A German youth has admitted to creating the Sasser computer virus during the first day of his trial in Germany.
Sven Jaschan is charged with computer sabotage, disrupting public services and illegally altering data.
The 19-year-old is being tried as a minor behind closed doors as he was 17 when he wrote the worm.
Sasser wrought havoc in many companies when the Windows worm struck in May 2004, swamping net links and making computers unusable.
As the day's legal proceedings started, Mr Jaschan "admitted to the alleged offences in every detail," said a court spokeswoman.
Defendants under German law do not enter formal pleas so the trial continues despite Mr Jaschan's confession.
The trial in Verden in northwest Germany is expected to last three days, with a possible verdict on Thursday.
He is likely to escape the maximum sentence of five years in prison since he is being tried as a juvenile.
He now works for a German security software company called Securepoint.
A $250,000 bounty offered by Microsoft produced a tip-off that led German police to Sven Jaschan in May 2004.
The German authorities have called a public broadcaster and three German city councils as witnesses to describe the disruption that the worm caused.
In the UK, Sasser forced staff at the Maritime and Coastguard Agency to return to manual map reading because computer systems were made unusable by the worm.
Check-in for some British Airways flights was also delayed thanks to Sasser.
Around the world, the Australian Railcorp trains stopped running because computer problems caused by Sasser made it impossible for drivers to talk to signalmen.
In Taiwan, more than 400 branches of the post office were forced to use pen and paper because Sasser crashed desktop PCs.
Soon after being arrested in May 2004 at his home in Waffensen, Mr Jaschan reportedly confessed that he was the creator of Sasser and some versions of the Netsky virus.
Anti-virus firm Sophos estimates that 70% of all the virus infections in the first half of 2004 could be blamed on Mr Jaschan's creations.
Statistics gathered by Sophos show that in the first six months of 2005 there were four variants of Netsky in the top 10 viruses and they accounted for 25.5% of all infections.
Unlike many other viruses, Sasser made its way from computer to computer without help from users. It got into Windows computers by exploiting a programming bug in the operating system.
Although Microsoft had released a patch for this loophole on 13 and 28 April 2004, many companies had not applied this protection before Sasser struck.