Michael Catanzaro has disclosed a command-injection vulnerability affecting a number of GTK-based PDF readers; exploits included: They contain a script for building malicious polyglot PDFs that are simultaneously both valid PDF files and also valid ELF binaries. When the user opens the PDF in the PDF viewer and clicks on a malicious link embedded in the PDF, the PDF abuses the command injection vulnerability to load itself as a GTK module using the `--gtk-module` command line flag. It can then execute arbitrary code via its library constructor. That flag was removed in GTK 4, which is why the vulnerability is much less serious for Papers than it is for Evince, Atril, and Xreader.