Introduction

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.

Below is a concise technical explanation, detection and mitigation steps, developer guidance for a proper fix, and incident response recommendations. The tone is practical and oriented to site owners, developers, and security teams operating in fast-moving publishing environments.

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 mitigations and limit privileges

Why stored XSS from a Contributor matters

Stored XSS is high impact because malicious input is persisted in the database, then executed when another user loads the page. Key considerations:

• Contributor accounts are common in editorial workflows in Hong Kong and elsewhere. Writers and volunteers often have these accounts.
• Contributors can create content or save block attributes. If block attributes are stored without validation, a Contributor can persist script-bearing payloads that later execute for Editors, Administrators, or visitors.
• Stored XSS can enable session theft, privilege escalation (via browser-initiated admin actions), content defacement, phishing redirection, or persistent malware delivery.

How this vulnerability works (technical overview)

The issue centers on a parameter called subHeadingTagName. It is intended to store an HTML tag name (for example, h2, h3). Correct handling requires strict validation against an allowlist of permitted tag names and proper escaping at output. In the vulnerable code path, input supplied by an authenticated Contributor is stored and later output without sanitization/escaping or validation, enabling script injection.

Typical problematic patterns that lead to this bug:

• Accepting arbitrary strings for a “tag name” and storing them directly.
• Rendering user input into HTML with little 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

• Submit a crafted value for subHeadingTagName that contains a script or on* attribute, relying on output that will not be sanitized.
• Because the value is stored, the payload will affect every visitor who loads that content — including Editors and Administrators who open it in the block editor or settings panel.
• 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.

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 depends on how the plugin uses the subHeadingTagName value: if it is rendered on the front-end to visitors or in the admin area to editors, the attack surface is larger.
• Secure cookie flags (HttpOnly, SameSite) and CSP headers may reduce some risks, but they are not a substitute for server-side validation and escaping.

Immediate risk reduction for site owners

If you run WordPress and have Radius Blocks installed, consider the following immediate actions.

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 or direct database queries to find suspicious entries (examples below in the detection section).

3. Put a WAF rule in place (virtual patch)

If you manage a Web Application Firewall (WAF) or have the ability to add server-side request filtering, add rules to block requests attempting to store script tags, event handlers, or invalid tag names into block attributes. See the “Sample WAF rules (conceptual)” section below for ideas.

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 reduce the impact of injected scripts.
• 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 plugin developer or maintain the site and can modify plugin code, apply these corrections.

1. Validate inputs using an allowlist

Only allow legitimate HTML tag names for subHeadingTagName, for example: h1, h2, h3, h4, h5, h6, p, span. Example in PHP:

<?php
$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:

• Use esc_attr() for attribute context.
• Use esc_html() when outputting text.
• For tag names used to build HTML tags, validate against an allowlist and then output safely.

<?php
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 saving endpoints perform appropriate checks:

• 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 sanitization with a strict allowed HTML list (wp_kses) rather than saving raw input:

<?php
$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.

Managed WAF and virtual patching (vendor-neutral)

When an official patch is not yet available, managed request filtering or a WAF can act as a temporary mitigation by blocking malicious requests and patterns. Typical mitigations include:

• Blocking POST/PUT requests to endpoints that include <script or encoded equivalents in form fields or JSON payloads.
• Denying values for tag name parameters that contain non-alpha characters, angle brackets, or event handler substrings (e.g., onerror, onclick).
• Normalizing payload encoding to detect obfuscated script tags (hex, double encoding) and blocking them.

Note: virtual patching reduces immediate attack surface but does not replace a proper code fix. After the plugin author releases an official update, apply it promptly.

Sample WAF rules (conceptual)

Below are conceptual signatures you can adapt. Test carefully to avoid false positives.

• Block requests where a field that should contain only a tag name contains angle brackets:
  Pattern: parameter value matches .*[<>].* — Action: block or sanitize.
• Enforce allowed tag names:
  Pattern: parameter value NOT matching ^(h[1-6]|p|span)$ — Action: block or remove parameter.
• Block common XSS tokens in JSON body or form data:
  Pattern: (<script|%3Cscript|javascript:|onerror=|onload=|onmouseover=|document\.cookie) — Action: block + alert.

Detection and clean-up if you suspect compromise

If you believe your site was exploited, perform an ordered investigation and remediation.

