Magento implements the Authorize.Net Direct Post payment method, using Authorize.Net's AIM (Advanced Integration Method) and DPM (Direct Post method) APIs, which use MD5 based hash.
Authorize.net will stop supporting MD5 based hash usage on March 14, 2019. Starting from this date, Magento Open Source, Magento Commerce and Magento Cloud merchants will not be able to process payments using Authorize.Net Direct Post payment method. To be able to continue successfully process payments using these methods, merchants need to apply the patch provided by Magento and replace the existing MD5 hash with a Signature Key in the Magento Admin configuration settings.
Now Apply Patch directly in vendor folder
This patch is applicable on all Magento 2.x and 1.x versions.
Alternatively you can use patched files from following address and replace them in your vendor folder.
If Need Run
php bin/magento setup:upgrade; php bin/magento setup:static-content:deploy -f;
I found this from github which explains the detail; https://gist.github.com/robolmos/7a3bf336516b18f8e0bd48a13d93e1ef
Basically, you have to change from the original patch file here;
Index: vendor/magento/module-authorizenet/Model/Directpost.php +++ vendor/magento/module-authorizenet/Model/Directpost.php (revision a0dade4d8bc15b651d8b1ea0e7e4e3580fb1d3ae) --- vendor/magento/module-authorizenet/Model/Directpost.php (revision 01fbfeba9bd743266199e30260376a9c1b95fbf5)
and they become this;
Index: Model/Directpost.php +++ Model/Directpost.php (revision a0dade4d8bc15b651d8b1ea0e7e4e3580fb1d3ae) --- Model/Directpost.php (revision 01fbfeba9bd743266199e30260376a9c1b95fbf5)
after the text replacement.