Authenticated Stored XSS in “User Language Switch” (≤ 1.6.10) — What WordPress Site Owners Need to Know and How to Protect Themselves

Published: 13 February 2026

執行摘要

On 13 February 2026 a stored Cross‑Site Scripting (XSS) vulnerability affecting the WordPress plugin User Language Switch (versions ≤ 1.6.10) was disclosed (CVE‑2026‑0735). The issue requires an authenticated administrator to save a specially crafted value via the plugin’s color picker option (tab_color_picker_language_switch), which is stored and later rendered without adequate escaping. Although an admin account is required to inject the payload, stored XSS can still have severe consequences: session theft, remote actions in an admin’s browser, persistent defacement, or backdoor installation. This write-up — provided in the tone of a Hong Kong security practitioner — explains the technical cause, detection steps, emergency mitigation and longer‑term hardening advice.

目錄

• 披露的內容
• Why stored XSS matters
• 技術根本原因
• 利用場景
• 偵測步驟
• Immediate mitigations
• Permanent fixes and coding best practices
• WAF and virtual patching (vendor-neutral)
• 硬化檢查清單
• 恢復和事件響應
• Appendix — developer reference snippets

披露的內容

• 受影響的插件： User Language Switch (WordPress)
• Affected versions: ≤ 1.6.10
• 漏洞類型：儲存型跨站腳本 (XSS)
• Parameter involved: tab_color_picker_language_switch
• Required privileges to inject: Administrator
• CVE： CVE‑2026‑0735
• Public disclosure date: 13 Feb 2026

The plugin stores a value submitted from the admin settings (the color picker). That value was not sufficiently sanitized on input and was output without correct escaping, allowing persistent script injection.

Why stored XSS matters (real‑world impact)

From a Hong Kong enterprise and SME perspective: even if the injection requires an admin to store a payload, the consequences are practical and dangerous.

• 會話盜竊: Malicious script can exfiltrate cookies and tokens.
• Privilege abuse: Script executing in an admin’s browser can trigger administrative actions via AJAX.
• Persistent defacement / SEO poisoning: Public pages may be altered to host spam, redirects, or ads.
• 惡意軟體傳遞: Injected JavaScript can load further payloads or redirects to exploit kits.
• Supply‑chain risk: Integrated services and API tokens exposed through in‑browser attacks.

Given common threats (phishing, credential reuse, weak MFA), treat admin‑level XSS as a high‑impact issue.

Technical root cause and how the bug arises

The core failures:

1. Input is stored without proper validation or sanitization. The plugin expects a hex color but does not enforce it.
2. Output is echoed into HTML without proper escaping for the context.
3. Insufficient server‑side checks and assumptions about client‑side controls.

Defensive rules that would have prevented this: strict type validation, sanitize at input, and escape at output (whitelist approach).

利用場景 — 誰面臨風險

• Single‑admin sites: A compromised or social‑engineered admin can introduce the payload and persist it.
• Multi‑admin sites: Any admin user with access to plugin settings can inject content that will later affect other admins.
• Public impact: If the color setting is rendered on public pages, visitors may also be affected.

如何檢測您的網站是否受到影響

Assume risk if you run the plugin at the affected versions. Detection steps:

1. 確認插件和版本： WP Admin → Plugins, or via WP-CLI:

   wp plugin list --status=active | grep user-language-switch

2. Search the database for suspicious values: 檢查 wp_options and plugin tables for tab_color_picker_language_switch or scripted content.

   SELECT option_name, option_value
   FROM wp_options
   WHERE option_name LIKE '%language_switch%'
      OR option_name LIKE '%tab_color%'
      OR option_value LIKE '%<script%'
      OR option_value LIKE '%onmouseover=%'
      OR option_value LIKE '%javascript:%'
   LIMIT 100;

3. Inspect the plugin settings page: Use a hardened browser or secondary admin account and inspect the color picker value.
4. Run malware scans: Use a reputable WordPress scanner or host‑level malware scanner to find suspicious stored payloads.
5. 審查管理員活動： Look for unexpected logins, new admin users, and changes around the disclosure date.

If you find payloads, do not execute them in a normal browser. Isolate the environment and preserve forensic evidence.

Immediate mitigations (emergency steps)

Containment should be rapid and pragmatic.

1. 限制管理員訪問： Rotate admin passwords, remove untrusted admin accounts, and enforce 2FA immediately.
2. 停用插件： Deactivate “User Language Switch” until it is patched or replaced. If removal is not possible, restrict access to the plugin settings page.
3. 應用虛擬修補： Use your WAF or host firewall to block POSTs where tab_color_picker_language_switch contains suspicious tokens (<, >, script, javascript:, event handlers) or does not match a hex color regex.
4. Scan and remove stored payloads: Locate and safely sanitize or remove malicious option/post values (see cleanup section).
5. Take backups for forensics: Snapshot DB and files before making destructive changes.
6. 使會話失效： Force logout all users and monitor for repeated attempts to access the vulnerable endpoint.

