Magento have released a new security notice regarding stores detected with Malware. This Malware is capable of stealing customers credit card information.
Whilst it is not clear how these exploits are being executed, it is strongly thought that unauthorised access to the admin area was gained.
To protect your admin area from unauthorised access, the following suggestions are a good place to start:
If you follow the steps above you will greatly reduce your chances of becoming a victim of malware.