I did some digging and the hacker posted which exploit he used. Apparently some boards allowed uploading PDF files, but the site never checked if the PDF file was an actual PDF file. Once a PDF file was uploaded it was passed to a version of Ghostscript from 2012 which would generate a thumbnail. So the attacker found an exploit where uploading a PDF with the right PostScript commands could give the attacker shell ac…
Reminds me of how people were crashing the PSP's XMB with BMP and TIFF files twenty years ago. I was just a kid, and began "pirating" every one of my classmates' consoles (some in exchange for a small amount of money). Good times.
This was a user-empowering application of the vulnerability. Obviously, a bug that allows root-level arbitrary code execution just by getting the user to load a single image could be used for some pretty bad stuff. (And perhaps was.)