CVE-2024-20498 Overview
CVE-2024-20498 is a denial of service vulnerability affecting the Cisco AnyConnect VPN server component in Cisco Meraki MX and Cisco Meraki Z Series Teleworker Gateway devices. An unauthenticated, remote attacker can exploit this flaw to cause a DoS condition in the AnyConnect service, disrupting VPN connectivity for legitimate users.
The vulnerability stems from insufficient validation of client-supplied parameters during SSL VPN session establishment. By sending specially crafted HTTPS requests to the VPN server, an attacker can trigger a server restart, terminating all established SSL VPN connections and forcing remote users to reauthenticate. A sustained attack can prevent new SSL VPN connections from being established entirely.
Critical Impact
Unauthenticated remote attackers can disrupt enterprise VPN connectivity for entire organizations, impacting business continuity and remote workforce operations.
Affected Products
- Cisco Meraki MX Series Security Appliances (MX64, MX65, MX67, MX68, MX75, MX84, MX85, MX95, MX100, MX105, MX250, MX400, MX450, MX600)
- Cisco Meraki Z Series Teleworker Gateways (Z3, Z3C, Z4, Z4C)
- Cisco Meraki vMX Virtual Appliance
Discovery Timeline
- October 2, 2024 - CVE-2024-20498 published to NVD
- June 4, 2025 - Last updated in NVD database
Technical Details for CVE-2024-20498
Vulnerability Analysis
This vulnerability is classified under CWE-415 (Double Free), indicating that the root cause involves improper memory management within the VPN server's session handling code. When processing SSL VPN session establishment requests, the AnyConnect VPN server fails to adequately validate parameters submitted by clients. This insufficient input validation allows an attacker to trigger memory corruption through crafted HTTPS requests.
The vulnerability is particularly concerning because it requires no authentication and can be exploited remotely over the network. An attacker simply needs network access to the VPN server's HTTPS endpoint to initiate the attack. The lack of user interaction requirements means automated exploitation is straightforward.
While the vulnerability only impacts availability (with no confidentiality or integrity implications), the effect on enterprise VPN infrastructure is significant. The AnyConnect VPN server restart terminates all active SSL VPN sessions, disconnecting all remote users simultaneously.
Root Cause
The vulnerability originates from a double-free memory corruption condition in the SSL VPN session parameter handling code. When certain malformed or crafted parameters are submitted during session establishment, the code path triggers improper memory deallocation, causing the same memory region to be freed twice. This double-free condition results in memory corruption that crashes the AnyConnect VPN server process.
The insufficient validation occurs at the HTTPS layer during the initial VPN handshake, before any authentication takes place. This allows completely unauthenticated attackers to trigger the vulnerability.
Attack Vector
The attack is conducted remotely over the network by sending crafted HTTPS requests to the VPN server endpoint. The attacker does not require any credentials, user accounts, or prior access to the target network. The attack flow consists of:
- The attacker identifies a Cisco Meraki device with AnyConnect VPN enabled
- The attacker crafts malicious HTTPS requests with specially formatted parameters
- These requests are sent to the VPN server's public-facing HTTPS port
- The server processes the malformed parameters, triggering the double-free condition
- The AnyConnect VPN server crashes and restarts
- All established VPN sessions are terminated, and users must reconnect
The vulnerability is self-recovering—when attack traffic stops, the VPN server recovers automatically without manual intervention. However, sustained attacks can maintain a persistent denial of service condition.
Detection Methods for CVE-2024-20498
Indicators of Compromise
- Frequent AnyConnect VPN server restarts observed in Meraki dashboard logs
- Multiple simultaneous VPN disconnection events across all users
- Spike in HTTPS connection attempts to the VPN server from single source IPs
- Users reporting recurring VPN disconnections requiring reauthentication
Detection Strategies
- Monitor Meraki event logs for repeated VPN server restart events
- Implement rate limiting on incoming HTTPS connections to VPN endpoints
- Analyze network traffic for anomalous patterns of malformed HTTPS requests
- Set up alerts for mass VPN session termination events
Monitoring Recommendations
- Configure Meraki dashboard alerts for VPN server health status changes
- Establish baseline metrics for normal VPN reconnection rates
- Monitor for unusual traffic patterns to VPN server HTTPS ports (typically 443)
- Review connection logs for sources generating high volumes of failed session establishments
How to Mitigate CVE-2024-20498
Immediate Actions Required
- Review and apply firmware updates from Cisco addressing this vulnerability
- Restrict access to VPN server endpoints using firewall rules where feasible
- Monitor for unusual VPN server behavior and prepare incident response procedures
- Consider enabling additional network-level protections such as DDoS mitigation
Patch Information
Cisco has released security updates addressing this vulnerability. Organizations should consult the Cisco Security Advisory for specific firmware versions that contain the fix. Meraki devices typically receive updates through the Meraki cloud dashboard, and administrators should verify their devices are running patched firmware versions.
Workarounds
- Implement network-level rate limiting on incoming connections to VPN endpoints
- Use upstream firewall or DDoS protection services to filter malicious traffic
- Consider geographic IP restrictions if VPN users are from known regions
- Monitor and temporarily block source IPs exhibiting attack patterns
# Example network access control (implement at perimeter firewall)
# Restrict VPN access to known corporate IP ranges where possible
# Rate limit new connections to the VPN endpoint
# Enable logging for connection attempts to aid in attack identification
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
