CVE-2024-20501 is a high-severity vulnerability affecting the Cisco AnyConnect VPN server component within Cisco Meraki MX and Cisco Meraki Z Series Teleworker Gateway devices. The vulnerability arises from insufficient validation of client-supplied parameters during the establishment of SSL VPN sessions. Specifically, an unauthenticated remote attacker can send specially crafted HTTPS requests to the VPN server, triggering an out-of-bounds write condition. This leads to a denial-of-service (DoS) state where the Cisco AnyConnect VPN server restarts, causing all active SSL VPN connections to drop and forcing remote users to reconnect and reauthenticate. If the attacker sustains the attack, they can prevent new VPN connections from being established, effectively disrupting remote access services. Importantly, the VPN server recovers gracefully once the attack traffic ceases, requiring no manual intervention. The vulnerability does not impact confidentiality or integrity directly but severely affects availability of VPN services. The CVSS 3.1 base score is 8.6 (high), reflecting the network attack vector, no required privileges or user interaction, and the critical impact on availability. No known exploits are currently reported in the wild, but the ease of exploitation and the critical role of VPN services in enterprise environments make this a significant threat. No specific affected firmware versions are listed, indicating that organizations using Cisco Meraki MX and Z Series devices with AnyConnect VPN should assume exposure until patches or mitigations are confirmed.

For European organizations, this vulnerability poses a substantial risk to business continuity and secure remote access infrastructure. Many enterprises and public sector entities in Europe rely on Cisco Meraki MX and Z Series devices to provide secure VPN connectivity for remote employees, contractors, and partners. A successful DoS attack could disrupt critical operations by severing VPN connections, delaying workflows, and potentially causing compliance issues if remote access is mandated for regulatory reasons (e.g., GDPR requirements for secure data access). The disruption could be particularly impactful for sectors with high remote workforce dependency such as finance, healthcare, government, and technology. Additionally, the ability to cause repeated VPN service outages without authentication lowers the barrier for attackers, increasing the likelihood of opportunistic or targeted attacks. While confidentiality and integrity are not directly compromised, the availability impact can indirectly affect security posture by forcing fallback to less secure access methods or causing operational delays.

Organizations should immediately verify the firmware versions of their Cisco Meraki MX and Z Series Teleworker Gateway devices and consult Cisco’s security advisories for available patches or firmware updates addressing CVE-2024-20501. Until patches are applied, network administrators should consider implementing the following mitigations: 1) Restrict access to the VPN server management interfaces and SSL VPN endpoints using firewall rules or access control lists (ACLs) to limit exposure to trusted IP ranges. 2) Monitor VPN server logs and network traffic for unusual HTTPS requests or repeated connection resets indicative of exploitation attempts. 3) Employ rate limiting or intrusion prevention systems (IPS) to detect and block malformed or excessive VPN connection attempts. 4) Educate remote users about potential service disruptions and establish contingency plans for critical remote access needs. 5) Coordinate with Cisco support for any recommended configuration changes or temporary workarounds. Proactive vulnerability management and incident response readiness are essential to minimize downtime and operational impact.