US prosecutors say a British computer expert has admitted to creating software that harvests bank details.
But Marcus Hutchins’ own lawyer says he denies six charges of creating and distributing the Kronos malware.
The 23-year-old from Ilfracombe, Devon, who helped stall the WannaCry cyber-attack which hit the NHS, was arrested on Wednesday in Las Vegas.
He was granted $30,000 (23,000) bail, but will spend the weekend in prison after not being able to pay on Friday.
As he left the courtroom Mr Hutchins was ordered to walk with his hands behind his back but he was not shackled.
No members of his family were present, but defence lawyer Adrian Lobo presented the judge with a bundle of letters.
She said they were from friends and relatives showing support for a client who had never been in trouble with the law in the US or the UK.
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Ms Lobo told the BBC: “He’s pled not guilty. He is standing by that and he fights the charges and we intend to fight the case in Wisconsin.”
She described the federal indictment against him as “pretty flimsy, it’s pretty slim compared to what we normally see in a United States indictment.”
Prosecutors told a Las Vegas court on Friday that Mr Hutchins had been caught in a sting operation when undercover officers bought the code.
They claimed the software was sold for $2,000 in digital currency in June 2015.
Dan Cowhig, prosecuting, also told the court that Mr Hutchins had made a confession during a police interview.
“He admitted he was the author of the code of Kronos malware and indicated he sold it,” said Mr Cowhig.
The lawyer claimed there was evidence of chat logs between Mr Hutchins and an unnamed co-defendant – who has yet to be arrested – where the security researcher complained of not receiving a fair share of the money.
At the scene
By James Cook, BBC North America correspondent
There was no missing Marcus Hutchins as he was brought into courtroom 3C of the US District Court in Las Vegas.
The “surfer who saved the world” was wearing a bright yellow custody-issue T-shirt and trousers along with luminous orange socks and sandals.
Judge Nancy Kobbe was sympathetic to the defendant’s plea to be released on bail, waving away a claim from a government lawyer that the cyber-security expert posed a risk to the public because he had gone shooting on a gun range popular with tourists.
Mr Hutchins was so softly spoken that several times Ms Kobbe had to ask him to raise his voice.
Ms Lobo said Mr Hutchins denied he was the author of the malware and said he would plead not guilty to all of the charges, which date between July 2014 and July 2015.
“He has dedicated his life to researching malware, not trying to harm people,” she said. “Use the internet for good is what he has done.
“He was completely shocked, this isn’t’ something he anticipated. He came here for a work-related conference and he was fully anticipating to go back home and had no reason to be fearful of coming or going from the United States.”
Mr Hutchins came to prominence in May this year after finding a “kill switch” to stop the WannaCry ransomeware attack that hit the NHS, as well as other organisations in 150 countries.
Also known as “MalwareTech” online, Mr Hutchins was hailed as an “accidental hero” after registering a domain name to track the spread of the virus, which actually ended up halting it.
Mr Hutchins, who works for Los Angeles-based computer security firm Kryptos Logi, had been in Las Vegas to attend the Black Hat and Def Con cyber-security conferences.
He was arrested at Las Vegas airport minutes before he was due to fly home.
District judge Nancy Koppe, who was presented letters of support from Mr Hutchins’ cyber-security colleagues, ordered his release on bail as he had no criminal history and because the allegations dated back two years.
However, friends and family were unable to raise the bond money before the court closed on Friday, so he will not be released until Monday.
The conditions of his bail include him not being allowed to access the internet and to stay in Clark County, Nevada, and within the Eastern District of Wisconsin, where he will appear in court on Tuesday.
He must also be monitored by GPS and surrender his passport.
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