The German computer expert Sven Jaschan, age 19, could face up to five years' imprisonment, but is expected to escape with a fine because he was a minor when he committed the offences.
Mr Jaschan, who is being tried behind closed doors in Verden because of his age at the time of the offence, "admitted to the alleged offences in every detail," the court spokeswoman Katharina Kruetzfeld said.
The worm raced around the world, causing computers to crash and reboot, making it impossible to work on them.
In Britain, staff at the Maritime and Coastguard Agency were forced to return to manual map reading because computer systems were made unusable. Check-in for some British Airways flights was delayed.
In Australia, trains stopped running and in Taiwan, more than 400 branches of the post office were forced to use pen and paper because Sasser crashed desktop computers.
Authorities say Mr Jaschan had already confessed to creating the worm at the time of his arrest in May 2004, and to modifying some versions of the Netsky worm. Mr Jaschan now works for a German security software company called Securepoint.
A verdict was expected tomorrow.Reuse content