Cisco Secure Firewall ASA and FTD: Brief Summary of CVE-2025-20243 Denial of Service Vulnerability

Introduction

Unexpected reloads of perimeter firewalls can disrupt remote access, halt business operations, and leave networks temporarily unprotected. CVE-2025-20243 highlights a critical input validation flaw in Cisco Secure Firewall ASA and FTD software, allowing unauthenticated attackers to trigger denial of service conditions through crafted HTTP requests to management and VPN web interfaces.

Cisco Secure Firewall ASA and FTD are among the most widely deployed network security appliances in enterprise and service provider environments. These platforms provide firewall, VPN, and advanced threat defense capabilities, protecting critical infrastructure across industries.

Technical Information

CVE-2025-20243 arises from improper validation of user-supplied input by the management and VPN web servers in Cisco Secure Firewall ASA and FTD software. The vulnerability is present when VPN web services are enabled on the device. An unauthenticated remote attacker can send specially crafted HTTP requests to the affected web server interface. Due to insufficient input validation, the device's processing logic may enter a loop with an unreachable exit condition, classified as CWE-835. This leads to resource exhaustion and forces the device to reload, resulting in a denial of service.

Key technical points:

• The attack vector is remote and does not require authentication.
• Only devices with VPN web services enabled are vulnerable.
• The flaw is triggered by crafted HTTP requests targeting the management or VPN web server interface.
• The underlying issue is a logic error in input validation, leading to infinite processing loops and device reloads.
• No public code snippets or proof of concept exploit are available.

Patch Information

To address the vulnerability in the SSL VPN feature of Cisco Adaptive Security Appliance (ASA) Software and Cisco Firepower Threat Defense (FTD) Software, Cisco has released software updates that rectify the memory management logic error responsible for the issue. (sec.cloudapps.cisco.com)

Steps to Apply the Patch:

1. Identify Affected Devices:

   • Determine if your devices are running a vulnerable release of Cisco ASA or FTD Software with the SSL VPN feature enabled.

2. Obtain the Fixed Software:

   • Access the Cisco Software Checker to identify the appropriate fixed software version for your device. (sec.cloudapps.cisco.com)

3. Upgrade the Software:

   • Follow the upgrade instructions specific to your device:
     • For Cisco ASA devices, refer to the Cisco Secure Firewall ASA Upgrade Guide.
     • For Cisco FTD devices, consult the Cisco Firepower Management Center Upgrade Guide.

Important Considerations:

• Backup Configuration: Before initiating the upgrade, ensure you have a current backup of your device's configuration.

• Review Release Notes: Examine the release notes for the fixed software version to understand any changes or additional steps required post-upgrade.

• Test in a Controlled Environment: If possible, apply the update in a test environment to verify stability before deploying it to production systems.

By promptly applying these updates, you can mitigate the risk associated with this vulnerability and maintain the security and reliability of your network infrastructure.

Affected Systems and Versions

• Cisco Adaptive Security Appliance (ASA) Software with SSL VPN feature enabled
• Cisco Firepower Threat Defense (FTD) Software with SSL VPN feature enabled
• Specific affected versions are not listed in the advisory. Users must check their running version and consult the Cisco Software Checker to determine if their release is vulnerable.
• Only devices with VPN web services enabled and accessible to remote attackers are affected.

Vendor Security History

Cisco has previously addressed similar denial of service vulnerabilities in its ASA and FTD product lines, including:

• CVE-2024-20353: DoS via web management and VPN services
• CVE-2024-20481: DoS in Remote Access VPN service

Cisco typically issues timely advisories and provides fixed software across multiple release trains. The recurrence of input validation flaws in web-facing features indicates ongoing challenges in secure development for complex network appliances, but Cisco maintains a mature and responsive security process.

References

• NVD entry for CVE-2025-20243
• MITRE CVE entry
• Cisco Security Advisory: cisco-sa-asaftd-vpn-dos-mfPekA6e
• Cisco Security Advisory: cisco-sa-asaftd-webvpn-dos-hOnB9pH4