Cisco AnyConnect VPN Server Vulnerability Allows Attackers to Trigger DoS

Cisco has disclosed a significant vulnerability in its AnyConnect VPN Server for Meraki MX and Z Series devices, allowing authenticated attackers to trigger denial-of-service (DoS) conditions.

The flaw (CVE-2025-20212) stems from an uninitialized variable during SSL VPN session establishment and affects over 20 hardware models across enterprise networks.

Vulnerability Overview

Exploiting this bug requires valid VPN credentials. Attackers can craft malicious attributes during session setup, forcing the VPN service to restart and disrupting active SSL VPN connections.

Sustained attacks could block new VPN sessions entirely, though services resume automatically after malicious traffic stops.

Affected Products

MX Series	Z Series
MX64, MX64W	Z3, Z3C
MX65, MX65W	Z4, Z4C
MX67, MX67C, MX67W
MX68, MX68CW, MX68W
MX75, MX84, MX85
MX95, MX100, MX105
MX250, MX400, MX450
MX600, vMX

Mitigation Steps

1. Verify AnyConnect VPN Status:
   • Dashboard > Security & SD-WAN (MX) or Teleworker Gateway (Z Series) > Client VPN > AnyConnect Settings.
   • Disabled configurations are not vulnerable.
2. Update Firmware:
   • MX/Z Series: Migrate to fixed versions (e.g., 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 Sessions:
   • Watch for repeated VPN reconnections or unexplained service restarts.

Technical Analysis

• 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 Releases

Firmware Branch	First Fixed Version
18.1	18.107.12
18.2	18.211.4
19.1	19.1.4

Cisco has confirmed that there are no workarounds for this issue, and only upgrading to a fixed release can resolve the vulnerability.

Administrators are advised to monitor their deployments carefully and upgrade affected Meraki devices promptly to ensure continued secure and reliable VPN functionality. Further details and firmware best practices can be found in Cisco’s official advisory.

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