If you are getting 100s of unauthorized transactions in your Authorize.net account, it means your Authorize.net transaction key got stolen.
There are reports of this kind of hack is going on out there.
You need to do this:
1) Scan your computer for a malware. Looks like a hacker got access to your email and/or your store control panel.
The best maleware scan we found is this one:
http://download.cnet.com/Malwarebytes-Anti-Malware/
Download a free version, do a full scan and let it clean your computer.
2) Ask your host to scan your website for suspicious files. If hackers got your FTP login and uploaded a backdoor, they will be able to get any information from your database without having to login to store’s control panel
3) Change your email account passwords
4) Change all administrator passwords in the store ( in Users -> User search ) and remove administrators you do not need.
5) Login to your Authorize.net account, change login password there and generate new API key.
6) Copy that new key and paste it in Settings -> Payment methods -> AuthorizeNet – SIM -> Configure.
7) Add your store IP address as the only allowed source of transactions in your Authorize.net account. This will stop any unauthorized transactions being posted from somewhere else even if hacker get your transaction key.
8) We can setup an additional level of authentication for your control panel in the store but it will not help if you have a keystroke logger spyware on your computer . Hacker will get that password right away so you need to clean your computer first. Contact your host after you did that and ask to add folder authentication to /admin and /provider folders.
9) If you need some third party contractor to work in your store, create a new separate administrator login for them and disable it after work is done. Too many stores have temporary logins with unsecured passwords (ex: temp123) and full access to the store enabled which is an invitation for hackers to do a lot of damage.
