How can I detect I'm inside an eval() call?

Question

Does there exist a string s such that

(new Function(s))();

and

eval(s);

behave differently? I'm trying to "detect" how a string is being evaluated.

Answer 1

Check for the arguments object. If it exists, you're in the function. If it doesn't it has been evaled.

Note that you'll have to put the check for arguments in a try...catch block like this:

var s = 'try {document.writeln(arguments ? "Function" : "Eval") } catch(e) { document.writeln("Eval!") }';
(new Function(s))();
eval(s);

Demo

Solution to nnnnnn's concern. For this, I've edited the eval function itself:

var _eval = eval;
eval = function (){
    // Your custom code here, for when it's eval
    _eval.apply(this, arguments);
};

function test(x){
    eval("try{ alert(arguments[0]) } catch(e){ alert('Eval detected!'); }");
}
test("In eval, but it wasn't detected");​

Answer 2

The current answer does not work in strict mode since you can't redefine eval. Moreover, redefining eval is problematic for many other reasons.

The way to differenciate them is based on the fact that well... one of them creates a function and what doesn't. What can functions do? They can return stuff :)

We can simply exploit that and do something with return:

// is in function
try {
     return true;
} catch(e) { // in JS you can catch syntax errors
     false; //eval returns the return of the expression.
}

So in example:

var s = "try{ return true; }catch(e){ false; }";
eval(s); // false
Function(s)(); // true
(new Function(s))(); // true, same as line above
(function(){ return eval(s); })(); // the nested 'problematic' case - false

Answer 3

evaled code can be detected by invoking an error and checking if the native stack-trace contains a lower row/column indicator.

if (typeof window === 'object') {
    // browser
    window.isEvilEval = function () {
        return parseInt(String(new Error().stack).split(':').pop(), 10) < 10;
    };
} else {
    // nodejs (must be global to be callable from within Function)
    global.isEvilEval = function () {
        return (
            new Error().stack
                .split('\n')
                .filter((l) => l.trim().startsWith('at eval') && l.indexOf('<anonymous>:1') > -1).length > 0
        );
    };
}

// test - directly in code => false
console.log(isEvilEval());

// test - in evil eval => true
eval('console.log(isEvilEval())');

// test scoped in function => false
Function('console.log(isEvilEval())')();