CVE-2024-20337: Cisco Secure Client XSS Vulnerability

CVE-2024-20337 Overview

A vulnerability in the SAML authentication process of Cisco Secure Client could allow an unauthenticated, remote attacker to conduct a carriage return line feed (CRLF) injection attack against a user. This vulnerability is due to insufficient validation of user-supplied input during the VPN session establishment process.

An attacker could exploit this vulnerability by persuading a user to click a crafted link while establishing a VPN session. A successful exploit could allow the attacker to execute arbitrary script code in the browser or access sensitive, browser-based information, including a valid SAML token. The attacker could then use the stolen token to establish a remote access VPN session with the privileges of the affected user.

Critical Impact

Successful exploitation allows attackers to steal valid SAML tokens and establish unauthorized VPN sessions, potentially gaining access to corporate networks with the privileges of targeted users.

Affected Products

• Cisco Secure Client (Windows)
• Cisco Secure Client (macOS)
• Cisco Secure Client (Linux)

Discovery Timeline

• 2024-03-06 - CVE-2024-20337 published to NVD
• 2025-07-22 - Last updated in NVD database

Technical Details for CVE-2024-20337

Vulnerability Analysis

CVE-2024-20337 is classified as CWE-93 (Improper Neutralization of CRLF Sequences), commonly known as HTTP Response Splitting or CRLF Injection. The vulnerability exists in how Cisco Secure Client processes user-supplied input during SAML-based authentication flows.

CRLF injection occurs when an application fails to properly sanitize carriage return (\r or %0D) and line feed (\n or %0A) characters from user input. In the context of SAML authentication, these characters can be injected to manipulate HTTP responses, potentially leading to script execution or information disclosure.

The attack requires user interaction—specifically, the victim must click a maliciously crafted link while in the process of establishing a VPN session. This social engineering component is necessary for exploitation but does not significantly diminish the risk given the prevalence of phishing attacks.

Root Cause

The root cause of this vulnerability is insufficient validation of user-supplied input in the SAML authentication handler. When processing SAML responses or redirects, the Cisco Secure Client does not adequately sanitize CRLF sequences, allowing an attacker to inject arbitrary HTTP headers or content.

This insufficient input validation allows attackers to break out of the intended response structure and inject malicious content that the browser interprets as legitimate response data.

Attack Vector

The attack vector is network-based and requires user interaction. An attacker would typically:

1. Craft a malicious URL containing CRLF sequences embedded in parameters processed during SAML authentication
2. Distribute this link to targeted users through phishing emails, malicious websites, or social engineering
3. When a victim clicks the link while establishing a VPN session, the injected CRLF sequences manipulate the HTTP response
4. The attacker's injected script executes in the victim's browser context, enabling theft of the SAML token
5. Using the stolen token, the attacker can authenticate to the VPN as the victim

The vulnerability allows for cross-site scripting (XSS) attacks within the authentication flow and can expose sensitive browser-based information including valid SAML tokens. However, individual hosts and services behind the VPN headend would still require additional credentials for successful access.

Detection Methods for CVE-2024-20337

Indicators of Compromise

• Unusual or malformed URLs containing encoded CRLF sequences (%0D%0A, %0D, %0A) in VPN authentication requests
• Suspicious SAML authentication attempts from unexpected geographic locations or IP addresses
• Multiple VPN sessions established from different locations using the same user credentials in short timeframes
• Browser console errors or unusual script execution during VPN authentication flows

Detection Strategies

• Deploy web application firewall (WAF) rules to detect and block requests containing CRLF injection patterns
• Monitor authentication logs for anomalous SAML token usage or session establishment patterns
• Implement network detection rules to identify crafted URLs targeting Cisco Secure Client authentication endpoints
• Use endpoint detection and response (EDR) solutions to monitor for suspicious browser activity during VPN authentication

Monitoring Recommendations

• Enable detailed logging for SAML authentication events in Cisco ASA or Firepower VPN headends
• Configure alerts for VPN sessions established shortly after receiving authentication from unusual sources
• Monitor for phishing campaigns targeting your organization that may distribute malicious VPN authentication links
• Review network traffic for patterns consistent with CRLF injection attempts

How to Mitigate CVE-2024-20337

Immediate Actions Required

• Update Cisco Secure Client to the latest patched version immediately
• Review VPN access logs for any suspicious authentication activity
• Alert users about potential phishing attempts involving VPN authentication links
• Implement additional authentication factors for VPN access where possible

Patch Information

Cisco has released security updates to address this vulnerability. Organizations should apply the patches as outlined in the Cisco Security Advisory. The advisory contains specific version information and upgrade paths for affected Cisco Secure Client installations across Windows, macOS, and Linux platforms.

Administrators should prioritize patching based on the network exposure of their VPN infrastructure and the sensitivity of resources accessible through VPN connections.

Workarounds

• Educate users to only initiate VPN connections through official Cisco Secure Client interfaces rather than clicking external links
• Implement network-level filtering to block URLs containing suspicious CRLF-encoded characters targeting authentication endpoints
• Consider implementing conditional access policies that restrict VPN authentication to trusted devices and locations
• Deploy email security solutions to filter phishing attempts containing malicious VPN authentication links

# Verify Cisco Secure Client version on Windows
"C:\Program Files (x86)\Cisco\Cisco Secure Client\vpncli.exe" version

# Verify Cisco Secure Client version on macOS/Linux
/opt/cisco/secureclient/bin/vpn version