One way "curl pipe sh" install scripts can be dangerous [proof of concept]

I have seen several sites recently that offer a one-line installation that looks something like this:

curl -s example.com/install | sh

While I appreciated the elegance, it set off warning bells. Despite this initial reaction I couldn't come up with a reason why it was any less secure than other installation methods. In fact, I read a very logical argument that this method was actually more secure from the highly respected Paul Irish:

Today I realized at least once case that could validate those initial warning bells. Take a look at this proof of concept:

curl -s https://jordaneldredge.com/projects/curl-proof/install/|sh

I'll wait while you paste the script URL into your browser and make sure it's not going to do anything naughty…

Now that you're sure the line is safe to run, paste it into your shell (I dare ya!). Or, if you are a big wimp, have it output to the console instead of sh like so:

curl -s https://jordaneldredge.com/projects/curl-proof/install/

As you can see Curl returned a different script than the one you saw in your browser. This is achieved by checking for Curl's user-agent and, when found, serving a different file.

Of course, the whole point is pretty much moot because the install script is probably installing lots of other code that you haven't reviewed, so you wouldn't be running this code unless you already trusted the author. However, be aware that you may be lulling yourself into a false sense of security because you appear to have the code right in front of you.