Cisco AnyConnect VPN Flaw Allows Attackers to Launch DoS Attacks

A newly disclosed vulnerability in Cisco’s AnyConnect VPN implementation for Meraki MX and Z Series devices poses a significant risk to enterprise networks, enabling unauthenticated attackers to disrupt remote access by triggering denial-of-service (DoS) conditions.

The flaw, tracked as CVE-2025-20271, carries a high CVSS score of 8.6, underscoring its potential to impact organizations that rely on Cisco Meraki gateways for secure remote connectivity.

CVE-2025-20271: Technical Details

The vulnerability stems from a variable initialization error that occurs when establishing SSL VPN sessions using client certificate authentication.

Attackers can exploit this flaw by sending specially crafted HTTPS requests to affected devices.

A successful exploit causes the Cisco AnyConnect VPN service to crash and restart, immediately disconnecting all active VPN users and forcing them to re-authenticate.

The vulnerability affects a broad range of Cisco Meraki MX and Z Series models, including but not limited to MX64, MX65, MX67, MX68, MX75, MX84, MX100, MX250, MX450, vMX, Z3, Z3C, Z4, and Z4C.

Devices are only vulnerable if they are running a susceptible version of the Cisco Meraki MX firmware and have AnyConnect VPN with client certificate authentication enabled.

Affected Devices and How to Check Exposure

Administrators can determine if their devices are impacted by following these steps:

• Log into the Cisco Meraki Dashboard.
• For MX devices: Navigate to Security & SD-WAN > Configure > Client VPN.
• For Z Series devices: Go to Teleworker Gateway > Configure > Client VPN.
• Under the AnyConnect Settings tab, if “Enabled” is selected, the device may be vulnerable.
• In the Authentication & Policy section, if “Certificate authentication” is enabled, the device is affected.

There are currently no workarounds for this vulnerability. Cisco has released software updates addressing the flaw across multiple firmware branches.

Administrators are strongly urged to update to the following fixed versions or later:

• 18.1.x: 18.107.13
• 18.2.x: 18.211.6
• 19.1: 19.1.8

Devices running firmware 16.2 or earlier are not affected. Notably, the MX400 and MX600 models will not receive fixes due to their end-of-life status.

Cisco’s Product Security Incident Response Team (PSIRT) discovered the vulnerability during internal testing, with no evidence of active exploitation to date.

However, given the ease of exploitation and the critical role of VPN services in today’s distributed work environments, prompt patching is essential. Organizations are advised to:

• Immediately apply the provided firmware updates.
• Monitor VPN connection logs for unusual activity.
• Plan hardware upgrades for unsupported models.

This incident highlights the ongoing risks associated with SSL VPN implementations and the importance of maintaining up-to-date security practices for remote access infrastructure.

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