7 ms·

I don't get it, it's not really unexpected if you pass the name of the executable (which just happens to be a script) via a flag... used explicitly for passing

• ushi 11y ago
  I agree. Every single example in this article makes perfect sense to me.
• strictfp 11y ago
  I think he wants to bring to peoples attention that if you give someone sudo rights for a specific program chances are that they will be able to elevate their privilegies.
• reubenmorais 11y ago
  I guess the author's point is don't ever, ever, ever mix shells and unsanitized user inputs, even if you think it can't possibly be harmful. In other words, even if it's not obvious, doing this creates a security vulnerability: tar bla bla ${user input}
  • krylon 11y ago
    Perl has a special "taint" mode (Ruby does, too, I think, but I don't know about the differences or similarties), where any data coming from outside the program - environment variables, network connections, files, etc. - is considered "tainted", i.e. treated as bad, until you validate it using regular expressions (or something along those lines). I have to admit I never tried it, but it sounds like a good idea, assuming it works as advertised, when dealing with potentially malicious input or paranoia.
    • JonnieCache 11y ago
      Rails has ActiveSupport::SafeBuffer for doing XSS protection, but ruby doesn't track external strings as dirty like that.
      • flgr 11y ago
        > but ruby doesn't track external strings as dirty like that. It does: irb(main):001:0> gets.tainted? foo => true You can even set $SAFE to 1 or higher to disallow the use of tainted strings in potentially dangerous commands: freya:~ flgr$ ruby -e '$SAFE = 1; system "tar bla bla #{gets}"' foo -e:1:in `system': Insecure operation: -r (SecurityError) from -e:1:in `<main>' See http://phrogz.net/programmingruby/taint.html http://phrogz.net/programmingruby/taint.html for more information.
      • vidarh 11y ago
        > but ruby doesn't track external strings as dirty like that. Yes, it does. E.g: x = STDIN.gets puts x.tainted? y = "hello "+x puts y.tainted? z = "foo" puts z.tainted? The two first will return "true", and the last will print "false". If you set the safe-level appropriately some methods will be disallowed for x and y above (e.g. eval()), though I would not trust that as comprehensive, but it's a lot better than nothing.
  • andrey-p 11y ago
    But if you're plugging in input like that, couldn't the malicious user just pass in something like && sudo rm -rf / anyway? Which would render the whole point of the article moot.
    • andrewaylett 11y ago
      Not every command gets run through the shell, and not using the shell was one of the prerequisites in the article.
    • ninkendo 11y ago
      There's simple ways to "escape" user input (as in, ensure the whole input string is interpreted as a single argument to this program) in ways that ensure you can't do simple &&'s or ;'s and execute a totally different command. But the point of the article is even if it's properly escaped, users can still do malicious things when input is passed to lots of standard UNIX utilities.
      • Gibbon1 11y ago
        What I wish, I wish there was a flag in unicode to declare characters as 'unsafe user input' so that system utilizes and databases can recognize unsafe user input and barf on it.
        • jacquesm 11y ago
          It would be a very rare case where a vulnerability related to in-band signalling can be fixed with more in-band signalling.
          • Gibbon1 11y ago
            You can if your encoding is explicit about what's in band and out of band. I get the feeling you've never written a protocol ever.
            • 11y ago
  • enno 11y ago
    This is a point that should have been made in the author's article. As it is, it presents a poorly defined problem without offering solutions (other than "be afraid").
• Retr0spectrum 11y ago
  It isn't much different from something like echo `somescript.sh`
• marcosdumay 11y ago
  Yes, it's unexpected and less commands should do that. Did you read the manpage of every utility on your computers to know that they won't execute a user supplied program? If you didn't you are up to nasty surprises. That $HOME trick alone would give privilege escalation to anybody that gets a user-level access to a server of mine. Gota change it.
  • tokenizerrr 11y ago
    How so? I might be missing something but as far as I can tell it's just executing the runme.sh as the user who ran the bash command. It would be virtually identical to ./runme.sh
• subway 11y ago
  I've seen a lot of novice sysadmins do silly things like rely on sudo's command restrictions, expecting this to actually contain a user. For those sysadmins, yeah, it's unexpected.
  • digi_owl 11y ago
    All security is a matter of degree, be it computing or physical. the most determined of attackers will always find a way in.
• 0x0 11y ago
  There's been a bit of attention recently on these things. The big issue is people using " * " on the command line without prefixing with an "--" argument or simply "./*". If someone has managed to sneak in particularly evil filenames (that look like -options), then the unsuspecting user may be in for a surprise, for example tar'ing up a public upload directory.
• yk 11y ago
  This way around is perfectly obvious. But assume that you are writing a website that prints a man page, for example by piping the output of man $URLPARAMETER to the user. In that case it is only obvious to check if man can execute arbitrary commands if you already know that man is not just a cat for man pages.