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

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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)>");