CVE-2025-20271 Overview
A vulnerability exists in the Cisco AnyConnect VPN server of Cisco Meraki MX and Cisco Meraki Z Series Teleworker Gateway devices that could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition. This vulnerability stems from variable initialization errors during SSL VPN session establishment, which can be exploited by sending specially crafted HTTPS requests to affected devices.
Critical Impact
Successful exploitation causes the AnyConnect VPN server to restart, terminating all active SSL VPN sessions and forcing users to reconnect. Sustained attacks can completely prevent new VPN connections, disrupting remote workforce connectivity.
Affected Products
- Cisco Meraki MX Series Security Appliances with AnyConnect VPN enabled
- Cisco Meraki Z Series Teleworker Gateway devices
- Devices running vulnerable firmware versions with SSL VPN functionality
Discovery Timeline
- June 18, 2025 - CVE-2025-20271 published to NVD
- June 23, 2025 - Last updated in NVD database
Technical Details for CVE-2025-20271
Vulnerability Analysis
This vulnerability is classified as CWE-457 (Use of Uninitialized Variable), a memory-related flaw that occurs when the Cisco AnyConnect VPN server fails to properly initialize variables during SSL VPN session establishment. When an attacker sends a sequence of crafted HTTPS requests to the VPN endpoint, the uninitialized variables can cause unexpected behavior in the VPN service, leading to a service crash.
The network-accessible nature of this vulnerability means attackers can exploit it remotely without any authentication or user interaction. The changed scope indicates that while the vulnerability exists in the VPN service, its impact extends to other components relying on VPN connectivity. The attack results in complete availability disruption for the AnyConnect VPN service while maintaining confidentiality and integrity of the system.
Root Cause
The root cause is improper variable initialization within the SSL VPN session establishment process. When specific sequences of HTTPS requests are processed by the AnyConnect VPN server, certain variables are accessed before being properly initialized, leading to undefined behavior that causes the VPN service to crash and restart.
Attack Vector
The attack is conducted remotely over the network by targeting the HTTPS interface of the Cisco AnyConnect VPN server. An attacker sends a specially crafted sequence of HTTPS requests to the affected device. The malformed requests trigger the variable initialization flaw during VPN session processing, causing the service to restart. All established SSL VPN sessions are immediately terminated, and legitimate users must reconnect and re-authenticate. A persistent attack can block all new VPN connections.
The vulnerability requires no authentication and no user interaction, making it particularly dangerous for organizations relying on Meraki devices for remote access VPN services.
Detection Methods for CVE-2025-20271
Indicators of Compromise
- Unexpected AnyConnect VPN service restarts visible in device logs
- Multiple simultaneous VPN session disconnections affecting all remote users
- Unusual patterns of HTTPS requests to the VPN endpoint from external IP addresses
- User complaints of repeated forced re-authentication to VPN services
Detection Strategies
- Monitor Meraki dashboard for AnyConnect VPN service restart events and availability metrics
- Implement network intrusion detection rules for anomalous HTTPS traffic patterns to VPN endpoints
- Configure alerting for high volumes of failed VPN connection attempts from single sources
- Review security logs for sequences of malformed HTTPS requests preceding service disruptions
Monitoring Recommendations
- Enable detailed logging on Meraki MX and Z Series devices for VPN service events
- Deploy traffic analysis to identify potential DoS attack patterns against VPN infrastructure
- Configure real-time alerts in Meraki dashboard for VPN service health degradation
- Establish baseline metrics for normal VPN connection patterns to identify anomalies
How to Mitigate CVE-2025-20271
Immediate Actions Required
- Review the Cisco Security Advisory for affected firmware versions
- Schedule firmware updates during maintenance windows to minimize user disruption
- Implement rate limiting on incoming HTTPS connections to VPN endpoints where possible
- Monitor VPN service availability closely until patches are applied
Patch Information
Cisco has released security updates to address this vulnerability. Administrators should consult the Cisco Security Advisory for specific firmware versions containing the fix. Meraki devices can be updated through the Meraki cloud dashboard by navigating to Organization > Firmware upgrades and selecting the patched firmware version for affected MX and Z Series devices.
Workarounds
- Restrict access to the AnyConnect VPN service to known IP ranges using firewall rules if feasible
- Implement web application firewall (WAF) rules to filter malicious HTTPS request patterns
- Consider temporary use of alternative VPN solutions until devices can be patched
- Enable geo-blocking for VPN endpoints if remote workforce is localized to specific regions
# Meraki dashboard verification steps
# 1. Navigate to Security & SD-WAN > Client VPN
# 2. Verify AnyConnect VPN is enabled and note current firmware
# 3. Check Organization > Firmware upgrades for available updates
# 4. Schedule upgrade to patched firmware version per Cisco advisory
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
