A source of side-loaded software is determined. An action may be performed in response to the determination of the source. In one case, the handling of an application on a mobile device may be based on whether the source of the application is trusted or untrusted. If a software application being newly-installed on a mobile device of a user is determined to be untrusted, installation or execution is blocked. In one approach, the determination of the source includes: determining whether a first source identifier of a first application matches a white list of source identifiers or a black list of source identifiers; and sending the first source identifier, a first application identifier, and a signature of authorship for the first application to a different computing device.