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Mark Pilgrim
2009-09-17 21:33:50 -04:00
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@@ -528,7 +528,7 @@ for guess in itertools.permutations(digits, len(characters)):
<p class=xxxl>eval() is EVIL
<p>Well, the evil part is evaluating arbitrary expressions from untrusted sources. You should only use <code>eval()</code> on trusted input. Of course, the trick is figuring out what&#8217;s &#8220;trusted.&#8221; But here&#8217;s something I know for certain: you should <b>NOT</b> take this alphametics solver and put it on the internet as a fun little web service. Don&#8217;t make the mistake of thinking, &#8220;Gosh, the function does a lot of string manipulation before getting a string to evaluate; <em>I can&#8217;t imagine</em> how someone could exploit that.&#8221; Someone <b>WILL</b> figure out how to sneak nasty executable code past all that string manipulation (<a href=http://www.matasano.com/log/1032/this-new-vulnerability-dowds-inhuman-flash-exploit/>stranger things have happened</a>), and then you can kiss your server goodbye.
<p>Well, the evil part is evaluating arbitrary expressions from untrusted sources. You should only use <code>eval()</code> on trusted input. Of course, the trick is figuring out what&#8217;s &#8220;trusted.&#8221; But here&#8217;s something I know for certain: you should <b>NOT</b> take this alphametics solver and put it on the internet as a fun little web service. Don&#8217;t make the mistake of thinking, &#8220;Gosh, the function does a lot of string manipulation before getting a string to evaluate; <em>I can&#8217;t imagine</em> how someone could exploit that.&#8221; Someone <b>WILL</b> figure out how to sneak nasty executable code past all that string manipulation (<a href=http://www.securityfocus.com/blogs/746>stranger things have happened</a>), and then you can kiss your server goodbye.
<p>But surely there&#8217;s <em>some</em> way to evaluate expressions safely? To put <code>eval()</code> in a sandbox where it can&#8217;t access or harm the outside world? Well, yes and no.