CVE-2023-6934

Executive Summary

CVE-2023-6934 is a Stored Cross-Site Scripting (XSS) vulnerability in the Limit Login Attempts Reloaded WordPress plugin, affecting all versions up to and including 2.25.26. The flaw resides in insufficient input sanitization and output escaping on user-supplied attributes within the plugin's shortcodes. Authenticated attackers with contributor-level permissions or higher can inject arbitrary web scripts that execute whenever a user accesses an injected page. This vulnerability is specific to the WordPress ecosystem and does not involve Apache or Nginx directly.

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 plugin is widely used for brute-force protection, making this a high-impact supply chain risk for WordPress sites.

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: 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

Threat Level: Medium. While the CVSS score is moderate, the ability for authenticated users to inject persistent scripts that execute for all visitors elevates the risk, especially on multi-author WordPress sites.

Deep Dive & Mechanics

The vulnerability exists in the limit-login-attempts-reloaded plugin's shortcode handling. The plugin registers shortcodes (e.g., [llar_login_form]) that accept user-supplied attributes. These attributes are not properly sanitized or escaped before being rendered in the HTML output.

Root Cause:

• Insufficient Input Sanitization: The plugin does not strip or encode malicious HTML/JavaScript from shortcode attributes.
• Insufficient Output Escaping: When rendering the shortcode, the plugin outputs attribute values directly into the page without using WordPress's escaping functions like esc_attr() or wp_kses().

Attack Flow:

1. An attacker with contributor-level access creates or edits a post/page.
2. The attacker inserts the vulnerable shortcode with a malicious attribute, e.g., [llar_login_form redirect_url="javascript:alert('XSS')"].
3. When any user (including admins) visits the page, the injected script executes in their browser context.

WordPress-Specific Context: This is a classic WordPress plugin vulnerability where shortcode attributes are mishandled. The attack is stored in the database and persists until the content is removed or patched.

Vulnerable vs. Patched Code

Vulnerable Code (<= 2.25.26):

// In shortcode handler, attributes are used directly without escaping
function llar_shortcode_login_form($atts) {
    $redirect_url = $atts['redirect_url'];
    return '<div class="llar-login-form" data-redirect="' . $redirect_url . '">...</div>';
}

Patched Code (Hypothetical):

function llar_shortcode_login_form($atts) {
    $redirect_url = esc_url($atts['redirect_url']); // Properly escape URL
    return '<div class="llar-login-form" data-redirect="' . $redirect_url . '">...</div>';
}

The patch would involve using esc_attr() for HTML attributes and esc_url() for URL attributes, along with wp_kses() for any rich content.

Exploitation & Lab Setup

Red Team (Attack Simulation)

Prerequisites:

• WordPress site with Limit Login Attempts Reloaded <= 2.25.26
• Contributor-level account or higher

Exploit Steps:

1. Log in as a contributor.
2. Create a new post or edit an existing one.
3. Switch to Text/Code editor and insert:

   [llar_login_form redirect_url="javascript:alert(document.cookie)"]
   

4. Publish or save the post.
5. When any user visits the post, the XSS payload executes.

Advanced Payload:

[llar_login_form redirect_url="\" onfocus=\"alert(1)\" autofocus=\"true"]

Blue Team (Detection & Defense)

Detection:

• Monitor for posts/pages containing shortcodes with suspicious attributes (e.g., javascript:, onerror=, onfocus=).
• Use WAF rules to block XSS attempts in shortcode attributes.

Defense:

• Immediately update the plugin to a patched version (if available).
• Restrict contributor-level permissions to trusted users only.
• Implement Content Security Policy (CSP) headers to mitigate XSS impact.

Indicators of Compromise (IOCs)

• File Indicators:

  • Modified posts/pages containing shortcodes with malicious attributes.
  • Plugin files: /wp-content/plugins/limit-login-attempts-reloaded/ (check for version <= 2.25.26)

• Network Indicators:

  • HTTP requests to /wp-admin/post.php or /wp-admin/admin-ajax.php with shortcode injection attempts.
  • Outbound connections from injected scripts (e.g., to attacker-controlled domains).

• Log Indicators:

  • WordPress audit logs showing post creation/editing by contributors with unusual shortcode usage.
  • Web server logs with XSS payloads in POST data.

Detection Engineering

YARA Rule

rule CVE_2023_6934_WordPress_XSS {
    meta:
        description = "Detects malicious shortcode attributes in Limit Login Attempts Reloaded plugin"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2023-6934"
    strings:
        $shortcode = /\[llar_login_form[^\]]*\]/ nocase
        $xss_payload = /(javascript:|on\w+\s*=|alert|prompt|confirm)/ nocase
    condition:
        $shortcode and $xss_payload
}

Sigma Rule

title: CVE-2023-6934 - Stored XSS in Limit Login Attempts Reloaded
id: cve-2023-6934-sigma
status: experimental
description: Detects stored XSS attempts via shortcode attributes in WordPress
author: Valters IT
logsource:
    category: webserver
    product: apache
detection:
    selection:
        http.method: 'POST'
        http.url: '/wp-admin/post.php'
        http.body|contains: 'llar_login_form'
        http.body|contains|all:
            - 'redirect_url'
            - 'javascript:'
    condition: selection
falsepositives:
    - Legitimate use of redirect URLs (rare)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2023-6934 - Stored XSS in Limit Login Attempts Reloaded"; flow:to_server; content:"POST"; http_method; content:"/wp-admin/post.php"; http_uri; content:"llar_login_form"; http_client_body; pcre:"/(javascript:|on\w+\s*=)/i"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Update Plugin: Check for a patched version of Limit Login Attempts Reloaded. If unavailable, consider disabling the plugin temporarily.
2. Restrict Permissions: Remove contributor-level access from untrusted users.
3. WAF Rules: Deploy web application firewall rules to block XSS payloads in shortcode attributes.

Long-Term Measures:

• Implement a Content Security Policy (CSP) with script-src 'self' to prevent inline script execution.
• Regularly audit WordPress plugins for known vulnerabilities.
• Use WordPress security plugins (e.g., Wordfence) to monitor for malicious activity.

Patch Status: UNKNOWN. Monitor the plugin's official page for updates.

Valters IT
Securing Digital Frontiers