Introduction
Unexpected reloads of perimeter firewalls can disrupt remote access, halt business operations, and leave networks temporarily unprotected. CVE-2025-20243 is an input validation flaw in Cisco Secure Firewall ASA and FTD software. It lets unauthenticated attackers cause a denial of service by sending crafted HTTP requests to the 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. They provide firewall, VPN, and advanced threat defense functions, and they protect critical infrastructure in many industries.
Technical information
CVE-2025-20243 comes 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 crafted HTTP requests to the affected web server interface. Because the input is not validated well enough, the device's processing logic may enter a loop with an unreachable exit condition (CWE-835). The loop exhausts resources and forces the device to reload, which causes the 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.
- Crafted HTTP requests to the management or VPN web server interface trigger the flaw.
- The underlying issue is a logic error in input validation, which leads to infinite processing loops and device reloads.
- No public code snippets or proof-of-concept exploit are available.
Patch information
Cisco has released software updates for Cisco Adaptive Security Appliance (ASA) Software and Cisco Firepower Threat Defense (FTD) Software that fix the memory management logic error in the SSL VPN feature responsible for the issue. (sec.cloudapps.cisco.com)
Steps to apply the patch:
- Determine whether your devices run a vulnerable release of Cisco ASA or FTD Software with the SSL VPN feature enabled.
- Use the Cisco Software Checker to find the correct fixed software version for your device. (sec.cloudapps.cisco.com)
- Upgrade the software, following the instructions for your device:
- For Cisco ASA devices, see the Cisco Secure Firewall ASA Upgrade Guide.
- For Cisco FTD devices, see the Cisco Firepower Management Center Upgrade Guide.
Before you upgrade:
- Make sure you have a current backup of the device's configuration.
- Read the release notes for the fixed software version to see what changed and whether any steps are required after the upgrade.
- If possible, apply the update in a test environment and confirm it is stable before you deploy it to production systems.
Affected systems and versions
- Cisco Adaptive Security Appliance (ASA) Software with the SSL VPN feature enabled
- Cisco Firepower Threat Defense (FTD) Software with the SSL VPN feature enabled
- The advisory does not list specific affected versions. Users must check their running version and consult the Cisco Software Checker to determine whether their release is vulnerable.
- Only devices with VPN web services enabled and accessible to remote attackers are affected.
Vendor security history
Cisco has fixed similar denial-of-service vulnerabilities in its ASA and FTD product lines before, including:
- CVE-2024-20353: DoS via web management and VPN services
- CVE-2024-20481: DoS in Remote Access VPN service
Cisco typically publishes advisories promptly and provides fixed software across multiple release trains. Input validation flaws keep recurring in web-facing features, which points to ongoing difficulty with secure development on network appliances this complex. Even so, Cisco's security process is mature and responsive.
