Summary: A reflected Cross‑Site Scripting (XSS) vulnerability (CVE-2024-11685) affecting the Kudos Donations WordPress plugin (versions ≤ 3.2.9) allows untrusted input passed through add_query_arg() to be reflected in output without sufficient escaping. The issue was fixed in version 3.3.0. This advisory explains the technical details, real-world risk, detection techniques, practical mitigations (including virtual patching with a WAF), secure coding fixes, and an incident response checklist — written from the perspective of Hong Kong security practitioners.

Table of contents

• What happened (short)
• Technical root cause: add_query_arg and lack of escaping
• Exploit scenarios and real risk
• CVSS, OWASP mapping, and priority
• How to detect if your site is vulnerable or has been targeted
• Immediate remediation (update, deactivate, isolate)
• Virtual patching: WAF rules you can deploy now
• Secure coding fixes — examples and best practices
• Incident response: what to do if you were compromised
• Long‑term hardening: processes and controls for plugin authors and site owners
• Final notes and recommended next steps

What happened (short)

A reflected Cross‑Site Scripting (XSS) vulnerability was identified in the Kudos Donations plugin for WordPress affecting versions up to and including 3.2.9 (CVE-2024-11685). The vulnerability occurs when user‑controlled data passed via query parameters is used to build a URL or output via add_query_arg() and then injected into a page without proper escaping or output encoding.

The plugin author released a patched version (3.3.0). If you run the affected plugin and cannot immediately update, apply mitigations to reduce risk — including temporary deactivation or virtual patching with your Web Application Firewall (WAF).

Technical root cause: add_query_arg and lack of escaping

Understanding the root cause is critical for both site operators and developers.

• add_query_arg(): This WordPress helper builds or modifies query strings/URLs. It accepts parameters and returns a URL with those parameters appended or updated. It is not inherently unsafe — security depends on how the returned URL or parameters are output to the browser.
• The mistake: Feeding raw input (for example, from $_GET) into add_query_arg() and then echoing the result directly into HTML without escaping. If an attacker can control values stored in the query string and those values are reflected into the HTML response, they can craft a URL containing JavaScript or HTML fragments that execute in the victim’s browser.
• Why escaping matters: add_query_arg() does not HTML‑escape its return value. The correct pattern is to sanitize inputs (server side) and always escape outputs (HTML contexts) using WordPress escaping functions (esc_html, esc_attr, esc_url) appropriate to the context.

Simplified vulnerable pattern:

// vulnerable: echoes a URL built with an unsanitized query value
$url = add_query_arg( 'message', $_GET['message'], home_url() );
echo '<a href="' . $url . '">Share this</a>';

Safe pattern:

// safer: sanitize input, build URL, and escape output
$message = isset($_GET['message']) ? sanitize_text_field( wp_unslash( $_GET['message'] ) ) : '';
$url = add_query_arg( 'message', rawurlencode( $message ), home_url() );
echo '<a href="' . esc_url( $url ) . '">Share this</a>';

Key takeaway: Always treat add_query_arg() return values as data that must be escaped for the output context, and sanitize/validate inputs at the earliest point possible.

Exploit scenarios and real risk

Reflected XSS payloads are not stored on the server — they are reflected in the generated response based on the incoming request. That makes attacks relatively simple to execute but usually relies on social engineering.

• Phishing an admin/editor: An attacker crafts a link containing malicious payloads in query parameters and convinces an authenticated administrator or editor to click it. If the plugin reflects malicious content on an admin page, the admin’s browser executes the script, potentially allowing cookie theft, session hijacking, or administrative actions.
• Targeting site visitors: If the reflection occurs on a public page, any visitor who clicks the crafted link could have the script execute in their browser — resulting in redirects, fake donation forms, ad insertion, or drive‑by downloads.
• Scope of impact: The vulnerability is exploitable without authentication by crafting a URL, but successful exploitation often requires a victim who clicks the link. Impact ranges from UI defacement and redirecting to cookie theft and account takeover.
• Secondary risks: Executed JavaScript can exfiltrate nonces or CSRF tokens, trigger administrative actions, or enable an attacker to install more persistent backdoors if subsequent requests modify site state.

