CVE-2025-20333: Cisco Secure Firewall Adaptive Security Appliance (ASA) and Secure Firewall Threat Defense (FTD) Buffer Overflow Vulnerability

Vulnerability summary

CVE-2025-20333 is a critical (CVSS 9.9) Classic Buffer Overflow vulnerability in Cisco Secure Firewall Adaptive Security Appliance and Secure Firewall Threat Defense. It is known to be exploited: CISA lists it, first on 25-09-2025. EPSS estimates a 71.21% chance of exploitation in the next 30 days, higher than 99.4% of scored vulnerabilities.

Timeline

1. 25-09-2025
   PublishedVulnerability first disclosed.
2. 25-09-2025
   Added to CISA KEVCisco Secure Firewall Adaptive Security Appliance (ASA) and Secure Firewall Threat Defense (FTD) Buffer Overflow Vulnerability
3. 26-09-2025
   CISA remediation dueApply mitigations per vendor instructions.
4. 26-02-2026
   Last modifiedVulnerability information updated.

Description

A vulnerability in the VPN web server of Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Cisco Secure Firewall Threat Defense (FTD) Software could allow an authenticated, remote attacker to execute arbitrary code on an affected device. This vulnerability is due to improper validation of user-supplied input in HTTP(S) requests. An attacker with valid VPN user credentials could exploit this vulnerability by sending crafted HTTP requests to an affected device. A successful exploit could allow the attacker to execute arbitrary code as root, possibly resulting in the complete compromise of the affected device.

• CWE-120Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')The product copies an input buffer to an output buffer without verifying that the size of the input buffer is less than the size of the output buffer.