A data-sending Trojan is a kind of Trojan virus that relays sensitive information back to its owner. This type of Trojan can be used to retrieve sensitive data, including credit card information, email addresses, passwords, instant messaging contact lists, log files and so on.

Some data-sending Trojans are not used for malicious acts, but they still relay information. Usually, this is to serve ads to the user. Hackers use data-sending Trojans to gain data about user activity on the Web, such as the sites visited and the ads or other icons clicked. Using this information, the Trojans serve the relevant ads to the user.