JavaScript SetOffset()

While doing a JavaScript exploit I encountered that there isn’t a convenient function in JavaScript to set specific code to a specific offset in a given string.

For example:

Given the string “1234567890”

If you want to change the substring “567” to “756” for it to become “1234765890”

you would need to do something like this:

var string = “1234567890”;
var LeftHalf = String.substr(0 ,  3);   // leftHalf = “1234”
var RightHalf = String.substr(3);  // RightHalf = “56789”
RightHalf = RightHalf.substr(3);  // RightHalf = “89” (remove "567")
String = LeftHalf + “765” + RightHalf;  // String = “1234765890”

Of course you could’ve just make a new string with the desired changes, but when you have a very long string and you’re making multiple modifications to different offsets of it, like you probably would making an exploit, it can become a tedious pain.

So to make this more consistent, time efficient, and less likely to make mistakes, I created a general function so you can modify your string with a single line.

function SetOffset(String, Offset, Injection)


    var LeftHalf = String.substr(0 , Offset);
    var RightHalf = String.substr(Offset);
    RightHalf = RightHalf.substr(Injection.length);
    var ModifiedString = LeftHalf + Injection + RightHalf;
    return ModifiedString;


With this new function, the above piece of code would become:

var String = “1234567890”
String = SetOffset(String, 3, “765”);

This code works perfectly with the Metasploit tools “Pattern_Create.rb” and “Pattern_Offset.rb”. You just need to copy this function somewhere in the beginning of your code and you can call it as needed.