Microsoft introduced the Background Intelligent Transfer Service (BITS) with Windows XP to simplify and coordinate downloading and uploading large files. Applications and system components, most notably Windows Update, use BITS to deliver operating system and application updates so they can be downloaded with minimal user disruption.
Applications interact with the Background Intelligent Transfer Service by creating jobs with one or more files to download or upload. The BITS service runs in a service host process and can schedule transfers to occur at any time. Job, file, and state information is stored in a local database.
How Attackers Use BITS
As is the case with many technologies, BITS can be used both by legitimate applications and by attackers. When malicious applications create BITS jobs, files are downloaded or uploaded in the context of the service host process. This can be useful for evading firewalls that may block malicious or unknown processes, and it helps to obscure which application requested the transfer. BITS transfers can also be scheduled allowing them to occur at specific times without relying on long-running processes or the task scheduler.