Cisco Secure Firewall Adaptive Security Appliance and Secure Firewall Threat Defense: memory corruption flaw under attack

What is affected

Cisco Secure Firewall Adaptive Security Appliance and Secure Firewall Threat Defense is mostly run by companies, schools and public bodies, not on home computers. You are unlikely to have it yourself, but organisations that hold your data may. Breaches that start with flaws like this one are how personal data ends up leaked.

Cisco Secure Firewall Adaptive Security (ASA) Appliance and Secure Firewall Threat Defense (FTD) Software VPN Web Server contain a buffer overflow vulnerability that allows for remote code execution. This vulnerability could be chained with CVE-2025-20362.

What an attacker can do

This is a memory corruption flaw. It lets an attacker corrupt the program's memory by feeding it specially crafted content: a web page, a file or a network message. The result ranges from a crash to the attacker running their own code.

How it is reached

Over the internet or network

Access the attacker needs

A basic user account

Does the victim have to do something?

No, works without the victim doing anything

What to do

1. At home: nothing to install. Keep your own devices updated and use a different password for every service, so that a breach at one organisation does not open your other accounts.
2. If you administer Cisco Secure Firewall Adaptive Security Appliance and Secure Firewall Threat Defense at work: apply the vendor's fix or mitigation now and review logs for signs of compromise, because the flaw was being exploited before it was listed.
3. If the product is past its support date and no fix exists, take it off the internet or retire it.

The KEV due date refers to the deadline by which FCEB agencies are expected to review and begin implementing the guidance outlined in Emergency Directive (ED) 25-03 (URL listed below in Notes). Agencies must follow the mitigation steps provided by CISA (URL listed below in Notes) and vendor’s instructions (URL listed below in Notes). Adhere to the applicable BOD 22-01 guidance for cloud services or discontinue use of the product if mitigations are not available.

Vendor advisories and fixes

• sec.cloudapps.cisco.comsec.cloudapps.cisco.com/security/center/resources/asa_ftd_continued_attacks
• sec.cloudapps.cisco.comsec.cloudapps.cisco.com/security/center/private/resources/asa_ftd_continued_attacks#Detail
• sec.cloudapps.cisco.comsec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-asaftd-webv

How urgent is it

CISA added this vulnerability to its Known Exploited Vulnerabilities catalogue on September 25, 2025, which means there is reliable evidence of attacks in the wild. US federal agencies must fix it by September 26, 2025, a deadline that has already passed. That deadline does not bind anyone else, but it shows how seriously the agency rates it.

The EPSS model estimates a 71% probability that this flaw will be exploited somewhere in the next 30 days. That is higher than 99.4% of all scored vulnerabilities.

Its CVSS severity score is 9.9 out of 10 (critical), as recorded in the US National Vulnerability Database.

Technical 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.