"People fiercely compete to prove their loyalty," Jang Jin-sung, who allegedly used to work in a North Korean propaganda unit, said, as quoted by the newspaper. The former official explained that hackers must leave evidence of a successful attack to receive a promotion.
An expert from South Korea, engaged in cyberwarfare work in Seoul, agreed that the trace was left deliberately, saying that concealing one's tracks is an easy task for the hackers.
In November 2014, Sony Pictures Entertainment was attacked by a group of hackers, self-named the "Guardians of Peace." As a result of the massive attack, over 11 terabytes of corporate information were stolen, including employees' passwords, salary details, emails and other sensitive information.
The United States blamed North Korea for the hack, saying it was in retaliation for Sony's plans to release a comedy film called "The Interview," depicting an assassination attempt on North Korean leader Kim Jong Un.