When I launched Diablo III, I didn’t expect the pornography I had been looking at hours previously to be splashed on the screen. But that’s exactly what replaced the black loading screen. Like a scene from hollywood, the game temporarily froze as it launched, preventing any attempt to clear the screen. The game unfroze just before clearing the screen, and I was able to grab a screenshot (censored with bright red):

Even though this happened hours later, the contents of the incognito window were perfectly preserved.

So how did this happen? A bug in Nvidia’s GPU drivers. GPU memory is not erased before giving it to an application. This allows the contents of one application to leak into another. When the Chrome incognito window was closed, it’s framebuffer was added to the pool of free GPU memory, but it was not erased. When Diablo requested a framebuffer of it’s own, Nvidia offered up the one previously used by Chrome. Since it wasn’t erased, it still contained the previous contents. Since Diablo doesn’t clear the buffer itself (as it should), the old incognito window was put on the screen again.

In the interest of reproducing the bug, I wrote a program to scan GPU memory for non-zero pixels. It was able to reproduce a reddit page I had closed on another user account a few minutes ago, pixel perfect:

Of course, it doesn’t always work perfectly, sometimes the images are rearranged. I think it has something to do with the page size of memory on a GPU:

This is a serious problem. It breaks the operating system’s user boundaries by allowing non-root users to spy on each other. Additionally, it doesn’t need to be specifically exploited to harm users – it can happen purely by accident. Anyone using a shared computer could be exposing anything displayed on their screen to other users of the computer.

It’s a fairly easy bug to fix. A patch to the GPU drivers could ensure that buffers are always erased before giving them to the application. It’s what an operating system does with the CPU RAM, and it makes sense to use the same rules with a GPU. Additionally, Google Chrome could erase their GPU resources before quitting

I submitted this bug to both Nvidia and Google two years ago. Nvidia acknowledged the problem, but as of January 2016 it has not been fixed. Google marked the bug as won’t fix because google chrome incognito mode is apparently not designed to protect you against other users on the same computer (despite nearly everyone using it for that exact purpose).