CVE-2024-20382: Cisco ASA Software XSS Vulnerability

CVE-2024-20382 Overview

CVE-2024-20382 is a reflected cross-site scripting (XSS) vulnerability in the VPN web client services feature of Cisco Adaptive Security Appliance (ASA) Software and Cisco Firepower Threat Defense (FTD) Software. The flaw stems from improper validation of user-supplied input to application endpoints. An unauthenticated, remote attacker can craft a malicious link that, when followed by a victim, executes arbitrary HTML or JavaScript in the browser under the context of the web services page. The vulnerability was published to the National Vulnerability Database on October 23, 2024, and is tracked under CWE-80: Improper Neutralization of Script-Related HTML Tags.

Critical Impact
Successful exploitation allows attackers to run arbitrary script code in the victim's browser session, enabling credential theft, session hijacking, or phishing against administrators and VPN users accessing the affected device.

Affected Products

Cisco Adaptive Security Appliance (ASA) Software — multiple releases across 9.8.x through 9.20.x trains
Cisco Secure Firewall Threat Defense (FTD) Software — releases 6.2.3.x through 7.4.2
Devices with the VPN web client services feature enabled

Discovery Timeline

2024-10-23 - CVE-2024-20382 published to NVD
2026-08-11 - Last updated in NVD database

Technical Details for CVE-2024-20382

Vulnerability Analysis

The vulnerability resides in the VPN web client services feature exposed by Cisco ASA and FTD appliances. This component serves the browser-based interfaces used by remote access VPN users, including portal and clientless SSL VPN pages. The affected endpoints accept user-supplied parameters that are reflected into HTML responses without proper neutralization of script-related characters.

Because exploitation occurs against a browser that trusts the appliance's origin, injected script executes with the same privileges as the legitimate web services page. Attackers can leverage this trust to read authenticated content, forge requests, or stage phishing overlays impersonating the VPN portal. The vulnerability is classified under CWE-80 and requires user interaction to trigger.

Root Cause

The root cause is missing or insufficient output encoding of attacker-controlled input reflected into HTML responses served by the VPN web client services module. Special characters such as <, >, ", and ' are not escaped before being written into the response body. As a result, script payloads embedded in request parameters are rendered as executable content by the victim's browser.

Attack Vector

Exploitation is network-based and does not require authentication, but does require the victim to click a specially crafted link. A typical attack chain proceeds as follows:

The attacker identifies an internet-reachable ASA or FTD device with VPN web client services enabled.
The attacker crafts a URL targeting a vulnerable application endpoint with a payload in a reflected parameter.
The attacker delivers the link via email, chat, or a malicious site to a user who trusts the appliance's hostname.
When the victim follows the link, the appliance reflects the payload and the browser executes attacker-controlled script in the context of the VPN portal origin.

Because the CVSS scope is changed, injected script can affect data and actions across the trust boundary of the web services page, including any authenticated session established with the appliance.

Detection Methods for CVE-2024-20382

Indicators of Compromise

HTTP requests to VPN web client service endpoints on the ASA or FTD management or VPN interface containing URL-encoded script tags, event handlers (onerror=, onload=), or javascript: URIs.
Unexpected inbound referrers pointing users to the appliance's VPN portal from third-party domains.
Browser console errors or Content Security Policy violations logged by clients accessing the VPN portal.

Detection Strategies

Inspect web server and load-balancer logs upstream of ASA/FTD for GET or POST parameters containing <script, %3Cscript, onerror=, or other XSS signatures directed at VPN portal paths.
Deploy a web application firewall (WAF) or IPS signature set that flags reflected script patterns in requests to the VPN web services URIs.
Correlate suspicious clickthroughs to the VPN portal with email gateway telemetry to identify phishing campaigns delivering weaponized links.

Monitoring Recommendations

Enable and centralize ASA/FTD syslog for HTTP and WebVPN subsystems, forwarding to a SIEM for pattern matching on suspicious query strings.
Monitor for anomalous administrator or VPN user session activity following clicks on external links, including new bookmarks, config downloads, or profile changes.
Track referrer and User-Agent anomalies on requests hitting the VPN web client services endpoints.

How to Mitigate CVE-2024-20382

Immediate Actions Required

Review the Cisco Security Advisory cisco-sa-asaftd-xss-yjj7ZjVq and identify affected ASA and FTD versions in your environment.
Apply the fixed software release identified by Cisco for your specific ASA or FTD train.
Restrict access to the VPN web client services interface to trusted networks where operationally feasible.
Educate VPN users and administrators to avoid clicking unsolicited links that reference the corporate VPN portal.

Patch Information

Cisco has released free software updates that address this vulnerability. Customers with valid service contracts should upgrade to a fixed release as documented in the Cisco Security Advisory. No workarounds that fully mitigate the issue are available from the vendor; upgrading is the recommended remediation.

Workarounds

Disable the VPN web client services feature on appliances where it is not required for business operations.
Place the VPN portal behind a WAF or reverse proxy configured to strip or block reflected script content in request parameters.
Enforce strict browser security controls such as Content Security Policy headers at upstream proxies and require modern browsers with XSS auditors enabled for VPN users.

bash

# Verify running ASA software version
show version | include Software

# Check whether WebVPN (VPN web client services) is enabled
show running-config webvpn

# Disable WebVPN on interfaces where it is not required
configure terminal
webvpn
  no enable outside
end
write memory