In addition to own new code, bundled copies of libraries in packages introduces net new attack surface which isn't patched via the regular distribution security patch process. The image decoding lib that allows remote code execution now exists in flatpaks independently from the one in /lib. Every flatpak vendor that contains it has to build and ship their own patched version of it. This is even more valid for any other libraries flatpaks include that don't exist on the system. The most widely used Linux OSes come with security patching processes, expectations and sometimes guarantees. This new attack surface breaks those and the solution is security sandboxing. This approach has been proven in mobile app packaging and distribution systems. Android is a great example where apps are not trusted by default and vulnerable ones rarely cause collateral damage on otherwise up-to-date Android systems. This is an objective problem with the out-of-band distribution model allowed by flatpak and snap or any similar system, whether you care about it or not personally. It's a well understood tradeoff in software development. It has to be addressed as adoption grows or we risk reducing Linux security to the levels of Windows where apps regularly bundle dependencies with no sandboxing whatsoever.