Assuming you’re talking about Nix the package manager,

Nothing is actually “installed as root”, except for stuff that the root user installs themselves. The Nix Store (where all the “derivations”/packages are stored) can be thought of more as a cache, just because a package is there doesn’t mean it’s used anywhere. Users don’t get to choose the cache “key” (i.e. the directory name in /nix/store) either - it is determined by all the build instructions and dependencies needed to build the package, and Nix doesn’t (well, at the very least shouldn’t) give users any control about the package build process after it starts.

When users install something, Nix fetches or builds that package into /nix/store - which doesn’t affect other users in any way - and then just symlinks that particular package into some user-owned directory in their $PATH (e.g. $HOME/.local/share/nix/profiles/default or so), which also doesn’t affect root or other users in any way.

So, basically, if some user installs malware only they are affected - the fact that it’s also in the shared /nix/store is irrelevant since there’s nothing in other user’s profiles or $PATH or whatever that references it in any way.

The most likely vulnerability is something like this:

1. The attacker guesses the nix store path of some package that root will use in the future (e.g. glibc from a more up-to-date Nixpkgs version compared to what root currently uses) - note that users can’t replace a derivation that’s already in the store, so they need to guess a future derivation path
2. The attacker forces Nix to build that package from source instead of fetching from a trusted substituter
3. The attacker finds a way to breach Nix’s sandbox during the build and inject their own backdoor into the resulting package ← this is the difficult part, there are currently no such known sandbox holes
4. The attacker then waits until root starts using that package in the store, at which point the backdoor becomes actively installed on the system.

I must add that this is theoretical and I don’t think has ever happened in practice on a multi-user system.

Sorry, I’m pretty bad at explaining stuff, hopefully it makes some sense :)

If you mean Nix, the package manager, then no packages are actually installed as root. Every package is installed in the Nix store, but programs that need root privileges can’t be run by any user.

One of the (many) great things about Nix is that users don’t need root privileges to install packages.

Do you want explain a bit more?

Linux is Linux. If you’re using the standard install packages like Pam, then it’s built specifically for multiple users.

Elaborate more if you could.

However, the packages are installed as root. As such I’m a bit wary of whether a user installing something could have the system run malware as root by installing a package.

I’m pretty sure the packages are built as unprivileged users in a sandbox, and you can also restrict what users can use certain Nix features.

The main security flaw with a multi-user Nix system is that everything you build ends up in a world-readable store.

Are you talking about NixOS?

You can install post user nix home manager distributions which are separate from the shared nix environment (not sure if that’s the right term)