On October 23, 2024, Cisco published an advisory for CVE-2024-20481, a denial-of-service vulnerability in the Remote Access VPN (RAVPN) service of Cisco Adaptive Security Appliance (ASA) and Cisco Firepower Threat Defense (FTD) software. The vulnerability allows an unauthenticated remote attacker to cause the RAVPN service to become unavailable through resource exhaustion.
What made this advisory notable was not just the vulnerability itself but the context: Cisco confirmed that the vulnerability was being exploited as part of large-scale, coordinated brute-force campaigns targeting VPN services across multiple vendors.
Vulnerability Details
CVE-2024-20481 has a CVSS score of 5.8, which places it in the "medium" severity range. The vulnerability exists due to resource exhaustion when the RAVPN service processes a large number of VPN authentication requests.
When an attacker sends a high volume of authentication attempts to the VPN service, the device can exhaust its memory and processing resources, causing the RAVPN service to stop accepting new connections. In some cases, a device reload is required to restore VPN service.
Affected products: Any Cisco ASA or FTD device with the RAVPN service enabled. This includes:
- Cisco ASA Software (all versions with RAVPN enabled)
- Cisco FTD Software (all versions with RAVPN enabled)
The vulnerability was patched in updated software releases across multiple ASA and FTD version tracks.
The Brute-Force Campaign Context
CVE-2024-20481 gained attention not as an isolated vulnerability but as a consequence of the massive VPN brute-force campaigns that intensified throughout 2024.
Starting in March 2024, Cisco Talos documented a significant increase in brute-force attacks targeting VPN services, SSH interfaces, and web application authentication endpoints. The campaigns targeted devices from multiple vendors including Cisco, Fortinet, SonicWall, and others.
Key characteristics of the campaign:
Scale: The brute-force attempts originated from thousands of source IP addresses, using TOR exit nodes, anonymizing proxies, and residential proxy services to distribute the traffic.
Targets: The attacks specifically targeted Remote Access VPN endpoints, focusing on commonly used usernames and password patterns. The attackers appeared to be testing credentials obtained from data breaches against VPN infrastructure.
Collateral damage: Even when the brute-force attacks failed to compromise accounts, the sheer volume of authentication requests caused resource exhaustion on VPN appliances, triggering CVE-2024-20481 and similar issues.
Persistence: The campaigns were not one-time events. Organizations reported sustained brute-force activity lasting weeks, with the source IPs rotating regularly to evade IP-based blocking.
Cisco attributed the activity to a large-scale credential stuffing operation, noting that the attacks used valid usernames (often email addresses) paired with commonly used passwords or credentials from known breaches.
Impact on Organizations
The practical impact went beyond simple denial of service:
VPN service disruption: Organizations that depended on Cisco ASA or FTD for remote access VPN found their VPN services becoming unavailable during peak brute-force activity. For organizations with significant remote workforces, this meant employees could not connect to corporate resources.
Security team burden: The massive volume of failed authentication attempts flooded security logs and generated thousands of alerts, creating alert fatigue and making it harder to identify genuinely malicious activity.
Account lockout cascading: Organizations with account lockout policies found that the brute-force attempts were locking out legitimate user accounts, compounding the denial-of-service effect.
Credential compromise: While the brute-force approach has a low success rate per attempt, the massive scale meant that some organizations did experience account compromises where employees had reused passwords from breached services.
Mitigation Strategies
For CVE-2024-20481 specifically:
- Update ASA/FTD software to patched versions.
- Enable threat detection for remote-access VPN on the ASA:
This feature, introduced in recent ASA versions, specifically detects and mitigates brute-force attacks against VPN services.threat-detection service remote-access-vpn
For brute-force protection generally:
- Enforce multi-factor authentication on all VPN accounts. MFA makes credential brute-forcing effectively useless, even if a password is guessed correctly.
- Implement connection rate limiting: Configure the ASA to limit the number of authentication attempts per source IP per time period.
- Use certificate-based authentication: Requiring client certificates for VPN authentication eliminates the brute-force attack surface entirely.
- Deploy a VPN-specific WAF or DDoS protection: Some organizations have placed their VPN endpoints behind DDoS mitigation services that can absorb and filter brute-force traffic.
- Block known malicious sources: Implement threat intelligence feeds that block connections from known TOR exit nodes, anonymizing proxies, and botnet infrastructure.
- Monitor for credential stuffing indicators: Alert on high volumes of failed authentication attempts, particularly those using diverse usernames from single source IPs.
Rate limiting configuration example for Cisco ASA:
# Limit concurrent unauthenticated VPN sessions
vpn-sessiondb max-anyconnect-premium-or-essentials-limit 500
# Configure threat detection
threat-detection basic-threat
threat-detection statistics access-list
The Larger VPN Security Problem
CVE-2024-20481 is symptomatic of a broader challenge with VPN infrastructure:
VPN endpoints are by definition internet-facing: Unlike most internal services that can be hidden behind firewalls, VPN services must be accessible from the internet. This makes them perpetual targets for scanning, brute-forcing, and vulnerability exploitation.
Authentication is the entire security model: VPN security depends almost entirely on the strength of the authentication mechanism. If the authentication is password-based, the service is vulnerable to brute-force. If the authentication mechanism has a vulnerability (like CVE-2024-20481's resource exhaustion), the entire security model collapses.
Operational dependencies create patching pressure: VPN is often the most operationally critical service an organization runs. Any disruption to VPN service directly impacts business operations, which creates organizational resistance to patching, rebooting, or reconfiguring VPN infrastructure.
The industry's long-term answer is Zero Trust Network Access (ZTNA), which replaces the "authenticate once, access everything" model of traditional VPNs with per-application authentication and authorization. But the transition from VPN to ZTNA is a multi-year journey for most organizations, and in the meantime, VPN infrastructure needs to be defended.
How Safeguard.sh Helps
VPN infrastructure is a critical supply chain component that requires continuous monitoring.
- Appliance version tracking catalogs your Cisco ASA and FTD firmware versions, ensuring you are alerted to vulnerabilities like CVE-2024-20481 when they are disclosed.
- Exposure monitoring identifies which network services, including VPN endpoints, are accessible from untrusted networks, helping you manage your attack surface.
- Patch compliance tracks whether security appliances are running patched firmware within your organization's SLA, flagging devices that fall behind.
- Risk scoring integrates active exploitation intelligence with your component inventory, ensuring that vulnerabilities under active exploitation like CVE-2024-20481 are prioritized appropriately in your remediation queue.
When brute-force campaigns are targeting VPN infrastructure globally, knowing which of your appliances are vulnerable and which are patched is not a report you want to generate manually.