According to the plea agreement, Jeffrey Lee Parson admitted that he created his worm - known as the B or teekids variant of Blaster - last August by modifying the original worm and adding a mechanism that allowed him to access infected computers, authorities said.
Government attorneys estimate that more than 48,000 computers were infected by Parson's worm, but his attorneys dispute the figure.
"Sending out a computer worm may be viewed as a harmless prank but the damage to individual comptuer users is very real and the penalties are very real," John McKay, U.S. attorney for western Washington, said in a statement.
Parson is looking at 18 to 36 months in prison, he added. Government attorneys also have asked for millions of dollars in restitution.
Sentencing is scheduled for Nov. 12.