Eval is Evil, Part Two

Eval is Evil, Part Two

  • Comments 6

As I promised, more information on why eval is evil.  (We once considered having T-shirts printed up that said "Eval is evil!" on one side and "Script happens!" on the other, but the PM's never managed to tear themselves away from their web browsing long enough to order them.)


Incidentally, a buddy of mine who is one of those senior web developer kinda guys back in Waterloo sent me an email yesterday saying "Hello, my name is Robert and I am an evalaholic".  People, it wasn't my intention to start a twelve step program, but hey, whatever works! 


As I discussed the other day, eval on the client is evil because it leads to sloppy, hard-to-debug-and-maintain programs that consume huge amounts of memory and run unnecessarily slowly even when performing simple tasks.  But like I said in my performance rant, if it's good enough, then hey, it's good enough.  Maybe you don't need to write maintainable, efficient code.  Seriously! Script is often used to write programs that are used a couple of times and then thrown away, so who cares if they're slow and inelegant?


But eval on the server is an entirely different beast.  First off, server scenarios are generally a lot more performance sensitive than client scenarios.  On a client, once your code runs faster than a human being can notice the lag, there's usually not much point in making it faster.  But  as I mentioned earlier, ASP goes to a lot of work to ensure that for a given page, the compiler only runs once. An eval defeats this optimization by making the compiler run every time the page runs! On a server, going from 25 ms to 40 ms to serve a page means going from 40 pages a second to 25 pages a second, and that can be expensive in real dollar terms. 


But that's not the most important reason to eschew eval on the server.  Any use of eval (or its VBScript cousins Eval, Execute and ExecuteGlobal) is a potentially enormous security hole. 



  var Processor_ProductList;

  var Software_ProductList;

  var HardDisk_ProductList;

  // ...

  CategoryName = Request.QueryString("category");

  ProductList = eval(CategoryName & "_ProductList");

  // ...


What's wrong with this picture?  The server assumes that the client is not hostile.  Is that a warranted assumption?  Probably not!  You know nothing about the client that sent the request.  Maybe your client page only sends strings like "Processor" and "HardDisk" to the server, but anyone can write their own web page that sends


((new ActiveXObject('Scripting.FileSystemObject')).DeleteFile('C:\*.*',true)); Processor


which will cause eval to evaluate


((new ActiveXObject('Scripting.FileSystemObject')).DeleteFile('C:\*.*',true)); Processor_ProductList


Obviously that's a pretty unsophisticated attack.  The attacker can put any code in there that they want, and it will run in the context of the server process.  Hopefully the server process is not a highly privileged one, but still, there's vast potential for massive harm here just by screwing up the logic on your server.


Never trust the input to a server, and try to never use eval on a server.  Eval injection makes SQL injection look tame!


To try and mitigate these sorts of problems, JScript .NET has some restrictions on its implementation of eval, but that's a topic for another entry.


  • Great information, but how about some advice on what to use as an alternative?  The point to Eval/Execute is to provide the ability to execute code that you wont know you need until the last minute.  I can see why Eval might be bad, but how would one go about it otherwise?

    Answers are the difference between being a thermometer & a thermostat.  Thermometers just let you know something is wrong.  A thermostat sees the problem & then takes action to actually do something to improve the situation.
  • How would one go about WHAT otherwise?  To answer the question, I'd have to know what eval/execute was being used for in the first place!

    That said, _most_ uses of eval that I've seen could be rewritten to use a lookup table, and the resulting code would be smaller, faster, easier to maintain, and have fewer security holes.  
  • We are using XAML to write validation rules which are evaluated on the fly using Eval. So how would you go about evaluating at runtime a condition whose values may change depending on the input of the user?

  • PingBack from http://rayfd.wordpress.com/2007/03/28/why-wont-eval-eval-my-json-or-json-object-object-literal/

  • PingBack from http://www.keyongtech.com/1169946-calling-a-sub

  • In answer to Eric's question, this worked for me in ASP classic, JScript:


    I found it at JSON .org and I saved it as json-sans-eval.asp and edited it so that it started with <% and ended with %>

    Then in my code where I was using eval I put this at towards the top (outside of the <% %>

    <!-- #include file="json_sans_eval.asp" -->

    Then instead of eval(X) is used jsonParse(X).

    Worked great.

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