1. Isolate and image

• Put the site into maintenance mode or block public access until triage completes.
• Create a full backup/image of the site and database for forensic purposes.

2. Identify the malicious payload

• Search the database for suspicious strings (script tags, encoded script tokens, event handler attributes).
• Check typical locations: wp_posts.post_content, wp_postmeta, wp_options, and user meta.
• WP-CLI examples:

wp db query "SELECT ID, post_title FROM wp_posts WHERE post_content LIKE '%<script%';"

wp db query "SELECT option_name FROM wp_options WHERE option_value LIKE '%<script%';"

3. Clean or restore

• If you have a clean backup, restoring is often the fastest remediation.
• If cleaning in place: remove only malicious payloads, replace plugin files with official clean versions, rotate administrator passwords and secret keys.

4. Investigate account misuse

• Review user accounts for unauthorized changes or newly created privileged accounts.
• Remove suspicious users and reset passwords.

5. Request professional incident response if needed

Engage a qualified incident response team for complex intrusions.

Hardening WordPress against Contributor-level XSS risks

• Principle of least privilege: only grant Contributor access when needed. Consider custom roles with reduced capabilities.
• Content moderation workflow: require Editors to review and sanitize contributed content before it is rendered.
• Block untrusted HTML: ensure users without unfiltered_html capability cannot submit raw HTML that will be rendered.
• Implement a restrictive CSP to reduce impact of injected scripts (use nonces for trusted inline scripts when absolutely necessary).
• Regular plugin audits: track installed plugins and update status. Unmaintained plugins are higher risk.

Guidance for plugin authors — best practices

• Validate against an allowlist for values from a small domain (like tag names).
• Sanitize on input and escape on output. Use WordPress APIs: esc_attr(), esc_html(), wp_kses(), sanitize_text_field().
• Implement capability checks and nonces on endpoints that accept user input.
• Add unit tests that simulate injection attempts and verify sanitization.
• Adopt defense-in-depth: server-side validation even if UI validates client-side.

Detecting this vulnerability during code review

Flag code that:

• Stores values that look like HTML or tag names without server-side validation.
• Echoes plugin options or block attributes directly into HTML contexts.
• Uses REST or AJAX endpoints without capability and nonce checks.
• Allows Contributors to save settings that affect the front-end without moderation.

Longer-term defensive strategies

• Adopt CSPs that limit script execution sources and disallow inline scripts where possible.
• Enforce centralized input validation libraries within plugins and themes.
• Reduce the number of plugins that control rendering structure (tag names, raw HTML).
• Consider feature flags to disable plugin features that require rendering dynamic HTML until they are hardened.

If your site was affected — an incident response primer

1. Triage: identify affected content and isolate the site.
2. Containment: block malicious accounts and requests (WAF rule or server filters).
3. Eradication: remove malicious payloads, update plugins, replace infected files.
4. Recovery: restore from a clean backup if necessary; change credentials and rotate secrets.
5. Lessons learned: adjust processes and implement checks to prevent recurrence.

Action checklist for site owners

• Inventory: Do you have Radius Blocks installed? Which version?
• Users: Audit Contributor accounts — disable unused accounts and enforce strong passwords.
• Backups: Ensure you have recent clean backups before making changes.
• WAF: Enable or configure request filtering rules blocking script tags and event attributes in saved parameters.
• Scan: Run a site scan for injected script tags and suspicious content.
• Patch: When the plugin author releases a new version, apply updates after testing.
• Monitor: Keep server and application logs for signs of attempted exploitation.

Responsible disclosure & coordination

If you discover vulnerabilities in plugins you use or maintain:

• Report them through the plugin developer’s security contact or official support channels.
• Provide clear reproduction steps, evidence, and suggested mitigations.
• If no timely response is available, notify your hosting provider and apply server-side mitigations while coordinating with the community.

A developer example: safe handling of subHeadingTagName

Example pattern that enforces an allowlist and always escapes output:

<?php
// 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 ) . '>' . esc_html( $content ) . '</' . esc_html( $tag ) . '>';
?>

Further reading and tools

• CVE-2025-5844 (reference)
• WordPress developer handbooks on data sanitization and escaping
• WP-CLI documentation for searching the database
• Content Security Policy (CSP) guides