Description

A critical flaw has been discovered in Cisco’s AnyConnect VPN functionality used with Meraki MX and Z Series devices, posing a serious threat to enterprise environments that depend on secure remote access. The vulnerability, tagged as CVE-2025-20271, has received a CVSS score of 8.6, indicating high severity. The flaw allows remote, unauthenticated individuals to interfere with VPN availability, causing sudden disconnection for all users and disrupting ongoing secure communications. The issue stems from a programming oversight in how the system processes SSL VPN sessions that rely on certificate-based client authentication. By crafting specific HTTPS requests, an attacker can intentionally trigger a system error. This causes the VPN process to crash and automatically restart, forcibly removing all connected users and requiring them to log in again, which can severely impact business continuity. This vulnerability impacts multiple models across the Meraki lineup, including but not limited to the MX64, MX65, MX100, MX250, MX450, vMX, Z3, and Z4. The devices are only considered vulnerable if they are running affected firmware versions and have AnyConnect enabled with certificate-based authentication configured. Devices still operating on firmware version 16.2 or earlier are not exposed. Additionally, legacy models like the MX400 and MX600 are excluded from updates due to their discontinued status. To resolve this issue, Cisco has released updated firmware across its supported platforms. No alternative fixes or configuration-based mitigations are available, making the update path the only viable solution. Organizations should also audit their VPN settings and monitor user access logs for abnormalities. While there is no current evidence of this vulnerability being exploited in real-world attacks, the ease of exploitation and potential for disruption mean that applying the patch should be treated as an urgent priority.