javascriptarraysjavascript-security

Array constructor "new Array()" can be overwritten and replaced with malicious code, how?

We can create an array in a couple of ways:

var myArray = new Array();

Or:

var myArray = [];

The second way is safer to use than the new Array() syntax, because the Array constructor can be overwritten and potentially replaced with malicious code.

I have seen above lines in many JavaScript books but I don't understand how an Array constructor can be overwritten and replaced with malicious code? I'm looking for an example of how someone can do it, so that I can understand the reality of the issue.

Solution

  • Somewhere in the code above:

    Array.prototype.forEach = function (e){
  console.log("something wrong there");
  return(e);
};

    Somewhere in the code below:

    var i = [1,2,3,4,5];
i.forEach(function(e){
  console.log(e);
});

    Output:

    >"something wrong there"

    As you can see, there is no difference how to initialize array variable. var i = []; just shorter notation.