Authenticated Contributor Stored XSS in Radius Blocks (= 2.2.1) — What WordPress Site Owners Need to Know and How WP‑Firewall Protects You

Дата: 2025-08-15
Автор: WP‑Firewall Security Team
Теги: WordPress, Security, WAF, XSS, Plugin Vulnerability, Radius Blocks, CVE-2025-5844

Note: This post is written from the perspective of the WP‑Firewall security team. It explains the recently reported stored Cross‑Site Scripting (XSS) vulnerability affecting the Radius Blocks plugin (versions <= 2.2.1, CVE‑2025‑5844), the risk to your site, developer fixes, and practical protection steps site owners and administrators can apply immediately — including virtual/managed firewall mitigations we provide.

Введение

On 14 August 2025 a stored Cross‑Site Scripting issue (CVE‑2025‑5844) affecting Radius Blocks (<= 2.2.1) was disclosed. The vulnerability allows an authenticated user with Contributor privileges (or higher) to store HTML/JavaScript content in a plugin parameter named subHeadingTagName. When that stored value is rendered without proper sanitization or escaping, it can execute in a victim’s browser — impacting site visitors and privileged users who view the affected output.

As a WordPress firewall and security provider, we want to give site owners an accurate, actionable, and developer‑aware explanation of this issue: how it works, the real-world risk, how to detect and mitigate it immediately, and what a long‑term developer fix should be. We’ll also explain how a managed WAF / virtual patching approach can provide immediate protection while you plan remediation.

Quick summary

• Vulnerability type: Stored Cross‑Site Scripting (XSS)
• Affected software: Radius Blocks plugin, versions <= 2.2.1
• CVE: CVE‑2025‑5844
• Required attacker privilege: Contributor (authenticated)
• Exploitability: Moderate — requires a Contributor account but the payload persists and can execute for other users later
• Severity / CVSS: Reported CVSS 6.5 (medium‑low) — meaningful impact, especially on multi‑author or editorial sites
• Official fix: Not available at disclosure time (apply virtual patching / mitigation and limit privileges)

Why stored XSS from a Contributor matters

Stored XSS is a high‑impact class of vulnerability because malicious input is persisted in the site database, then executed when another user loads the page. The notable aspects here:

• Contributor accounts are common in editorial workflows. Many news sites, blogs, and organizations grant Contributor access to writers or volunteers.
• While Contributors typically cannot publish, they can create content or interact with interfaces where plugin settings or block attributes are saved. If a Contributor can inject JavaScript into plugin data that later displays to Editors, Administrators, or site visitors, the consequences can be severe.
• Stored XSS can be used for session theft, privilege escalation (via CSRF + admin actions triggered in the browser), content defacement, redirection to phishing pages, or persistent malware injection.

How this vulnerability works (technical overview)

The issue centers on a parameter called subHeadingTagName in the Radius Blocks plugin. That parameter is intended to store a tag name (for example, h2, h3, etc.) used when rendering sub‑headings. The correct handling for such a parameter is strict validation against an allowlist of permitted tag names (and escaping before output). In the vulnerable code path, input supplied by an authenticated Contributor is stored and later output without proper sanitization/escaping or validation, enabling script injection.

Typical problematic patterns that lead to this kind of bug:

• Accepting arbitrary strings for “tag name” and storing them directly.
• Rendering user input into HTML with minimal or no escaping (e.g., echoing a value into a tag name or attribute context).
• Missing capability or nonce checks on REST/AJAX endpoints used to save block attributes.

What an attacker with Contributor access could do

An authenticated Contributor could:

• Submit a crafted value for subHeadingTagName that contains a script or on* attribute, relying on output that will not be sanitized, so the browser executes it when the page, post, or block is rendered.
• Because the value is stored, the payload will affect every visitor who loads that content — including Editors and Administrators who might open it in the block editor or a settings panel.
• The attacker can embed client‑side code that performs redirection, steals cookies or session tokens (if HttpOnly flags are missing), or triggers browser‑initiated requests that perform privileged actions on behalf of an authenticated admin (CSRF‑style).

Important contextual notes

• This is not an unauthenticated RCE or SQL injection: an attacker needs a logged‑in account with Contributor privileges or higher.
• The impact on a site depends on how the plugin uses the subHeadingTagName value: if it is rendered in the front‑end to visitors or in the admin area to editors, the attack surface is larger.
• The presence of secure cookie flags (HttpOnly, SameSite) and modern CSP headers may reduce some risks, but you must not rely solely on those defenses.

