Introduction
A single crafted HTTP request can force a Cisco firewall to reload, which disrupts network traffic and exposes the organization to denial of service. CVE-2025-20263 is a buffer overflow vulnerability in Cisco Secure Firewall Adaptive Security Appliance (ASA) and Firepower Threat Defense (FTD) Software, in a component that enterprises and service providers around the world rely on.
Cisco is a dominant vendor of networking and security appliances. ASA and FTD devices handle perimeter defense for millions of networks across a very large number of enterprise, government, and critical infrastructure environments.
Technical information
CVE-2025-20263 is caused by insufficient boundary checks in the web services interface of Cisco ASA and FTD Software. It is classified as CWE-680 (Integer Overflow to Buffer Overflow). When the web services interface receives certain crafted HTTP requests, an improper integer calculation during buffer allocation can produce a buffer too small for the incoming data. The overflow that follows corrupts memory and makes the device reload, and that reload is the denial of service.
Key technical points:
- Attackers do not need to authenticate. Any remote attacker can exploit the flaw by sending a specially crafted HTTP request to the web services interface.
- The root cause is insufficient boundary checking on input data, specifically in the logic that calculates buffer sizes for HTTP request handling.
- Exploitation reloads the system, which interrupts firewall operations and may affect every connected network.
- No public code snippets or PoCs are available for this vulnerability.
Patch information
Cisco has released software updates for Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Cisco Secure Firewall Threat Defense (FTD) Software that fix the insufficient boundary checks in the web services interface. (sec.cloudapps.cisco.com)
Understanding the patch
The flaw is in how the web services interface processes specific HTTP requests. The updates strengthen validation of incoming HTTP requests, so data is checked properly before it is processed. That removes the buffer overflow conditions that could reload the system and cause a denial of service (DoS).
Implementing the patch
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Identify your current software version
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Open your device's command-line interface (CLI).
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Run this command to display the current software version:
show version -
Note the version number displayed.
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Determine the appropriate fixed release
- Check the Cisco Security Advisory for the first fixed release that corresponds to your current software version. (sec.cloudapps.cisco.com)
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Download the fixed software
- Go to the Cisco Software Download Center.
- Log in with your Cisco credentials.
- Find the fixed software release for your device model and current software version.
- Download the matching software image.
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Upgrade the software
- Follow the standard upgrade procedure for your device:
- Transfer the new software image to your device.
- Verify the integrity of the image.
- Set the device to boot from the new image.
- Reload the device to complete the upgrade.
- Follow the standard upgrade procedure for your device:
Post-upgrade verification
After the upgrade, confirm that the device is running the updated software:
- Run the
show versioncommand again to confirm the software version. - Monitor the device logs for unexpected errors or issues.
Affected systems and versions
- Cisco Secure Firewall Adaptive Security Appliance (ASA) Software: the Cisco Security Advisory lists the specific affected versions
- Cisco Secure Firewall Threat Defense (FTD) Software: the Cisco Security Advisory lists the specific affected versions
- The vulnerability affects systems with the web services interface enabled. See the advisory for exact version numbers and configurations.
Vendor security history
Cisco has fixed similar buffer overflow and input validation vulnerabilities in ASA and FTD before, including CVE-2016-1287 and other web services interface flaws. Cisco usually publishes advisories and patches promptly, but memory safety bugs keep recurring, which shows how hard secure development remains for network appliances this complex.
