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Here is the relevant portion of the text from page 13: (I believe this is fair use)

Trojan horses
While we are talking about obfuscation, it is worth talking about a very insidious way of including executable code within strings. Normally, when Perl sees a string such as "$a", it does variable interpolation. But you now know that "a" can be replaced by a block as long as it returns a reference to a scalar, so something like this is perfectly acceptable, even within a string:
print "${foo()}";
Replace foo() by system ('/bin/rm *') and you have an unpleasant Trojan Horse.
print "${system('/bin/rm *')}"
Perl treats it like any other function and trusts system to return a reference to a scalar. The parameters given to system do their damage before Perl has a chance to figure out that system doesn't return a scalar reference.

Moral of the story: Be very careful of strings that you get from untrusted sources. Use the taint-mode option (invoke Perl as perl -T) or the Safe module that comes with the Perl distribution. Please see the Perl documentation for taing checking, and see the index for som pointers to the Safe module.

At the very best this is correct but unclear... at the worst it is just plain wrong. The above implies (but doesn't exactly state) that this would wreak havoc on your machine:
$a = q|{system('/bin/rm -rf *')}|; print "$a";
but, as has been stated above, the output of this snippet is simply the value of $a.... no external commands are executed.

Unless I'm overlooking something, this looks like an Errata to me.

-Blake


In reply to Re: Trojan Horse? (taint mode) by blakem
in thread Trojan Horse? (taint mode) by IraTarball

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