Immediate risk reduction for site owners

If you run WordPress and have Radius Blocks installed, take these immediate steps:

1. Limit Contributor access temporarily
   • Restrict who has Contributor accounts. Disable or remove unused Contributor accounts.
   • If your workflow allows, temporarily downgrade or lock Contributor accounts until the site is patched or mitigated.
2. Audit recent content and settings
   • Search for suspicious content in posts, postmeta, widget options, and plugin options where block attributes may be stored. Look for strings containing <script, javascript:, onerror=, onload=, or unusual HTML inserted into tag settings.
   • Use WP‑CLI, database queries, or your database admin tools to find suspicious entries (see detection guidance below).
3. Put a WAF rule in place (virtual patch)
   • If you run a managed firewall or WAF (application layer), enable a rule blocking requests that attempt to save parameters with script tags, event handlers, or unusual tag names into block attributes. See WAF rule ideas below.
4. Harden site security
   • Enforce strong admin/editor passwords and enable two‑factor authentication for administrator/editor users.
   • Apply Content Security Policy (CSP) headers to help limit script execution origins.
   • Ensure cookies use secure flags (HttpOnly, Secure, SameSite).
5. Monitor logs & user activity
   • Watch for anomalous behavior from Contributor accounts (unexpected saves, changed profiles, posts containing HTML).
   • Check web server access logs for POST requests to REST endpoints or admin‑ajax that include suspicious payloads.

Developer guidance — how to fix the plugin (recommended patch)

If you are the developer of the plugin or maintain a site where you can patch plugin code, apply these corrections:

1. Validate inputs using an allowlist

   $allowed_tags = array('h1','h2','h3','h4','h5','h6','p','span');
   $tag = isset( $data['subHeadingTagName'] ) ? strtolower( trim( $data['subHeadingTagName'] ) ) : 'h3';
   if ( ! in_array( $tag, $allowed_tags, true ) ) {
       $tag = 'h3'; // fallback safe default
   }
   

2. Sanitize and escape at output

   Escape any dynamic values before echoing into HTML:

   • Использовать esc_attr() for attribute context,
   • esc_html() when outputting text,
   • For tag names used to build HTML tags, validate to ensure they are in the allowlist and then output them safely.

   printf( '<%1$s class="%2$s">%3$s</%1$s>',
       esc_html( $tag ),
       esc_attr( $class ),
       esc_html( $content )
   );
   

3. Enforce capability and nonce checks on REST and AJAX endpoints

   Ensure the saving endpoints perform:

   • current_user_can('edit_posts') or a suitable capability check,
   • check_ajax_referer() (or WP REST nonce checks) to avoid CSRF/unauthorized saves.
4. Avoid storing unsanitized HTML in options/meta

   If storing HTML is required, use WP’s built‑in sanitization with a strict allowed HTML list (wp_kses) rather than saving raw input.

   $allowed_html = array(
       'a' => array( 'href' => true, 'title' => true ),
       'strong' => array(), 'em' => array(),
       // ... limited tags only
   );
   $safe_html = wp_kses( $input_html, $allowed_html );
   

5. Unit tests and code review

   Add tests that attempt to inject XSS vectors and assert they are sanitized. Review all points where user input can be stored or rendered.

How WP‑Firewall protects sites (virtual patching & detection)

As a security provider, we layer multiple mitigations to protect our customers while developers prepare or release an official patch. Key protections we deploy:

1. Virtual patching (WAF rules)
   • Block or sanitize requests that include suspicious values in parameters that map to block attributes (including subHeadingTagName).
   • Rule examples we use internally:
     • Block POST/PUT requests to known plugin endpoints that include <script or encoded equivalents (%3Cscript) in form fields or JSON payloads.
     • Deny values for tag name parameters that contain non‑alpha characters, angle brackets, or event handler substrings (e.g., onerror, onclick).
     • Enforce strict allowlists on the subHeadingTagName parameter using pattern matching or an explicit list.
   • These rules apply to both admin and front‑end submission endpoints to stop storage of malicious content.
2. Request normalization and detection
   • Normalize payload encoding to detect obfuscated script tags (hex, double encoding) and block them.
   • Detect and block attribute injection patterns (e.g., ">).
3. Monitoring and alerting
   • Alert site owners when blocked requests match the stored XSS signatures or when suspicious Contributor activity is detected.
4. Granular lockdown options
   • Allow administrators to restrict Contributor roles from accessing certain endpoints or block writes to plugin settings via WAF.

// Example: sanitize a subheading tag name before saving or output
function sanitize_subheading_tag( $input_tag ) {
    $allowed_tags = array( 'h1','h2','h3','h4','h5','h6','p','span' );
    $t = strtolower( trim( (string) $input_tag ) );

    if ( in_array( $t, $allowed_tags, true ) ) {
        return $t;
    }

    return 'h3'; // safe default
}

// Output usage
$tag = sanitize_subheading_tag( $saved_value );
echo '<' . esc_html( $tag ) . ' class="' . esc_attr( $class ) . '">' . esc_html( $content ) . '</' . esc_html( $tag ) . '>';