Cisco AnyConnect VPN Server Vulnerable to DoS Exploits

Cisco has revealed a critical flaw (CVE-2025-20212) in its AnyConnect VPN Server for Meraki MX and Z Series devices, enabling authenticated attackers to cause denial-of-service (DoS). The issue arises from an uninitialized variable during SSL VPN session setup and impacts over 20 enterprise hardware models.

Cisco AnyConnect VPN Vulnerability Disrupts SSL VPN Connections

Cisco has identified a critical vulnerability in its AnyConnect VPN Server for Meraki MX and Z Series devices. The flaw allows attackers with valid VPN credentials to disrupt active SSL VPN connections by sending malicious session attributes. This forces the VPN service to restart, potentially blocking new sessions until the attack stops.

Affected Devices

MX Series: MX64, MX64W, MX65, MX65W, MX67, MX67C, MX67W, MX68, MX68CW, MX68W, MX75, MX84, MX85, MX95, MX100, MX105, MX250, MX400, MX450, MX600, vMX

Z Series: Z3, Z3C, Z4, Z4C

Cisco recommends updating affected devices to mitigate the risk. Stay vigilant and ensure your systems are protected.

Mitigation Steps for Cisco AnyConnect VPN Vulnerability

1. Check AnyConnect VPN Status:
Go to Dashboard > Security & SD-WAN (MX) or Teleworker Gateway (Z Series) > Client VPN > AnyConnect Settings.
If AnyConnect is disabled, your device is not affected.

2. Update Firmware:
Upgrade to a fixed version: 18.107.12 for 18.1, 19.1.4 for 19.1.
Critical: MX400/MX600 users must replace hardware or isolate vulnerable devices.

3. Monitor VPN Sessions:
Look for frequent VPN reconnections or unexpected service restarts.

Updating your firmware and monitoring for unusual activity can help protect your network from disruptions.

Technical Analysis of Cisco AnyConnect VPN Vulnerability

Issue: CWE-457 – Uninitialized variable in SSL VPN session handling.
Attack Vector: Network-based (AV:N) with low complexity (AC:L).
Impact: High availability risk (A:H) but no data compromise (C:N/I:N).

Fixed Firmware Versions

Cisco confirms no workarounds exist—only upgrading to a fixed release can mitigate the issue. Administrators should update affected Meraki devices immediately and monitor deployments to maintain secure VPN functionality. For more details, refer to Cisco’s official advisory.