A client reported issues with payments for subscriptions as they have renewals beginning each year.

I looked into the notifications and observed two frequent log entries in addition to completed payments.

Log Type: IPN notificaton invalid hash / Fraud attempt
Reason code: ARB-Silent: ARB silent post Hash check failed

Log Type: Error returned by gateway (single payment)
Reason code: 11

I realized I did not have an MD5 hash set in Authorize.net nor the gateway setting in CBSubs, so I hope that fixes the first issue, but please advise if it's something else. If this is required input to avoid the error/notification perhaps the field in the gateway settings should be mandatory?

I'm not sure what the cause for #2 is. Authorize.net says that Error 11 is the result of a duplicate transaction being submitted . Why would that be happening? I assumed users are possibly being click-happy, but it has happened 25 times since Dec. 31, which seems very high.

Thanks for answering Krileon. A few more payments were made since I set the hash value in Authorize.net and updated the payment gateway setting to match. There are still notices for every transaction, even the completed ones, that say:
IPN notificaton invalid hash / Fraud attempt
ARB-Silent: ARB silent post Hash check failed

Any ideas? I'm sure there was no typo, but I have re-set a new hash value in both places to test further. I wondered if a space in the hash value was an issue, but it's fine according to Authorize.net :

Note that the MD5 Hash Value can be up to 20 characters long, including upper- and lower-case letters, numbers, spaces, and punctuation. More complex values will be more secure.

There have been more transactions since changing and updating the hash value in both Authorize.net and the payment gateway setting in CBSubs. The same error message occurs with each transaction.

Everything is up to date. I'm at a loss. Any ideas?