Solution: Check out the latest update with patches from the Magento site regarding this issue:
https://support.magento.com/hc/en-us/articles/360024368392 This solves the issue! For community editions download the patch from https://magento.com/tech-resources/download for magento 1 check for Authorize.net Direct Post Signature Key patch in Release Archive and for version 2 in Magento Open Source Patches - 2.x
Authorize.Net phases out MD5 based hashes used for transaction response verification. Phase 1 means that merchants are no longer able to configure or update their MD5 Hash settings in the Merchant Interface. This will have severe impacts on merchants who use or are interested in using the Authorize.Net Direct Post method in Magento. So if you are not using Authorize.Net Direct Post method in magento you don't have to worry.
If you want to fix the directpost method you will have to do redo the hash calculation with the new APISignatureKey :
$hash_data = "^" .$merchantApiLogin . "^" . $transactionId . "^" . $amount."^" ;
$local_hash = hash_hmac("sha512", $hash_data, pack("H*", $APISignatureKey));
and then the hash validation with transHashSHA2 instead of x_MD5_Hash.