Cisco AnyConnect VPN Server Vulnerability Let Attacker Trigger DoS Condition

Cisco disclosed a critical security vulnerability affecting Cisco Meraki MX and Z Series devices, which presents significant risks to enterprise networks. 

The vulnerability tracked as CVE-2025-20212 and associated with allows authenticated remote attackers to trigger denial of service (DoS) conditions by manipulating SSL VPN session attributes. 

This high-severity flaw, which has a CVSS base score of 7.7, impacts organizations that rely on Cisco AnyConnect VPN for secure remote access solutions.

According to Advisory, the vulnerability stems from an uninitialized variable during SSL VPN session establishment.

Cisco AnyConnect VPN Server Vulnerability

An attacker with valid VPN user credentials can exploit this vulnerability by supplying crafted attributes during the SSL handshake process. 

When successfully exploited, the AnyConnect VPN service abruptly restarts, terminating all established VPN sessions and forcing remote users to reconnect and re-authenticate.

The vulnerability allows manipulation of the session state as the server processes incoming requests and reads Cisco advisory. 

The CVSS vector CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:N/I:N/A:H/E:X/RL:X/RC:X indicates that while authentication is required, the attack complexity is low, making exploitation relatively straightforward in real-world network environments.

A sustained attack could prevent new SSL VPN connections from being established, potentially causing prolonged service outages until malicious traffic subsides. 

The system is designed to recover automatically without manual intervention once attack traffic ceases, but the business impact during exploitation remains significant with possible interruption of critical operations.

Network administrators must verify whether their systems are vulnerable by checking if Cisco AnyConnect VPN is enabled. 

This can be done for Cisco Meraki MX devices by logging into the Dashboard, navigating to “Security & SD-WAN > Configure > Client VPN” and selecting the AnyConnect Settings tab. 

For Z Series devices, administrators should check under “Teleworker Gateway > Configure > Client VPN.” If the “Enabled” radio button is active, immediate action is required.

The summary of the vulnerability is given below:

Particularly vulnerable are numerous Meraki models including MX64, MX65, MX67, MX68, MX75, MX84, MX85, MX95, MX100, MX105, MX250, MX400, MX450, MX600, vMX, and Z Series devices (Z3, Z3C, Z4, Z4C). 

However, MX64 and MX65 are only affected when running firmware releases 17.6 and later.

Mitigation & Fixed Releases

Cisco has released software updates to address this issue. The fixed firmware versions include:

Customers with earlier versions must migrate to a fixed release. Importantly, no workarounds exist, making timely updates the only viable defense.

Industry experts emphasize that this vulnerability highlights the inherent complexities in securing modern VPN implementations. 

The integration of both Client VPN (L2TP/IPsec) and AnyConnect VPN (SSL/DTLS) services in Meraki devices introduces additional security challenges, requiring vigilant firmware management.

Organizations must prioritize these updates to protect critical network infrastructure and prevent service disruptions. 

With remote work remaining prevalent, VPN services constitute essential business infrastructure, making their security paramount. 

Cisco recommends following firmware best practices and ensuring devices have sufficient memory and compatible hardware configurations before upgrading.

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