403 error - mismatch webhook signatures

[sparkpressstudios]

I am experiencing a persistent issue with webhook signature verification between my WordPress site and Square’s webhook system. Despite following the provided documentation for signature verification, the signatures generated on my end consistently do not match the ones provided by Square. This mismatch is leading to a 403 Forbidden response, which I believe is due to Square rejecting the request based on invalid signatures. I would greatly appreciate your help in diagnosing and resolving this issue.

Details:

• Platform: WordPress (WooCommerce)
• Environment: Production
• Webhook Endpoint: https://sparkpressstudios.com/wp-json/wc-square-sync/v1/webhook
• Error Received:
  • Status code: 403 Forbidden
  • Logs show mismatched webhook signatures.

Steps I’ve Taken:

1. Webhook Payload: I log the payload received from Square before any modifications are made:

• Payload is read directly from php://input to ensure it is not altered.
• The exact payload is used for signature generation and comparison.Example payload (as received from Square):

json

Copy code

{
  "merchant_id": "MLNTRH0XQCY55",
  "type": "order.created",
  "event_id": "a1416bf9-f4ab-3460-b018-2294038f5a3e",
  "created_at": "2024-09-21T04:33:20Z",
  "data": {
    "type": "order_created",
    "id": "htIpUQ6VcuE65GJrvMi7oSTPBnPZY",
    "object": {
      "order_created": {
        "created_at": "2024-09-21T04:33:19.734Z",
        "location_id": "LHTBF9SBMYCK5",
        "order_id": "htIpUQ6VcuE65GJrvMi7oSTPBnPZY",
        "state": "OPEN",
        "version": 1
      }
    }
  }
}

2. Signature Generation:

• I compute the expected signature using the raw payload with the HMAC-SHA256 algorithm and the secret key obtained from the Square Developer Dashboard.
• After computing the HMAC, it is base64-encoded before comparing it with the received signature.

3. Code for Signature Calculation:

php

Copy code

private function verify_square_webhook_signature($payload, $signature) {
    $webhook_secret = get_option('wc_square_sync_webhook_secret');

    if (empty($webhook_secret)) {
        error_log('Webhook verification failed: No webhook secret set.');
        return false;
    }

    // Compute the HMAC using raw payload and the secret key
    $computed_hmac = hash_hmac('sha256', $payload, $webhook_secret, true);
    $expected_signature = base64_encode($computed_hmac);

    // Log for troubleshooting
    error_log('Expected Signature: ' . $expected_signature);
    error_log('Received Signature: ' . $signature);

    // Perform timing-safe comparison
    return hash_equals($expected_signature, $signature);
}

4. Mismatch Example: Here’s an example of the mismatch between the expected and received signatures, as logged:

• Expected Signature: 4ZTw531aV5htqajQBU3fzIYsk3Ad7mQ00inc7B7htUw=
• Received Signature: pXUK+6S0wbyrkeDi/J7laPSS+o431eijIx13X2U4MWw=

5. Ensured the Following:

• The webhook secret used for signature generation matches exactly what is set in the Square Developer Dashboard.
• The payload is logged and used as received without any modifications (no trimming, whitespace, or encoding changes).
• The HMAC is calculated using the sha256 algorithm, and the result is base64-encoded.

Things I Have Checked:

• Webhook Secret: Verified that the webhook secret used in my code is accurate and matches what’s provided in the Square Developer Dashboard.
• Payload Logging: Ensured that the payload is logged before any modification.
• Server Time: There are no significant time discrepancies that could cause issues, but I am open to further investigation here if needed.

Questions:

1. Could there be any differences in how Square calculates and sends the signature compared to how I am calculating it on my end?
2. Are there any known issues with how payloads are delivered that could cause this type of mismatch?
3. Is there anything specific about the 403 Forbidden response that might provide additional insight into why this mismatch is happening?

I would appreciate any guidance or troubleshooting steps you can provide.

[Bryan-Square]

It looks like you’ve already done a lot of the right things, but let’s see if we can identify where things might be going wrong.

1. Webhook Secret Consistency:

• Ensure that the webhook secret is copied exactly as it appears in the Square Developer Dashboard. Any extra whitespace or hidden characters can cause mismatches.

2. Payload Handling:

• Verify that the payload read from php://input is exactly as received from Square. Sometimes, even minor changes like newline characters can affect the signature.

3. Time Synchronization:

• Although you’ve mentioned there are no significant time discrepancies, ensure that your server’s time is synchronized with an NTP server. This is usually not the issue, but it’s good to rule out.

4. Signature Calculation:

• Double-check that the payload is being read correctly and not modified in any way before signature calculation.

Additional Debugging Steps

1. Raw Payload Logging:

• Log the raw payload as soon as it is read from php://input to ensure no modifications are happening inadvertently.

$payload = file_get_contents('php://input');
error_log('Raw Payload: ' . $payload);

2. Recalculate Signature Manually:

• Use an external tool or script to manually calculate the HMAC-SHA256 signature of the payload using your webhook secret. This can help verify whether your code’s calculation is accurate.

3. Compare with Square’s Documentation:

• Double-check Square’s webhook signature verification documentation to ensure there are no steps or nuances missed.

Example Code Adjustment

Here is a refined version of your function with additional logging for debugging:

private function verify_square_webhook_signature($payload, $signature) {
    $webhook_secret = get_option('wc_square_sync_webhook_secret');

    if (empty($webhook_secret)) {
        error_log('Webhook verification failed: No webhook secret set.');
        return false;
    }

    // Compute the HMAC using raw payload and the secret key
    $computed_hmac = hash_hmac('sha256', $payload, $webhook_secret, true);
    $expected_signature = base64_encode($computed_hmac);

    // Log for troubleshooting
    error_log('Raw Payload: ' . $payload);
    error_log('Expected Signature: ' . $expected_signature);
    error_log('Received Signature: ' . $signature);

    // Perform timing-safe comparison
    return hash_equals($expected_signature, $signature);
}