Urgent: WP Statistics <= 14.15.4 — Unauthenticated Stored XSS via User‑Agent Header (CVE-2025-9816) — What You Need to Know and How to Protect Your Sites

Author: Hong Kong Security Expert
Date: 2025-09-27
Tags: WordPress, security, XSS, WP Statistics, WAF

Summary: A stored Cross‑Site Scripting (XSS) vulnerability (CVE-2025-9816) was disclosed in the WP Statistics plugin affecting versions <= 14.15.4. The issue is exploitable by unauthenticated attackers via a malicious User‑Agent header and was patched in version 14.15.5. This article explains the risk, high-level exploitation vector, detection and remediation options, and practical hardening advice from a Hong Kong security expert perspective.

Table of contents

• What happened (short)
• Why this vulnerability is serious
• How the vulnerability works (high-level, non-actionable)
• Where WP Statistics stores/outputs visitor metadata (what to check)
• Risk scenarios and real-world impact
• Detection — signs your site might be targeted or compromised
• Immediate mitigation — what to do in the next hour
• Recommended long-term remediation and hardening
• WAF / virtual‑patching guidance (rules you can apply)
• Incident response playbook if you suspect exploitation
• How to safely clean stored malicious payloads
• Monitoring and prevention — operational practices
• Appendix — vulnerability metadata and references

What happened (short)

On 27 September 2025 a stored Cross‑Site Scripting (XSS) vulnerability impacting WP Statistics versions up to and including 14.15.4 was publicly disclosed (CVE-2025-9816). The vulnerability allows an unauthenticated attacker to inject JavaScript by sending a crafted HTTP request with a malicious User‑Agent header. WP Statistics persisted parts of that header in its tracking data and later rendered the stored value to users, resulting in persistent (stored) XSS. The vendor fixed the issue in WP Statistics 14.15.5.

Why this vulnerability is serious

From an operational security standpoint, this is a high‑impact issue for several reasons:

• Unauthenticated: No account is required — any requester can attempt exploitation.
• Stored/persistent: The payload can be saved and later executed repeatedly, increasing exposure.
• Wide visibility: Execution context may include admin dashboards, public reports, or widgets that expose hundreds or thousands of users.
• Privilege escalation: Stored XSS executing in an administrative context can facilitate account takeover, backdoors, or data theft.
• Automation risk: Known vulnerabilities are easily scanned and mass‑exploited; delays raise risk rapidly.

How the vulnerability works (high-level, non-actionable)

At a technical level (without exploit details), the issue follows the classic stored XSS pattern:

1. The plugin collects visitor metadata (including the User‑Agent string) from incoming HTTP requests.
2. That metadata is stored in plugin‑managed tables for statistics and reporting.
3. In affected versions, the stored User‑Agent value was later rendered into HTML pages without adequate server‑side sanitization/encoding for HTML contexts.
4. If a malicious string containing JavaScript or event handlers is stored and later rendered, that script executes in the viewer’s browser when viewing the page — producing persistent XSS.

Because the User‑Agent header is client‑supplied, an attacker only needs to send requests with a crafted header to persist payloads. No authentication is needed.

Where WP Statistics stores or outputs visitor metadata (what to check)

WP Statistics uses custom database tables and options to store analytics data. Exact table names vary by installation prefix. Typical locations to inspect:

• Plugin database tables that store visitor requests (IP, timestamp, User‑Agent).
• Admin pages that list recent visits, device/browser lists, or raw User‑Agent strings.
• Frontend pages where WP Statistics shortcodes or widgets are used.

Immediate audit checklist:

• Inspect admin pages provided by WP Statistics, especially lists and reports showing User‑Agent strings.
• Review frontend pages with WP Statistics shortcodes/widgets.
• Search the plugin’s database tables for suspicious characters in User‑Agent fields.
• Check access and error logs for User‑Agent headers containing angle brackets or event handler attributes.

Risk scenarios and impact examples

• Public visitor hijack/redirects: Malicious scripts on public pages can alter content, redirect visitors, or show overlays.
• Admin account compromise: If payloads execute in the admin dashboard, an attacker may exfiltrate cookies or perform actions using an admin’s browser session.
• Defacement and SEO poisoning: Injected scripts can add spam links or content, damaging search ranking and reputation.
• Malware distribution: Scripts may load secondary payloads to infect visitors or perform cryptojacking.
• Mass exploitation: Automated scanners can weaponize disclosures and batch‑infect many sites.

Detection — signs your site might be targeted or compromised

Look for these indicators:

• Unexpected JavaScript in admin pages or statistics reports (inspect page source).
• Browser console errors or unusual network requests when viewing WP Statistics pages.
• User‑Agent fields in plugin tables containing “<“, “>”, “script”, “onerror”, “onload”, or “javascript:”.
• Spikes in outgoing traffic to third‑party domains after viewing affected pages.
• Unauthorized admin users created, posts altered, or settings changed soon after admin views.
• Alerts from malware scanners or security tools indicating stored XSS payloads.

Immediate mitigation — what to do in the next hour

The following are safe, immediate actions to reduce risk:

1. Update: Apply the vendor patch (upgrade WP Statistics to 14.15.5) as soon as you can — this is the definitive fix.
2. If you cannot update immediately:
   • Deactivate WP Statistics to stop further storage and rendering of new payloads.
   • Remove WP Statistics shortcodes/widgets from public pages.
3. WAF controls: If you operate a WAF or have access to request filtering, add conservative rules to block or challenge requests with explicit HTML/JS markers in User‑Agent headers (guidance below).
4. Limit admin access: Temporarily tighten admin access — use IP allowlists, VPNs, or require 2FA for admins.
5. Audit and clean: Scan the plugin tables for suspicious User‑Agent entries and neutralize them (see cleaning section).
6. Rotate sessions: Force password resets or invalidate admin sessions to reduce risk from exfiltrated cookies.

