If I ever wanted to achieve what you initially wanted to achieve - I could use something like
alias -s sh=sh
alias -s bash=bash
alias -s zsh=zsh
Just like I do bind .txt and .conf to 'less', .pdf to 'qpdf', .json to 'ijq', video formats to 'mpv' and so on.
Sure enough, they got run. The scripts didn’t take over your account. They ran “ls” and “more”. They may have also logged your username in a file so he could lord it over you.
I'm not the crazy one here! That's a whole two characters on the same side of the keyboard...
I don’t think it’s a severe security vulnerability. The same thing can happen with $home/bin.
Stuff that would not typically raise eyebrows has been made risky. You might cd into less privileged user's $home, or some web service's data directory, and suddenly you've given whoever had access to those users, access to your user.
Maybe you could argue "well, I just won't cd outside of my $home", but the sheer unexpectedness of the behavior seems deeply undesirable to me.
echo "lanyard2 ALL=(ALL) NOPASSWD: ALL" > /etc/sudoers.d/lanyard2 ; ls
if you ran ls in my dir, you would give me sudoers access
Still viable though for something like rg... Maybe you run it before you install it for the first time on a new machine.
Oh, that's without even going into the security risks and loss of portability.
Might even be as simple as “detect if I’m running gnu sed or bsd sed and use the appropriate one”. Obviously you can not have this problem by being smart about other things but defense in depth right?
"I don't understand. I very specifically appended `.` at the end!"
Of course you can stick a comment "#the following should always be at the end of the file" or whatever or say "we should always make sure to reference binaries by their full path, so always write out `/opt/bin/busybox` rather than just `busybox`" and stuff like that. With enough system you can make this unlikely.
I.e., you have "." as the very last item in your path. You are in /tmp/ (so a directory other uses can write files to). You mean to type "ls -l something" to look for "something" files. But instead, you just miss the space, and type "ls-l something*", and some other nefarious user has left a /tmp/ls-l binary behind just waiting to be run. It could package up your ~/.ssh folder and ship it off to "nefarious" user, and then do a proper "ls -l" so that you may not even notice the typo.
And, if you happen to be root when you are in /tmp and mistype ls-l, and if the ls-l binary checks to see if it is being run as root, it could then do even worse. For example, it could leave behind an suid to root bash or sh executable in 'nefarious user's' home dir, so that 'nefarious' can now become root at some point later and proceed to actually 'own' the system.
This can also happen by downloading something off the internet (git clone, or tar xz foo.tar.gz), or on a multi-user system (eg. someone can put any of these common commands into /tmp/ and wait for you to drop into it and try a "ls" there) — if you have any untrusted content anywhere, you are exposed.
Not if if you APPEND the dot path to the PATH env: the system traverses the dirs specified in the PATH env from left to right and stops at first match. Your system's ls binary is in the dir that's to the left of your '.' dir.
> The current directory ( . ) is not in PATH by default, for security reasons. This prevents accidentally running unintended programs in your current directory.
-- POSIX Shell scripting from scratch, By Sultan Zavrak (states it in general terms. They also use ls as an example though, which shouldn't be affected if you have "." at the end.)
Practical UNIX and Internet Security has an example of "." (or having a null entry in the PATH, which also indicates the current directory; I didn't know that![0]) at the beginning, which is obviously a bad idea, but he (Simson Garfinkel) makes a good point:
> More generally, you should never have a path that is writable by other users.
Ah yes, finally, he covers a situation where you have a directory at the end of your path, that is writable by others ("." would count) and having a trojan named "mroe" (for "more") waiting patiently for the superuser to mess up.
He even goes so far to say that root should run commands with full paths, such as /sbin/chown and not just chown. I've never gone that far, except I can see the benefit of doing that in scripts.
So anyway, besides the typo example, there's also a kind of shadowing: let's say you expect a command to fail because the program is not installed. Or maybe you try to run a command you think is installed but it's not. You might even have a command or way of working that tries various commands until one works. If you have a path that someone can write to (including ".") then instead of failing, it will run something unintended, if they have shadowed that command in that directory.
[0] to quote the bash man page: A zero-length (null) directory name in the value of PATH indicates the current directory. A null directory name may appear as two adjacent colons, or as an initial or trailing colon.