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Comment by daeken | Hacker News Reader

> Are there ready examples of what makes the usage of Perl a security liability?

The `open` function used on untrusted input allows arbitrary code execution (I've gotten privilege execution via setuid perl scripts many times this way, as well as getting a shell on the box via web apps allowing this).

While there are many other common things I saw in real world apps, e.g. perl scripts using backticks for command execution and allowing me to run anything I wanted, the `open` one is by far the worst. It's insanely pervasive and incredibly easy to do.

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The `open` function used on untrusted input allows arbitrary code execution...

... only if you use the insecure open form. The secure open form has been available and recommended since the release of Perl 5.6.0 in March 2000--twelve years ago.

People who write insecure code, when the language makes it just as easy to write secure code, are to blame for insecure code.

> People who write insecure code, when the language makes it just as easy to write secure code, are to blame for insecure code

We're not discussing who's to blame, we're discussing whether there's anything to assign blame for.

Can you name a practical language in which it's not possible, by default, to perform an unsafe operation with untrusted user input?

I can easily use Haskell's type system to disallow the use of UnsafeUserInput in my database abstraction layer, but that requires me to use my types pervasively and correctly.

The question is not whether it is POSSIBLE.

If your web application is opening and closing files based on user input, without checking them first, you have bigger issues. Blaming this on the language seems bizarre.

True but perl makes it amazingly easy to exploit.

Suppose that you know the source contains this line:

  open(file_handler, "$userinput");

How much work and what assumptions do you need to exploit this? In perl it's that easy:

  $userinput = "cat /etc/passwd |zenity --text-info |";
  open(file_handler, "$userinput");

Do you usually try $UserInput $Userinput $userInput $user_input etc. when you do that?

It couldn't run your code if it had an uninitialized variable in it, right?

It's vulnerable because the user controls the content of $userinput, the variable name doesn't matter. Real code would look like this:

  #!/usr/bin/perl

  print "Enter filename: ";
  $filename = <STDIN>;
  open(file_handler, "$filename");

If you see this code, you know that you can execute any command by giving it at the filename prompt and appending a '|' character. For example giving cat /etc/passwd |zenity --text-info | will popup a dialog with the content of /etc/passwd.

Oh I see, I thought you were talking about something that would be caught by the interpreter if you put

    use strict;

and

    use warnings;

in there. Thank you for taking the time to put in an example, I understand you better now.

> Suppose that you know the source contains this line

Then you have a massive damn problem, and you could set up the same strawman about almost any language - if you're starting from the premise that you are piping arbitrary user input to a system call unimpeded, all bets are off.

Shall we try some other examples? Let's say you accept input from a user, and dump it unchecked in to output shown to the user via Ruby. OH NOES! Ruby is insecure because it supports CSRF! Or, let's say you use Python and pass in input from a user unimpeded straight to the database without using the quoting mechanisms! OH NOES! Python is insecure because it enables SQL injections! etc etc etc

But wait! you say... Sensible programming languages have specific features to stop these kinds of attacks! And you're right. That's why Perl has taint mode... for when you're dumb enough to pass user input straight to open: http://perldoc.perl.org/perlsec.html#Taint-mode

I'm in no secret quest to stain Perl's reputation you know... Just illustrating how easy it is to exploit unchecked user input in the open() function. It just happens that this particular exploit is way easier in Perl than any other language I know.

Of course we should not extrapolate from this to make a judgment on the language. Of course it's nice that security mechanisms are available to alleviate this issue.

Those sound like two good leads, thank you!

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