Unauthenticated Reflected XSS via innerHTML in AVideo

Summary

AVideo contains a reflected XSS vulnerability that allows unauthenticated attackers to execute arbitrary JavaScript in a victim's browser. User input from a URL parameter flows through PHP's json_encode() into a JavaScript function that renders it via innerHTML, bypassing encoding and achieving full script execution.

Root Cause

The vulnerability is caused by two issues working together:

1. Source: Unescaped user input passed to JavaScript (videoNotFound.php)

File: view/videoNotFound.php line 49

if (!empty($_REQUEST['404ErrorMsg'])) {
    echo 'avideoAlertInfo(' . json_encode($_REQUEST['404ErrorMsg']) . ');';
}

PHP's json_encode() with default flags only escapes quotes (" → \") and backslashes. It does NOT escape HTML special characters (<, >, /). The resulting string contains raw HTML tags that are passed directly to JavaScript.

2. Sink: innerHTML renders HTML tags as executable DOM (script.js)

File: view/js/script.js

function avideoAlertInfo(msg) {            // line ~1891
    avideoAlert("", msg, 'info');           // calls ↓
}

function avideoAlert(title, msg, type) {   // line ~1270
    avideoAlertHTMLText(title, msg, type);  // calls ↓
}

function avideoAlertHTMLText(title, msg, type) {  // line ~1451
    var span = document.createElement("span");
    span.innerHTML = msg;                  // line 1464 — XSS SINK
    swal({ content: span });
}

innerHTML parses the string as HTML. Any <img>, <svg>, or other HTML tags with event handlers are instantiated as real DOM elements, triggering JavaScript execution.

Data Flow

URL parameter (?404ErrorMsg=PAYLOAD)
    → $_REQUEST['404ErrorMsg']
    → json_encode()          ← does NOT escape < > /
    → avideoAlertInfo()
    → avideoAlert()
    → avideoAlertHTMLText()
    → span.innerHTML = msg   ← renders HTML tags, executes JS

---

Proof of Concept

https://localhost/view/videoNotFound.php?404ErrorMsg=<img src=x onerror=alert(document.domain)>

<img width="1918" height="1035" alt="image" src="https://github.com/user-attachments/assets/20077ce2-5b49-4bd3-a7df-ab48be786cc1" />

The page renders:

avideoAlertInfo("<img src=x onerror=alert(document.domain)>");

Which flows to span.innerHTML = "<img src=x onerror=alert(document.domain)>". The browser creates an <img> element, src=x fails to load, onerror fires alert(document.domain).

Affected Code

The innerHTML sink exists in 4 functions. Any future code that passes user input to avideoAlertInfo(), avideoAlertWarning(), avideoAlertDanger(), or avideoAlertSuccess() will create additional XSS vectors.

Remediation

Fix 1: Escape HTML in PHP (source fix)

// view/videoNotFound.php line 49
// BEFORE (vulnerable):
echo 'avideoAlertInfo(' . json_encode($_REQUEST['404ErrorMsg']) . ');';

// AFTER (fixed):
echo 'avideoAlertInfo(' . json_encode($_REQUEST['404ErrorMsg'], JSON_HEX_TAG | JSON_HEX_AMP) . ');';

JSON_HEX_TAG converts < → \u003C and > → \u003E, preventing HTML injection.

Fix 2: Use textContent instead of innerHTML (sink fix, recommended)

// view/js/script.js - all alert functions
// BEFORE (vulnerable):
span.innerHTML = msg;

// AFTER (fixed):
span.textContent = msg;

textContent treats the string as plain text — HTML tags are displayed literally, never parsed or executed.

Fix 3: Add Content-Security-Policy header (defense in depth)

Content-Security-Policy: default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'

Impact

• Session hijacking — steal PHPSESSID cookie (not HttpOnly by default)
• Account takeover — use stolen session to change password or email
• Phishing — inject a realistic login form inside the SweetAlert modal
• Worm propagation — inject self-spreading payloads via comments/messages
• Admin compromise — send crafted link to admin, steal session, gain full control