The kid, from Rhondda Cynon Taff, who can’t be named, was captured upon the arrival of a Justin Bieber show at the Principality Stadium on 30 June.
The 17-year-old has been discovered blameworthy of five dread related charges at Birmingham Crown Court.
The trial heard he was found with a “suffering letter” when captured, which uncovered subtle elements of his arranged assault.
The court heard the A-level understudy, who is white and British, had set up an Instagram account empowering jihad and supporting al-Qaeda. Police found the secret word for the record was “truck assault”.
One post read: “May Allah expedite fear mongering to Cardiff 30th June.”
Police likewise discovered he had directed various online inquiries into how to complete a vehicle slamming assault and how to wound and slaughter.
Different targets he looked into included Cardiff Castle, the New Theater, the Capitol strip mall, the Central Library and Bridgend’s McArthur Glen shopping outlet.
His web history included scans for “Isis executing video”, “how to make a fear assault” and “what does getting shot feel like”.
The court heard he likewise looked into the murder of Fusilier Lee Rigby and data on the most proficient method to take an auto. His cell phone contained pictures of the truck assaults in Nice and Berlin.
Whenever captured, the kid was found with a gutting blade and a mallet in his school rucksack, and the “affliction letter” said he was a “fighter of the Islamic state” and “more assaults will take after”.
He had likewise told police in a meeting he had been conversing with somebody called “Al Baghdadi” on the web, who had advised the kid he would go to damnation as he didn’t trust in Islam.
Be that as it may, his barrier advodate Delroy Henry contended he was not arranging an assault but rather had a “dumb enthusiasm for the violent”.
The kid advised the jury he needed to perceive that it was so natural to look into fear related themes on the web.
The kid was discovered liable of one charge of participating in the readiness of a psychological oppressor act, two charges of empowering fear mongering, and a further two charges of having psychological militant data.
Condemning will occur on 10 January.
Sue Hemming, from the Crown Prosecution Service, stated: “This present young person’s conduct over numerous months leaves almost certainly that he proposed to kill and debilitate whatever number individuals as could be expected under the circumstances in an assault reminiscent of the occurrence on Westminster Bridge.
“He was additionally posting fanatic substance online that could have urged others to confer fear based oppressor acts and downloading guidelines on the most proficient method to complete ‘solitary wolf’ assaults.
“He will now appropriately confront the possibility of a generous jail sentence.”
Det Supt Lee Porter, of Wales Extremism Counter Terrorism Unit, said the examination “averted facilitate offenses being submitted” and cautioned the general population to be “careful”.
He included: “This case has featured the progressing worries with youngsters accessing fanatic material on the web and how rapidly that can prompt radicalisation.”