CVSS, OWASP mapping, and priority

• CVE: CVE-2024-11685
• CVSS (example): 7.1 — AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:L (context dependent)
• OWASP Top 10 mapping: A3 (Injection/XSS)
• Priority: Treat as high-risk for sites using the vulnerable plugin, especially where administrators or non-technical staff could be tricked into clicking crafted links. The environment and user roles determine real-world severity.

How to detect if your site is vulnerable or has been targeted

1. Inventory: Check plugin versions on all sites.

   wp plugin list --format=json | jq '.[] | select(.name=="kudos-donations")'

   If version ≤ 3.2.9, consider the site vulnerable until updated.

2. Web server and application logs: Search access logs for request URLs containing suspiciously encoded sequences like <script>, onerror=, javascript:, svg/onload, or percent‑encoded variants (%3Cscript%3E). Look for repeated, unusual query strings accessing plugin endpoints.
3. WordPress database check: Search wp_posts and wp_options for inserted scripts or unexpected <script> tags.

   SELECT ID, post_title FROM wp_posts WHERE post_content LIKE '%<script%';

4. File integrity: Compare plugin files with a clean copy of the plugin. Look for unexpected modifications or added files under wp-content/uploads or plugin directories. Use checksums or file comparison tools.
5. Browser-side indicators: Users reporting redirects, unexpected popups, or strange form behaviors after clicking links. Monitor for unexplained login attempts or new admin users.
6. Automated scanning: Run your security scanner or plugin scanner for known signatures of this plugin vulnerability. Look for suspicious query strings targeting plugin assets.

If you detect suspicious activity, follow the incident response steps later in this advisory.

Immediate remediation (what to do right now)

Prioritise these actions in order:

1. Update the plugin immediately — the plugin author published version 3.3.0 which addresses this issue. Updating is the canonical fix.
2. If you cannot update immediately:
   • Deactivate the plugin temporarily from the dashboard or via WP‑CLI:

     wp plugin deactivate kudos-donations

   • Or place the site in maintenance mode while planning the update.
3. Virtual patching via your WAF — deploy rules that block malicious payloads in query strings and protect plugin endpoints. See the Virtual patching section below for example rules.
4. Restrict access to plugin admin endpoints — restrict /wp-admin/ and plugin paths by IP where feasible; use .htaccess/Nginx rules to block requests to plugin files if deactivation is not possible.
5. Monitor for signs of exploitation — increase logging, rotate admin passwords, and invalidate sessions if compromise is suspected.
6. Plan a code review — review integrations that pass user input into add_query_arg or other output contexts.

Virtual patching: WAF rules you can deploy now

Virtual patching is the fastest way to reduce exposure while you apply permanent fixes. Below are practical rule templates you can implement in your WAF. Test in staging before enabling in production to avoid blocking legitimate traffic.

1. Block obvious <script> tokens in query string

   if (query_string matches /(%3C|<)\s*script/i) {
       block_request(403, "XSS payload detected in query string");
   }

2. Block common inline JS patterns in query string

   if (query_string matches /(javascript:|onerror=|onload=|<svg|eval\(|document\.cookie|window\.location)/i) {
       block_request();
   }

3. Whitelist expected characters for known parameters

   If a parameter such as “message” should only include simple text, enforce a strict pattern:

   if param "message" exists and not matches /^[\w \-.,]{0,200}$/ then block_request();

4. Path‑based virtual patch

   if request_path matches /wp-content/plugins/kudos-donations/i and query_string contains suspicious_payload then block;

5. Detect encoding evasion

   if query_string contains /%25(3C|3c)/ then block();

6. Heuristic scoring

   Assign scores to suspicious tokens; if the aggregate score exceeds a threshold, block or challenge the request (CAPTCHA).

7. Alerting and logging

   Log and alert on blocked requests with full request payload for forensic analysis.

Note: These are generic rule templates. Use the controls of your WAF to create, test and deploy such rules with fine‑grained exceptions. Start restrictive around plugin endpoints, then relax rules as you validate legitimate traffic.

Secure coding fixes — examples and best practices

Developers maintaining plugins or themes should apply these concrete fixes and principles to eliminate reflected XSS risks.

