this post was submitted on 13 Mar 2025
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You shouldn't install software from someone you don't trust anyway because even if the installation process is save, the software itself can do whatever it has permission to.
"So if you trust their software, why not their install script?" you might ask. Well, it is detectable on server side, if you download the script or pipe it into a shell. So even if the vendor it trustworthy, there could be a malicious middle man, that gives you the original and harmless script, when you download it, and serves you a malicious one when you pipe it into your shell.
And I think this is not obvious and very scary.
I presume you mean if you download the script in a browser, vs using curl to retrieve it, where presumably you are piping it to a shell. Because yeah, the user agent is going to reveal which tool downloaded it, of course. You can use curl to simply retrieve the file without executing it though.
Or are you suggesting that curl makes something different in its request to the server for the file, depending on whether it is saving the file to disk vs streaming it to a pipe?
It is actually a passive detection based of the timing of the chunk requests. Because curl by default will only request new chunks when the buffer is freed by the shell executing the given commands. This then can be used to detect that someone is not merely downloading but simultaneously executing it. Here's a writeup about it:
https://web.archive.org/web/20250209133823/https://www.idontplaydarts.com/2016/04/detecting-curl-pipe-bash-server-side/
You can also find some proof-of-concept implementations online to try it out yourself.
Wow, thanks for this. That is very helpful context. And thanks for your original post too, or I'd never have asked.
Oh, you're welcome, kind person :)
Irrelevant. This is just an excuse people use to try and win the argument after it is pointed out to them that there's actually no security issue with
curl | bash.It's waaaay easier to hide malicious code in a binary than it is in a Bash script.
You can still see the "hidden" shell script that is served for Bash - just pipe it through
teeand then into Bash.Can anyone even find one single instance of that trick ever actually being used in the wild (not as a demo)?
I never tried to win any argument. Hell I was not even aware that I'm participating in one. I just wanted to share the info, that even if the vendor is absolutely trustworthy and even if you validated the script by downloading and looking at it, there's still another hole that's not obvious to see.
Yes it's unlikely, but again, I never said it were. There are also arguments you can run curl with, to tell it to do the download first and then push it through the pipe afterwards, though I don't know them by heart now.
It won't cost you anything to set those parameters, when you insist to use curl | bash, just in the off chance that someone's trying to do what I mentioned.
But I'm also someone who usually validates their downloads with a checksum so maybe I'm just weird. Who knows.