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Clarify on eval usage

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felixge committed Oct 8, 2012
1 parent 4680b21 commit 18eaffc07cd8c9d440627a8ca301c5b9abb4ecaf
Showing with 7 additions and 1 deletion.
  1. +7 −1
@@ -234,7 +234,7 @@ solving the same problem as me:
* big switch statements = bad
* function calls are *really* cheap
* buffering / concatenating buffers is ok
-* eval is awesome
+* eval is awesome (when using its twin `new Function()`, `eval()` itself sucks)
Here is the eval example:
@@ -261,6 +261,12 @@ code += '};\n';
var parseRow = new Function('columns', 'parser', code);
+This optimization turned out to be a huge success, and it's what allowed my
+new parser to gain another 20% of performance after being very fast already.
+Of course this could turn into a security problem, but that can be easily fixed
+by escaping the `` properly.
### Data analysis
The next thing you should do is analyze your data. And for the love of god,

5 comments on commit 18eaffc

Maybe I'm missing something, but if you are going to eval (or use new Function, which is essentially the same, it parses arbitrary code) on data from an untrusted source, it doesn't matter what has been escaped?

It's not about SQL exploits, it's about parsing whatever text you add to that 'code' variable, which might contain malicious javascript.


felixge replied Oct 8, 2012

eval() gives the executed code access to the current scope, new Function does not. This has additional security and performance implications.

Ok, I realise you don't get access to the current scope, but surely an attacker could send this:

while (true) {}

And a more malicious attacker could probably use child_process.spawn() to open up a reverse shell back to their machine.


felixge replied Oct 9, 2012

Ok, I realise you don't get access to the current scope, but surely an attacker could send this:

Please read lines 267 - 268 in the patch above.

I understand now. Although I still think it's probably best to steer clear of things like this where there is any possibility of getting code executed. Thanks for clearing up the issue.

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