Overview
The cybersecurity community is currently dealing with a significant vulnerability, CVE-2025-20271, that has a profound impact on Cisco AnyConnect VPN servers, specifically the Cisco Meraki MX and Cisco Meraki Z Series Teleworker Gateway devices. As a widely-used VPN solution, Cisco AnyConnect VPN servers are a crucial component for businesses worldwide in ensuring secure, remote access for their employees. This vulnerability could potentially compromise systems and cause data leakage, impacting business continuity and potentially exposing confidential data.
Vulnerability Summary
CVE ID: CVE-2025-20271
Severity: High (8.6 CVSS Score)
Attack Vector: Network
Privileges Required: None
User Interaction: None
Impact: Denial of Service condition causing failure of all established SSL VPN sessions, potential system compromise or data leakage.
Affected Products
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Product | Affected Versions
Cisco Meraki MX | All versions
Cisco Meraki Z Series Teleworker Gateway | All versions
How the Exploit Works
The vulnerability is rooted in a flaw during the initialization of variables when an SSL VPN session is being established. Attackers can exploit this vulnerability by sending a sequence of specially-crafted HTTPS requests to the affected device. A successful exploit could allow the attacker to cause the Cisco AnyConnect VPN server to restart, resulting in the failure of all established SSL VPN sessions. During this period, remote users would be forced to initiate a new VPN connection and re-authenticate. If the attacker sustains the attack, it could prevent new SSL VPN connections from being established, rendering the Cisco AnyConnect VPN service unavailable for all legitimate users.
Conceptual Example Code
Below is a conceptual example of how an attacker might exploit this vulnerability through a malicious HTTP request:
POST /vpn/endpoint HTTP/1.1
Host: affected_device.example.com
Content-Type: application/json
{
"session": "new_ssl_vpn",
"payload": "crafted_sequence_that_causes_restart"
}This malicious payload, when sent repeatedly, could cause the VPN server to restart and disrupt all active SSL VPN sessions.
Mitigation Guidance
Cisco has released a patch to address this vulnerability. It is highly recommended to apply this patch as soon as possible. As a temporary mitigation, organizations can employ a Web Application Firewall (WAF) or Intrusion Detection System (IDS) to identify and block the specific pattern of malicious HTTPS requests associated with this exploit. However, this should not replace applying the vendor-provided patch, which is the most effective solution for this vulnerability.