This is quite a general question, you may want to make it more specific - there's always a huge risk of human error when it comes to security for example (people downloading content to a USB stick or sending it in an e-mail, etc) ... but in terms of security of the data held on the repository you may want to review this page of the documentation which describes our support for "encryption at rest"
How are you planning to host Alfresco? Will it be on an intranet behind a firewall or are you making it accessible to the broader internet. Have you read any of the available documentation on security? There's more useful information here.
