Cisco Secure Firewall Adaptive Security Appliance and Secure Firewall Threat Defense Software VPN Web Server Denial of Service Vulnerability

TL;DR 📌

A high-severity vulnerability has been identified in the Remote Access SSL VPN service of Cisco Secure Firewall Adaptive Security Appliance (ASA) and Secure Firewall Threat Defense (FTD) Software. This flaw could allow an authenticated attacker to create or delete files on the underlying operating system, potentially leading to a denial of service (DoS) condition. Cisco has released software updates to address this issue, and there are no available workarounds.

What happened 🕵️‍♂️

A vulnerability exists in the Remote Access SSL VPN service for Cisco Secure Firewall ASA and FTD Software due to insufficient input validation when processing HTTP requests. An authenticated attacker could exploit this vulnerability by sending crafted HTTP requests, allowing them to create or delete arbitrary files on the operating system. If critical system files are manipulated, it could result in new VPN sessions being denied and existing sessions being dropped, necessitating a manual reboot of the affected device to recover.

Affected products 🖥️

• Cisco Secure Firewall Adaptive Security Appliance (ASA) Software
• Cisco Secure Firewall Threat Defense (FTD) Software

Fixed software 🔧

Upgrade to the first fixed release in your train (or later):

Workarounds 🧯

There are no workarounds that address this vulnerability.

Risk in context 🎯

The vulnerability has a CVSS score of 8.5, indicating a high severity risk. It requires authentication as a VPN user, which limits exposure but still poses a significant risk if exploited. Organizations should prioritize applying the available software updates to mitigate this risk.

Fast facts ⚡

• Vulnerability ID: CVE-2025-20251
• CVSS Score: 8.5 (High)
• Impact: Denial of Service (DoS)
• Authentication Required: Yes (authenticated VPN user)
• Exploitation: No public exploitation reported yet

For leadership 🧭

This vulnerability presents a High risk to our network security, with a CVSS score of 8.5. The exposure is primarily driven by the requirement for authenticated access, which limits the potential attack surface. However, if exploited, it could lead to significant service disruption due to the denial of service condition.

Remediation ask: Patch affected systems within 7 days to mitigate the risk.

Operational impact: Expect a brief maintenance window with no configuration drift anticipated.

Now / Next / Later:

• Now: Review affected systems and identify those that require updates.
• Next: Schedule and apply the necessary software updates.
• Later: Monitor for any signs of exploitation or unusual activity related to this vulnerability.