My Spider Sense warns me that using eval() to parse incoming JSON is a bad idea. I'm just wondering if JSON.parse() - which I assume is a part of JavaScript and not a browser-specific function - is more secure.
6 Answers
You are more vulnerable to attacks if using eval: JSON is a subset of Javascript and json.parse just parses JSON whereas eval would leave the door open to all JS expressions.
2 Comments
JackLThornton
Sorry, Matheus, I have to agree. The problem is when you are using eval() to interpret "user input" - which is ANY source external from your JavaScript (including returned values from servlets or other web services you've called). You cannot guarantee users have not entered malicious JavaScript either directly into your client app, or indirectly because of unvalidated data stored in the server's database and then passed on to your program via an AJAX-style call. You may still need to validate individual fields to avoid "confused deputy" attacks, but using JSON.parse is a good first step.
V. Bozz
@Hydro Short proof of concept: try
eval('alert(1)');.All JSON.parse implementations most likely use eval()
JSON.parse is based on Douglas Crockford's solution, which uses eval() right there on line 497.
// In the third stage we use the eval function to compile the text into a
// JavaScript structure. The '{' operator is subject to a syntactic ambiguity
// in JavaScript: it can begin a block or an object literal. We wrap the text
// in parens to eliminate the ambiguity.
j = eval('(' + text + ')');
The advantage of JSON.parse is that it verifies the argument is correct JSON syntax.
4 Comments
nickf
yeah, except that the line right before that verifies that it's a safe and valid string.
jbo5112
I tested
JSON.parse() in Firefox 28 and Chromium 33 on my Linux Mint system. It was 2x as fast as eval() in Firefox and 4x as fast in Chromium. I'm not sure what source code you're posting, but they're not the same thing in my browsers.
mjs
@plodder "advantage" probably is not cheap to do that verifying.
Mohammad Alhashash
Modern browsers provides native
JSON.parse() implementation which is safer and faster than eval()-based parsers.Not all browsers have native JSON support so there will be times where you need to use eval() to the JSON string. Use JSON parser from http://json.org as that handles everything a lot easier for you.
Eval() is an evil but against some browsers its a necessary evil but where you can avoid it, do so!!!!!
1 Comment
reducing activity
It is no longer true, and even in times where it used to be true you should use a proper parser instead.
There is a difference between what JSON.parse() and eval() will accept. Try eval on this:
var x = "{\"shoppingCartName\":\"shopping_cart:2000\"}"
eval(x) //won't work
JSON.parse(x) //does work
See this example.
2 Comments
Charles HETIER
eval doesn't work because it parses strings as code statements and thus, consideres "{...}" as a code expression instead of a value declaration expression. if you remove the ambiguity ("[{....}]" for example), there is no doubt on the nature of the expression and eval will creates an array containing the parsed objet
Jeff Lowery
Yes. Traditionally, x would be wrapped by parentheses: eval("("+x+")"). What I said still stands: there's no ambiguity when using JSON.parse().
If you parse the JSON with eval, you're allowing the string being parsed to contain absolutely anything, so instead of just being a set of data, you could find yourself executing function calls, or whatever.
Also, JSON's parse accepts an aditional parameter, reviver, that lets you specify how to deal with certain values, such as datetimes (more info and example in the inline documentation here)
Comments
JSON is just a subset of JavaScript. But eval evaluates the full JavaScript language and not just the subset that’s JSON.
2 Comments
Kevin Major
Right, I know that. Are you implying that JSON.parse() ONLY evaluates JSON and fails on all other incoming data? Or is it simply a wrapper for: var myObject = eval('(' + responseText + ')'); ??
Gumbo
@Kevin Major: Yes, the natively implemented
JSON.parse (directly implemented into the JavaScript engine) parses only JSON. But other non-natively implementations use do some sanity checking and then use eval for performance reasons.
JSON.parseis faster thaneval, at least in V8 (Chromium's JS engine). Source.