tl;dr they had hardcoded their Elevenlabs admin keys into their client, which allowed anyone to access the entire history of TTS responses for every Rabbit R1 user
They didn't hardcode it into the client (source: I have personally RE'd their client code[0]), they hardcoded it into their server-side code, which certain security researchers claim[1] to have (had) access to.
https://rabbitu.de/articles/security-disclosure-1
tl;dr they had hardcoded their Elevenlabs admin keys into their client, which allowed anyone to access the entire history of TTS responses for every Rabbit R1 user