately, configure connection rate limiting, and block malicious brute-force IP addresses
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
Upgrade to the latest recommended fixed releases (e.g., ASA 9.16.4.57, 9.18.4.22, 9.20.2.10 or later / FTD 7.2.9, 7.4.2.1 or later) via the Cisco software management console. Below is an example of upgrading the ASA image via CLI:
# Download the patched ASA image from TFTP server and set boot path
copy tftp://192.168.1.100/asa9184-22-laddr.spa disk0:/asa9184-22-laddr.spa
configure terminal
boot system disk0:/asa9184-22-laddr.spa
write memory
reload2. Temporary Workaround & Hardening Configuration
Prior to patching, mitigate brute-force attempts by restricting simultaneous logins on tunnel groups and implementing rate-limiting policies.
# Limit simultaneous logins for the default WebVPN group
configure terminal
tunnel-group DefaultWEBVPNGroup general-attributes
vpn-simultaneous-logins 5
exit
# Apply Control Plane Policing (CoPP) to rate-limit incoming traffic
class-map COPP-CLASS
match any
policy-map COPP-POLICY
class COPP-CLASS
police input 1000000 150000 conform-action transmit exceed-action drop3. Log Audit & IoC Verification Commands
Monitor ASA/FTD system logs for high frequencies of authentication failure messages (such as %ASA-6-113015 or %ASA-3-713047) originating from identical source IPs.
# Check CPU, memory status, and search for authentication failure logs
show cpu usage
show memory detail
show local-userdb active
show log | include 113015Stop Exposing VPN Ports to the Internet — Switch to Zero-Trust Access
Eliminate public-facing VPN concentrators and firewall management ports with identity-first Zero-Trust Network Access.
Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
This vulnerability (CVE-2024-20481) is a resource exhaustion flaw within the Remote Access VPN (RAVPN) service of Cisco ASA and FTD software. The root cause lies in improper resource management (CWE-772) during the handling of incoming VPN authentication requests. The system fails to properly release allocated memory or session structures when processing high volumes of connection attempts, allowing unauthenticated attackers to deplete available system resources.
2. Exploitation Mechanics & Attack Scenario
An attacker leverages automated tools or botnets to send a massive volume of VPN authentication requests to the target device's RAVPN interface (typically TCP port 443). Because the device allocates resources to evaluate each request before authentication completes, a sustained flood of invalid credentials quickly saturates the session tables and memory. This results in a complete denial of service for legitimate users attempting to establish VPN connections, and may exhaust control plane resources to the point where a physical or logical device reload is required to restore functionality.
3. Infrastructure & Business Impact
This vulnerability is actively exploited in the wild as part of large-scale brute-force campaigns targeting VPN services, and is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. A successful attack disrupts secure remote access for employees, severely impacting business continuity and operational productivity. Furthermore, the resulting instability on the perimeter firewall can compromise overall network monitoring and gateway control capabilities.
Official Vendor Advisories & References
A vulnerability in the Remote Access VPN (RAVPN) service 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) of the RAVPN service. This vulnerability is due to resource exhaustion. An attacker could exploit this vulnerability by sending a large number of VPN authentication requests to an affected device. A successful exploit could allow the attacker to exhaust resources, resulting in a DoS of the RAVPN service on the affected device. Depending on the impact of the attack, a reload of the device may be required to restore the RAVPN service. Services that are not related to VPN are not affected. Cisco Talos discussed these attacks in the blog post Large-scale brute-force activity targeting VPNs, SSH services with commonly used login credentials.
Frequently Asked SecOps Questions (CVE-2024-20481)
FAQQ1. How critical is CVE-2024-20481 and is it actively exploited in the wild?
CVE-2024-20481 is rated CVSS 5.8 (MEDIUM) with a 30-day EPSS exploitation probability of 15.9%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2024-20481?
Primary impacted vendors include cisco across adaptive security appliance, firepower threat defense. Scope summary: All versions of Cisco ASA and FTD Software where Remote Access VPN (RAVPN) service is enabled (services unrelated to VPN are unaffected)
Q3. What temporary mitigations or workarounds exist for CVE-2024-20481 before patching?
[Mitigation] Apply official Cisco security patches immediately, configure connection rate limiting, and block malicious brute-force IP addresses Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
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