How hard is it to open a file?

The biggest security issue found was a complete sandbox escape, originating from the fact that flatpak run, the command-line tool to start a Flatpak application, accepted path strings, since flatpak run is assumed to be run by a trusted user. The problem lay in a D-Bus service sandboxed applications could use to create subsandboxes, and this service was built around, you guessed it, flatpak run. The issues in question, including this complete sandbox escape, have been addressed and fixed, but they highlight exactly the dangers that can come from opening files. This subsandboxing approach in Flatpak is built on assumptions from fifteen years ago, and times have changed since then. If you’re a programmer who deals with opening files, you might want to take a look at your own code to see if similar issues exist.