Authorize.net and PayPal have both made recent changes to the protocols they accept for processing credit card orders. As a result, many Zen Cart sites will suddenly find that people cannot check out as their credit cards will not process (we are seeing this happening a lot in Q1-Q2 2015)
This will happen to any site where the SSL is running SSL 3.0 or TLS 1.0.
How to know of your site is affected?
Contact our Support Desk on 1-855-572-9790 or Email Us