> 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.

> 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 for more information.