GPUs effectively have general-purpose compute now and they can DMA bytes into and out of system memory. At a basic level, sneaky on-GPU code run by an attacker could grab pixels off your screen (text from your emails, etc), but it's also possible for the GPU-based attack code to harvest data from system memory or even mess with data in order to attack a process running on the CPU.

Graphics APIs are defined and implemented in memory-unsafe languages on the CPU side, C/C++, and will read from and write to memory based on data and pointers you pass to them without any provisions for untrusted data. -> instant pwnage

(Not to say GPU code execution is safe, but you wouldn't need it to ruin the users day)

First of all, your code passes through the driver, so you can exploit vulnerabilities there.

Even after that, the GPU can do DMA requests, so you can abuse that to read/write arbitrary memory locations.

ok makes sense. didn't know it could access arbitrary memory