Recommended long-term remediation and hardening

Use this incident as an opportunity to strengthen site security:

• Keep plugins and WordPress core up to date promptly.
• Apply principle of least privilege: reduce the number of admin accounts and review roles regularly.
• Require strong authentication (2FA) for all privileged users.
• Implement Content Security Policy (CSP) to reduce XSS impact.
• Deploy security headers: X‑Content‑Type‑Options, X‑Frame‑Options, Referrer‑Policy, Strict‑Transport‑Security.
• Develop secure rendering practices: encode and sanitize untrusted data server‑side (esc_html(), esc_attr(), wp_kses() for WordPress developers).
• Restrict access to analytics dashboards — require authentication for reporting pages where possible.
• Maintain regular backups and test restores.
• Subscribe to vulnerability disclosure channels and maintain an update cadence.

WAF / virtual‑patching guidance (rules you can apply)

If you operate a WAF or can add request filtering, apply conservative defensive patterns. The goal is to reduce immediate risk without blocking legitimate traffic.

High‑priority, conservative rules:

• Challenge or block requests where the User‑Agent header contains explicit HTML/script markers (case‑insensitive): “<script”, “</script”, “onerror=”, “onload=”, “javascript:”.
• Rate limit or challenge (CAPTCHA) high‑volume IPs sending suspicious User‑Agent values.
• Block or challenge requests with extremely long User‑Agent headers.
• Consider sanitising or stripping angle brackets from header values before they are stored or logged (if this can be done safely at the filtering layer).
• Whitelist well‑known browser tokens (Mozilla, Chrome, Safari, Edge) and closely monitor other User‑Agent strings rather than outright denying them.

Operational notes:

• Start in logging/detection mode for 24 hours to assess false positives before blocking.
• Prefer challenging (CAPTCHA) to outright blocking for marginal matches.
• Document rules and maintain logs for forensic review.

Incident response playbook if you suspect exploitation

1. Contain
   • Deactivate WP Statistics if you cannot guarantee sanitization or patch immediately.
   • Block or challenge suspicious traffic via request filtering.
   • Temporarily restrict admin access (IP allowlist, VPN, or mandatory 2FA).
2. Forensic triage
   • Export WP Statistics tables for offline analysis.
   • Search for malicious payloads in User‑Agent and related fields for markers such as “<“, “>”, “script”, “onerror”, “onload”, “javascript:”.
   • Review web server access logs for requests with unusual User‑Agent values.
   • Check for lateral movement: new admin users, modified files, unexpected cron jobs.
3. Eradicate
   • Remove or neutralize malicious entries in the database (see cleaning steps).
   • Search and clean any backdoors or modified files; if unsure, restore from a known‑good backup.
4. Recover
   • Reinstall the patched plugin (14.15.5 or later).
   • Rotate admin passwords and revoke active sessions.
   • Reintroduce plugin functionality gradually while monitoring logs and detection systems.
5. Lessons learned
   • Document the incident timeline, root cause, and remediation steps.
   • Update processes to reduce recurrence (patch cadence, detection rules, backups).

How to safely clean stored malicious payloads

When cleaning stored payloads, work from backups and avoid data loss.

Safe cleaning approach:

• Export affected WP Statistics tables as a backup before modifying anything.
• Locate rows where User‑Agent fields contain suspicious markers (e.g., ‘<‘, ‘>’, ‘script’).
• Neutralize payloads instead of deleting rows to preserve analytics: replace ‘<‘ with ‘&lt;’ and ‘>’ with ‘&gt;’, or truncate long User‑Agent strings and add a sanitized flag.
• If unsure about direct DB edits, export suspicious rows, deactivate the plugin, and consult a qualified incident response specialist.
• After cleaning, clear caches and verify that no payload executes in admin pages or frontend widgets.

Important: Always work on a copy/backup first; direct DB edits without backups are risky.

Monitoring and prevention — operational practices

• Centralise logs: forward web server and request‑filter logs to a logging service or SIEM for correlation and retention.
• Monitor for spikes in access to WP Statistics pages or unusual headers.
• Maintain a predictable patching cadence and a staging process to validate updates before production rollout.
• Enforce strong authentication and review admin users quarterly.
• Limit plugin exposure: reduce the number of plugins that render untrusted input and keep your plugin footprint minimal and actively maintained.

Appendix — vulnerability metadata and references

• Affected product: WP Statistics (WordPress plugin)
• Vulnerable versions: <= 14.15.4
• Fixed in: 14.15.5
• Vulnerability type: Stored Cross‑Site Scripting (XSS)
• CVE: CVE-2025-9816
• Privilege required: None (Unauthenticated)
• Published: 27 September 2025

Further reading and sources:

• Review vendor release notes for WP Statistics 14.15.5 for implementation and migration details.
• Check hosting provider advisories for any provider‑specific mitigation steps.
• Refer to reputable resources on XSS and WAF tuning for guidance on minimizing false positives.

Final words from a Hong Kong security expert

Stored XSS vulnerabilities are among the most dangerous web flaws because of their ease of exploitation and potential impact in administrative contexts. The single most effective immediate action is to update WP Statistics to version 14.15.5. If you cannot update right away, deactivating the plugin and applying conservative filtering rules to suspicious User‑Agent headers will reduce exposure.

If you manage multiple sites or client environments, consider engaging a trusted security practitioner to help with audit, remediation, and tuned request filtering. Timely patching, layered defenses, and well‑drilled incident response processes materially reduce risk.

If you need further clarification on any step in this advisory, reply with details about your hosting environment and I can provide targeted, non‑vendor recommendations suitable for Hong Kong and international hosting contexts.