CVE-2024-20341: Cisco ASA Software XSS Vulnerability

CVE-2024-20341 Overview

CVE-2024-20341 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 link that submits malicious input to the affected application. When a targeted user follows the link, the attacker can execute arbitrary HTML or script code in the browser within the context of the web services page. This weakness is tracked under CWE-79 and CWE-80.

Critical Impact
Successful exploitation lets an unauthenticated attacker execute arbitrary script in the victim's browser session, enabling credential theft, session hijacking of VPN portal users, and phishing against remote-access users of Cisco ASA and FTD appliances.

Affected Products

Cisco Adaptive Security Appliance (ASA) Software across the 9.8, 9.12, 9.14, 9.15, 9.16, 9.17, 9.18, 9.19, and 9.20 trains
Cisco Secure Firewall Threat Defense (FTD) Software versions 6.2.3 through 7.4.1.1
Devices exposing the VPN web client services feature (for example, Clientless SSL VPN or AnyConnect web portal)

Discovery Timeline

2024-10-23 - CVE-2024-20341 published to the National Vulnerability Database
2026-08-11 - Last updated in NVD database

Technical Details for CVE-2024-20341

Vulnerability Analysis

The vulnerability resides in HTTP request handlers exposed by the VPN web client services feature on Cisco ASA and FTD devices. These endpoints render user-controlled parameters back to the browser without adequate output encoding or input filtering. Because the flaw is reflected and requires user interaction, exploitation typically starts with a targeted phishing link that references the affected appliance's public hostname or IP.

When the victim clicks the crafted URL, the appliance echoes the attacker-supplied payload into the response HTML. The browser then parses the injected markup as legitimate content originating from the trusted VPN portal. Any script executes with the origin of the appliance, giving the attacker access to cookies, tokens, and DOM content bound to that origin.

The scope change captured by the vulnerability's CVSS vector reflects that injected code runs in the browser context rather than on the appliance itself. However, the attacker still gains an authenticated foothold on the VPN portal from the victim's perspective, which is often the entry point into corporate networks.

Root Cause

The root cause is missing or inadequate sanitization of user-supplied input passed to application endpoints of the VPN web services. The affected handlers do not neutralize HTML control characters before reflecting parameter values in the response body, matching the pattern described in CWE-79 (Improper Neutralization of Input During Web Page Generation) and CWE-80 (Improper Neutralization of Script-Related HTML Tags).

Attack Vector

Exploitation is network-based and does not require prior authentication, but it does require the victim to click a malicious link. A typical attack chain works as follows:

The attacker identifies an internet-reachable Cisco ASA or FTD device with VPN web client services enabled.
The attacker crafts a URL to a vulnerable endpoint that embeds a JavaScript payload in a reflected parameter.
The attacker delivers the link through phishing, chat, or watering-hole content aimed at VPN users or administrators.
When the target loads the URL, the appliance returns the payload inline. The browser executes it in the origin of the VPN portal.
The payload can rewrite the login form, capture credentials, or exfiltrate session cookies to an attacker-controlled host.

Refer to the Cisco Security Advisory for ASA and FTD XSS (cisco-sa-asaftd-xss-yjj7ZjVq) and Cisco ERP Alert #75300 for authoritative technical details.

Detection Methods for CVE-2024-20341

Indicators of Compromise

HTTP GET or POST requests to VPN web services endpoints containing URL-encoded <script>, javascript:, onerror=, or onload= substrings in query parameters or form fields
Referer headers pointing at external attacker-controlled domains preceding hits on the ASA or FTD web portal
Unusual outbound requests from client browsers to unfamiliar domains immediately after visiting the VPN portal
Sudden spikes in failed logins or credential-reuse attempts against the VPN following user clicks on suspicious links

Detection Strategies

Inspect web application firewall (WAF) and reverse proxy logs in front of ASA and FTD devices for reflected XSS payload signatures targeting VPN portal paths.
Deploy Snort or Suricata rules that match on script tags, event handler attributes, and JavaScript URI schemes in requests destined for the WebVPN URI space.
Correlate email gateway and DNS telemetry to flag URLs pointing at your ASA and FTD hostnames that contain HTML control characters or long encoded parameters.

Monitoring Recommendations

Enable and centralize HTTP access logging on ASA and FTD appliances and forward logs to a SIEM for retention and correlation.
Monitor DNS resolution and outbound HTTP requests from user endpoints for anomalies following visits to the VPN portal to catch data exfiltration channels created by injected scripts.
Track authentication and session activity on the VPN portal for anomalies such as impossible-travel logins or new device fingerprints that could indicate stolen session material.

How to Mitigate CVE-2024-20341

Immediate Actions Required

Identify all Cisco ASA and FTD devices exposing VPN web client services and inventory their running versions against the fixed releases listed by Cisco.
Apply the fixed software releases published in the Cisco ASA and FTD XSS advisory.
Educate VPN portal users to access the portal only through bookmarked URLs and to report unsolicited links referencing the corporate VPN.
Rotate session tokens and require re-authentication on portals that were exposed prior to patching.

Patch Information

Cisco has released fixed software addressing this vulnerability. Consult the Cisco Security Advisory cisco-sa-asaftd-xss-yjj7ZjVq and the companion Cisco ERP Alert #75300 for the exact fixed release trains for your device model and current version. Cisco also documents related XSS issues in the ASA XSS advisory and the FMC XSS advisory, which should be reviewed together when planning upgrades.

Workarounds

Where feasible, disable the VPN web client services feature on appliances that do not require browser-based access, and force users onto native VPN clients.
Restrict access to the VPN portal by source IP, geo, or client certificate to reduce exposure to untrusted networks while upgrades are staged.
Place a WAF or reverse proxy in front of the portal with rules that strip or block HTML control characters in query strings and form parameters targeting WebVPN endpoints.
Enforce strict Content-Security-Policy, X-Frame-Options, and HttpOnly/Secure cookie flags at any upstream proxy to reduce the impact of reflected script execution.

bash

# Example: verify current ASA software version and WebVPN status
show version | include Software
show running-config webvpn

# Example: disable WebVPN globally as a temporary risk-reduction step
configure terminal
webvpn
 no enable outside
end
write memory