1. Sanitize early, escape late
   • Sanitize inputs when they enter your application (sanitize_text_field, absint, sanitize_email, etc.).
   • Escape outputs depending on context:
     • HTML body text: esc_html()
     • Attribute values: esc_attr()
     • URLs: esc_url()
     • JavaScript data structures: wp_json_encode() with esc_js()
2. Use proper functions when building URLs

   $val = isset($_GET['val']) ? sanitize_text_field( wp_unslash( $_GET['val'] ) ) : '';
   $url = add_query_arg( [ 'val' => rawurlencode( $val ) ], home_url() );
   echo '<a href="' . esc_url( $url ) . '">Link</a>';

3. Avoid echoing raw $_GET/$_REQUEST content

   // BAD
   echo $_GET['foo'];
   
   // GOOD
   echo esc_html( sanitize_text_field( wp_unslash( $_GET['foo'] ) ) );

4. Use nonces and capability checks on admin actions

   Verify a nonce and check current_user_can() for actions via GET or POST intended for authenticated users.

5. Content Security Policy (CSP)

   Use CSP headers to reduce impact of injected scripts. Example:

   Content-Security-Policy: default-src 'self'; script-src 'self' 'nonce-...'; object-src 'none';

   CSP reduces exploitability but is not a silver bullet.

6. Example: Fixing vulnerable code

   Vulnerable:

   // echoing raw URL with untrusted input
   $link = add_query_arg('note', $_GET['note'], site_url());
   echo '<a href="' . $link . '">Read note</a>';

   Fixed:

   $note = isset($_GET['note']) ? sanitize_text_field( wp_unslash( $_GET['note'] ) ) : '';
   $link = add_query_arg( 'note', rawurlencode( $note ), site_url() );
   echo '<a href="' . esc_url( $link ) . '">Read note</a>';

Incident response: what to do if you were compromised

If you have evidence of exploitation, act methodically.

1. Contain
   • Take the site offline or enable maintenance mode (short term).
   • Rotate admin passwords and revoke sessions (force all users to reauthenticate).
2. Preserve evidence
   • Export logs, database dumps and copies of suspected files for forensic analysis.
   • Do not overwrite or truncate logs.
3. Eradicate
   • Remove malicious files, backdoors and unauthorized admin users.
   • Replace modified plugin files with clean copies from the official plugin package.
   • Reinstall WordPress core and plugins from trusted sources.
4. Recover
   • Restore from a verified pre‑compromise backup.
   • Apply all patches and configuration changes.
5. Post‑recovery actions
   • Rotate API keys and secrets used by the site.
   • Notify stakeholders and customers if required by policy or regulation.
   • Conduct root cause analysis and strengthen controls to prevent recurrence.
6. Consider a professional security review — if unsure about scope or persistence of compromise, engage a security specialist for a thorough audit.

Long‑term hardening for plugin authors and site owners

Prevention saves time and reputational cost. Recommended long-term measures:

• For plugin authors: Follow “sanitize input, escape output”, use automated security testing in CI (static analysis and dynamic tests), reduce output contexts where raw values are printed, and provide clear changelogs and security release notes.
• For site owners: Keep WordPress core, themes and plugins up to date; use a WAF that supports virtual patching for zero‑day coverage; enforce strong admin practices (2FA, least privilege, session management); regularly backup and test restores; conduct periodic security scans and patch management.

Final notes and recommended next steps

1. Check all sites and update the Kudos Donations plugin to version 3.3.0 (or later) immediately.
2. If you cannot update right away, put the site into maintenance mode and deploy WAF rules that block malicious query string payloads targeting the plugin.
3. Review code patterns described here and ensure all add_query_arg() usages sanitize and escape appropriately.
4. If you suspect compromise, follow the incident response checklist above or engage a security professional for an investigation.

Reflected XSS vulnerabilities are often exploited through crafted links. Rapid patch management, careful virtual patching, and secure development practices substantially reduce the likelihood of successful exploitation.

If you need hands-on help implementing WAF rules, detection checks, or an incident response plan for multiple sites, engage a qualified security consultant with WordPress experience.

Stay vigilant and act swiftly — in our experience in the Hong Kong security community, every hour of delay increases the window of opportunity for attackers.

— Hong Kong Security Experts