Permanent fixes & coding best practices

For plugin authors and developers, apply these secure coding practices:

1. 保存時清理： For color values, use sanitize_hex_color() or similar to enforce allowed format.
2. 輸出時進行轉義： 使用 esc_attr(), esc_js(), or context‑appropriate escaping when printing values into HTML or JS.
3. Settings API sanitizers: 使用 register_setting() 與一個 sanitize_callback.
4. 能力檢查： 驗證 current_user_can( 'manage_options' ) (or a suitable capability) before processing POSTs.
5. Whitelist data types: If the UI expects a hex color, reject anything else at server side.
6. 自動化測試： Add tests that assert malicious payloads are rejected and outputs are escaped.

WAF and virtual patching (vendor‑neutral guidance)

When an upstream patch is not yet available, virtual patching via a WAF is a practical stopgap. Suggested virtual patch approaches:

• Block or sanitize POSTs that try to set tab_color_picker_language_switch containing characters or tokens outside a safe hex color pattern (e.g. reject content containing <, >, script, javascript:, onerror=, ，等等）。.
• Apply a regex rule that allows only ^#?[A-Fa-f0-9]{3,6}$ for this parameter.
• Enable monitoring and alerting for requests that target the plugin settings page or carry suspicious payloads.
• Use host‑level protections where possible (webserver rules, mod_security rules, or reverse proxy filters) if WAF management is available through your host.

Note: virtual patches reduce exposure but are not a substitute for a proper code fix. Remove the rule only after the plugin is patched and updated safely.

Detection and cleanup: sample queries and safe removal

Work on a copy of the database when possible and preserve a forensic snapshot.

Read‑only detection query:

-- Search for suspicious patterns in options table
SELECT option_id, option_name, LEFT(option_value, 200) AS sample
FROM wp_options
WHERE option_value LIKE '%<script%'
   OR option_value LIKE '%javascript:%'
   OR option_value LIKE '%onmouseover=%'
   OR option_name LIKE '%tab_color_picker%'
LIMIT 100;

Safe PHP sanitization approach:

// replace dangerous content in option safely
$opt_name = 'tab_color_picker_language_switch';
$value = get_option( $opt_name );

if ( $value && preg_match( '/<[^>]+>/', $value ) ) {
    // remove tags and keep hex color fallback
    $clean = sanitize_hex_color( strip_tags( $value ) );
    if ( empty( $clean ) ) {
        $clean = '#000000';
    }
    update_option( $opt_name, $clean );
}

If you find injected scripts elsewhere (posts, usermeta), export the records, sanitize or replace with safe defaults, and rotate credentials. When in doubt, isolate and consult incident response professionals.

Hardening checklist (practical steps every WordPress admin should follow)

1. 補丁管理： Keep core, themes, and plugins updated. Deactivate unmaintained plugins with known issues.
2. 最小特權： Minimize admin accounts and use role separation.
3. 存取控制： Enforce strong passwords and 2FA; restrict admin access by IP if feasible.
4. Backups & monitoring: Maintain frequent backups and monitor admin actions and file integrity.
5. Security headers & CSP: Implement Content Security Policy to reduce impact of injected scripts where practical.
6. WAF & scanning: Deploy a managed WAF or host‑level protections and schedule periodic malware scans.
7. 事件響應計劃： Prepare standard procedures for compromise: isolate, snapshot, scan, clean, restore, and communicate.

Recovery and incident response after compromise

1. Isolate the site (maintenance mode, restrict access).
2. Take full backups (DB + files) and preserve timestamps.
3. Scan for persistence mechanisms: changed plugin/theme files, mu-plugins, new admin users, unexpected cron jobs, unknown files.
4. Remove or clean injected code, but keep a forensic copy.
5. Rotate all credentials (WP accounts, DB, FTP, hosting panel).
6. Reinstall software from known good sources and rebuild compromised components.
7. Conduct a full post‑incident audit and harden systems to prevent recurrence.

If you lack internal capacity, engage a reputable incident response provider experienced in WordPress environments.

最後的備註

Do not under‑estimate admin‑accessible XSS. Even when an admin is required to save the payload, attacker techniques such as phishing and lateral compromise make this a real-world threat. Use layered defenses: secure coding, least privilege, 2FA, network restrictions, WAF coverage, and ongoing scanning. Quick containment followed by careful cleanup and credential rotation is the proven approach.

Appendix — developer reference snippets

Hex color validation function:

function is_valid_hex_color( $color ) {
    return preg_match( '/^#?[A-Fa-f0-9]{3}([A-Fa-f0-9]{3})?$/', trim( $color ) );
}

Sanitize callback example:

function wps_sanitize_color_callback( $value ) {
    $value = sanitize_text_field( $value );
    $clean = sanitize_hex_color( $value );
    if ( empty( $clean ) ) {
        return '#000000';
    }
    return $clean;
}

register_setting( 'wps_settings_group', 'tab_color_picker_language_switch', array(
    'sanitize_callback' => 'wps_sanitize_color_callback',
) );

Conceptual WAF rule: block POSTs where tab_color_picker_language_switch contains < or > or text that does not match the hex color regex.

Need a concise remediation plan?

If you would like a short, prioritized remediation checklist tailored to your installation, reply with the following (only if you intend to involve a security team):

• WordPress admin URL (for planning; do not post credentials)
• WordPress version
• “User Language Switch” plugin status/version

A qualified security professional can prepare a step‑by‑step plan (containment, virtual patch suggestions, safe cleanup steps and tests) for your site.