Grand Theft Auto 6 leak: who hacked Rockstar and what was stolen?

A major data breach has given the world an early look at Grand Theft Auto 6. Why is this such bad news for the developer?

In the early hours of 18 September, a poster on GTAForums going by the name teapotuberhacker posted about 90 videos, totalling 50 minutes of footage from an in-development version of forthcoming video game Grand Theft Auto 6, from Rockstar Games. The footage has since proliferated around social media and the wider internet. Shortly after the initial announcement, the hacker left a message on the forum claiming they wanted to negotiate a deal" with Rockstar for the return of unreleased data - including the source code for Grand Theft Auto 5 and the in-development version of Grand Theft Auto 6.