CVE-2025-20271 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 root cause is the use of an uninitialized variable during the establishment of SSL VPN sessions. Specifically, when an SSL VPN session is initiated, certain variables are not properly initialized, which can be triggered by an attacker sending a carefully crafted sequence of HTTPS requests to the affected device. This flaw allows an unauthenticated, remote attacker to cause a denial of service (DoS) condition by forcing the Cisco AnyConnect VPN server to restart unexpectedly. The immediate consequence is the termination of all active SSL VPN sessions, forcing legitimate remote users to reconnect and re-authenticate. If the attacker sustains the exploit, the VPN server may remain unavailable, preventing any new SSL VPN connections from being established. This effectively disrupts remote access capabilities for all users relying on the Cisco AnyConnect VPN service on these devices. The vulnerability does not impact confidentiality or integrity directly but severely impacts availability. The CVSS 3.1 base score is 8.6, reflecting the high impact on availability, ease of exploitation (no authentication or user interaction required), and the broad scope of affected systems. No known exploits are reported in the wild yet, but the vulnerability is publicly disclosed and could be targeted by threat actors due to the critical role of VPNs in secure remote access infrastructure.

For European organizations, this vulnerability poses a significant risk to business continuity and secure remote access operations. Cisco Meraki MX and Z Series devices are widely deployed in enterprise, government, and critical infrastructure networks across Europe to provide secure VPN connectivity for remote workers. A successful DoS attack exploiting this vulnerability could disrupt remote workforce productivity by disconnecting active VPN sessions and preventing new connections, potentially halting access to internal resources and cloud services. This is especially critical for sectors with high remote access dependency such as finance, healthcare, public administration, and technology. Extended downtime could lead to operational delays, increased support costs, and potential regulatory compliance issues related to availability of critical services. Additionally, the disruption could be leveraged as part of a larger attack campaign to distract or delay incident response efforts. Given the unauthenticated nature of the exploit and the lack of user interaction requirements, the threat actor could launch attacks at scale, impacting multiple organizations simultaneously.

Apply Cisco's official firmware updates and patches for Meraki MX and Z Series devices as soon as they become available to address the uninitialized variable issue. Implement network-level protections such as rate limiting and filtering of HTTPS requests to the VPN server to detect and block suspicious or malformed traffic patterns that could trigger the vulnerability. Deploy intrusion detection and prevention systems (IDS/IPS) with signatures or heuristics tuned to detect anomalous SSL VPN session establishment attempts. Segment VPN infrastructure from other critical network segments to contain potential impact and facilitate rapid isolation in case of an attack. Establish robust monitoring and alerting on VPN server health and session stability to enable rapid detection of service restarts or connection failures. Prepare incident response playbooks specifically for VPN service disruptions, including communication plans for remote users and fallback access methods. Consider temporary deployment of alternative remote access solutions or redundant VPN gateways to maintain connectivity during patching or in case of exploitation attempts.