GitHub Security Advisory: GHSA-h54m-c522-h6qr
Package: wwbn/avideo (COMPOSER)
Severity: MODERATE (CVSS 5.3)
CWE: CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
Vulnerable versions: <= 26.0
Patched in: unpatched

Summary: AVideo Vulnerable to Wallet Balance Double-Spend via TOCTOU Race Condition in transferBalance

Details:
## Summary

The `transferBalance()` method in `plugin/YPTWallet/YPTWallet.php` contains a Time-of-Check-Time-of-Use (TOCTOU) race condition. The method reads the sender's wallet balance, checks sufficiency in PHP, then writes the new balance — all without database transactions or row-level locking. An attacker with multiple authenticated sessions can send concurrent transfer requests that all read the same stale balance, each passing the balance check independently, resulting in only one deduction being applied while the recipient is credited multiple times.

## Details

The vulnerable code path in `plugin/YPTWallet/YPTWallet.php:450-517`:

```php
// Line 473-474: READ - fetch current balance (plain SELECT, no FOR UPDATE)
$senderWallet = self::getWallet($fromUserId);
$senderBalance = $senderWallet->getBalance();
$senderNewBalance = $senderBalance - $amount;

// Line 477: CHECK - verify sufficient funds in PHP
if ($senderNewBalance < 0) {
    return false;
}

// Line 486-487: WRITE - set new balance (plain UPDATE)
$senderWallet->setBalance($senderNewBalance);
$senderWalletId = $senderWallet->save();

// Line 497-502: Credit receiver (also plain SELECT + UPDATE)
$receiverWallet = self::getWallet($toUserId);
$receiverBalance = $receiverWallet->getBalance();
$receiverNewBalance = $receiverBalance + $amount;
$receiverWallet->setBalance($receiverNewBalance);
$receiverWalletId = $receiverWallet->save();
```

The `getWallet()` method (`YPTWallet.php:244`) calls `Wallet::getFromUser()` (`Wallet.php:69-84`) which executes a plain `SELECT * FROM wallet WHERE users_id = $users_id` with no `FOR UPDATE` clause. The `save()` method (`Wallet.php:105`) calls `ObjectYPT::save()` (`Object.php:293`) which executes a plain `UPDATE` — no transaction wrapping.

**Race window**: Between the SELECT (step 1) and UPDATE (step 3), all concurrent requests see the same original balance. Each independently computes `original - amount`, passes the check, and writes back. The last writer wins for the sender (only one deduction effective), but the receiver gets credited once per request.

**Why concurrent requests succeed**: PHP's file-based session locking serializes requests per session. However, an attacker can create multiple login sessions (different PHPSESSID cookies) for the same user account. Each session has its own lock and can execute concurrently. Each session needs its own captcha, but the captcha validation in `objects/captcha.php:58-73` compares `$_SESSION['palavra']` without unsetting it after validation, allowing unlimited reuse within each session.

Entry point: `plugin/YPTWallet/view/transferFunds.json.php:39` calls `YPTWallet::transferBalance(User::getId(), $_POST['users_id'], $_POST['value'])` — requires only `User::isLogged()` and a valid captcha.

## PoC

```bash
# Prerequisites: Attacker has a registered account with $10 wallet balance
# Accomplice has a registered account (recipient)

TARGET="https://target-avideo-instance"
ACCOMPLICE_ID=123  # recipient