CVE-2025-20243: Cisco Secure Firewall ASA DoS Vulnerability

CVE-2025-20243 Overview

A vulnerability in the management and VPN web servers of Cisco Secure Firewall ASA Software and Secure Firewall Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to cause the device to reload unexpectedly, resulting in a Denial of Service (DoS) condition.

This vulnerability is due to improper validation of user-supplied input on an interface with VPN web services. An attacker could exploit this vulnerability by sending crafted HTTP requests to a targeted web server on an affected device. A successful exploit could allow the attacker to cause a DoS condition when the device reloads.

Critical Impact

Unauthenticated remote attackers can force device reloads through malicious HTTP requests, disrupting VPN services and network security operations.

Affected Products

• Cisco Secure Firewall ASA Software
• Cisco Secure Firewall Threat Defense (FTD) Software
• Devices with VPN web services enabled

Discovery Timeline

• 2025-08-14 - CVE-2025-20243 published to NVD
• 2025-08-15 - Last updated in NVD database

Technical Details for CVE-2025-20243

Vulnerability Analysis

This vulnerability is classified under CWE-835 (Loop with Unreachable Exit Condition, also known as "Infinite Loop"). The flaw exists within the management and VPN web servers of Cisco's firewall products, where improper input validation allows attackers to trigger an uncontrolled loop condition that exhausts system resources and forces a device reload.

The vulnerability is particularly concerning because it affects critical network infrastructure components. When exploited, the firewall device reloads unexpectedly, temporarily disrupting all traffic inspection, VPN connections, and network security enforcement. This creates a window of vulnerability where protected network segments may be exposed or users lose access to VPN-protected resources.

Root Cause

The root cause of CVE-2025-20243 is improper validation of user-supplied input on interfaces with VPN web services enabled. When processing certain crafted HTTP requests, the web server enters an infinite loop (CWE-835) that cannot be exited under normal conditions. This uncontrolled loop eventually leads to resource exhaustion or triggers a watchdog mechanism that forces the device to reload.

The vulnerability specifically affects the HTTP processing logic in the VPN web services component, where boundary conditions and input length validations are insufficient to prevent malicious input from causing abnormal program flow.

Attack Vector

The attack can be executed remotely over the network without requiring authentication or user interaction. An attacker needs network access to the management or VPN web server interfaces of an affected Cisco ASA or FTD device.

The exploitation process involves sending specially crafted HTTP requests to the targeted web server. These malicious requests contain input that triggers the infinite loop condition in the web server's request processing logic. Once triggered, the device becomes unresponsive and eventually reloads, causing service disruption for all connected users and traffic passing through the firewall.

This is a network-accessible vulnerability that can be exploited by any attacker with connectivity to the affected web services interfaces, making it particularly dangerous for internet-facing VPN concentrators and management interfaces.

Detection Methods for CVE-2025-20243

Indicators of Compromise

• Unexpected device reloads or reboots without scheduled maintenance windows
• Crash logs or core dumps indicating web server process failures
• Abnormal HTTP traffic patterns targeting VPN web service endpoints
• System logs showing repeated service restarts or watchdog timer triggers

Detection Strategies

• Monitor syslog messages for crash events related to the web server or VPN services
• Implement network-based intrusion detection to identify malformed HTTP requests targeting Cisco ASA/FTD devices
• Configure SNMP traps for unexpected device reloads and service interruptions
• Review crash dump analysis for patterns consistent with infinite loop conditions

Monitoring Recommendations

• Enable detailed logging on management and VPN web server interfaces
• Implement continuous availability monitoring for critical firewall infrastructure
• Configure alerting for multiple consecutive device reloads within short timeframes
• Establish baseline HTTP traffic patterns to detect anomalous request volumes or characteristics

How to Mitigate CVE-2025-20243

Immediate Actions Required

• Review the Cisco Security Advisory for affected versions and patch availability
• Apply available security updates to affected Cisco ASA and FTD devices
• Restrict access to management and VPN web server interfaces using access control lists (ACLs)
• Implement rate limiting on web service interfaces where possible

Patch Information

Cisco has released a security advisory addressing this vulnerability. Administrators should consult the Cisco Security Advisory (cisco-sa-asaftd-vpn-dos-mfPekA6e) for specific version information, fixed software releases, and upgrade paths. Organizations should prioritize patching internet-facing ASA and FTD devices with VPN web services enabled.

Workarounds

• Limit access to management interfaces by configuring strict ACLs to allow only trusted IP addresses
• Disable unnecessary VPN web services on interfaces that do not require them
• Deploy web application firewalls (WAFs) or intrusion prevention systems (IPS) to filter potentially malicious HTTP traffic
• Implement high-availability configurations to minimize service disruption during potential exploitation

# Example: Restrict management access to trusted networks only
access-list management-acl extended permit tcp 10.0.0.0 255.255.255.0 host <management-ip> eq https
access-list management-acl extended deny ip any host <management-ip>