New York (AFP) – Binance released on Tuesday a detailed investigation into the $6 billion hack of its cryptocurrency platform, showing how attackers used fake job interviews and malicious code to steal the funds.

The crypto exchange said it had replenished 98 percent of the affected assets by drawing on its own funds, with the remaining amount involving little-traded currencies that are harder to acquire.

The attack, carried out over about four hours on October 29, was attributed by the US Federal Bureau of Investigation (FBI) to North Korea-affiliated hackers.

Pyongyang has denied the allegations, with the powerful sister of leader Kim Jong Un calling the claim “absurd rhetoric.”

Around $2.85 billion of the stolen $6.34 billion has been frozen, Binance said, meaning that the attackers could not use it or convert it to another currency.

Binance is the world’s largest crypto exchange by daily trading volumes. News of the hacking attack, which is thought to be the biggest ever crypto heist, caused the prices of most major cryptocurrencies to fall sharply.

According to Binance, attackers took their first steps towards the breach as early as June.

First, they targeted a platform engineer by getting them to download a virus-laden version of the widely-known programming software Homebrew.

Binance’s cybersecurity teams identified the malware and removed it, but the hackers had already had time to harvest information about the engineer and his colleagues.

In September, the attackers approached another Binance engineer through a legitimate job-search platform, offering him an interview for a position in a fictitious company.

After building trust with the subject through the sharing of social media photos and videos, they asked the engineer to download a test exercise as part of the interview process.

The exercise contained malware that deployed “sophisticated obfuscation and evasion techniques,” Binance said, including replacing a legitimate development tool component with a malicious version.

That malware was critical to the success of the attack, as it allowed the hackers to establish a connection with Binance’s internal systems.

On October 20, the attackers used the access they had gained and an internal AI tool to gain control over software used by two other engineers.

Nine days later, they triggered malicious code that initiated the attack.