This is a program I wrote in JavaScript to validate some inputted credentials, with an intentional vulnerability. The credentials_validator function takes an object as input, with usernames and passwords. It returns a function, which takes a username and password and returns true if the credentials are valid.

Your challenge is to find a username and password which, when passed to the function returned by credentials_validator, cause it to return true without knowledge of the credentials ahead of time.

This is a , so the first valid answer wins.


var credentials_validator = (credentials) => {
    if (typeof credentials !== "object")
        throw new TypeError("'credentials' must be an object");
    for (var username in credentials)
        if (typeof credentials[username] === "object")
            throw new TypeError("Passwords may not be objects or 'null'");
    return (username, password) => {
        if (typeof password === "object")
            throw new TypeError("Passwords may not be objects or 'null'");
        if (!(username in credentials))
            return false;
        if (credentials[username] === password)
            return true;
        return false;


Run with:

    var validator = credentials_validator({
        "J. Doe": "P4ssword",
        "R. Wolf": "1234",
        "X. Ample": "wxyz#1"

    validator("J. Doe", "P4ssword"); // true
    validator("R. Wolf", "123"); // false

  • \$\begingroup\$ I'm not sure if the difficulty of this challenge is too low. Couldn't sandbox it for obvious reasons :p \$\endgroup\$ Apr 26, 2021 at 18:31
  • 1
    \$\begingroup\$ Suggested test case: var validator = credentials_validator(Object.create(null));. Not sure if this is intended to be a valid input or not but the existing answer doesn't work for it. \$\endgroup\$
    – Etheryte
    Apr 26, 2021 at 20:56
  • \$\begingroup\$ @Etheryte I didn't mention this in the question because I didn't think about it, but assume the property names and values are within reason. The credentials object will just be ordinary names and passwords, not {} or null or { "isNaN": () => true }. \$\endgroup\$ Apr 26, 2021 at 22:29

1 Answer 1


username = 'toString', password = {}.toString

Try it online!

Note: Due to a certain username pointed out by Wezl in the comments, this answer is not valid for the current challenge.

Why this works:

Among other properties, Object.prototype.toString is inherited from Object.prototype, even if it wasn't a part of the object literal given to the validator. Since any object inherits the same function {}.toString is the same as the toString of the credentials object.

  • 3
    \$\begingroup\$ Well that was fast :p \$\endgroup\$ Apr 26, 2021 at 18:41
  • 3
    \$\begingroup\$ What about var v = credentials_validator({ "toString": "purple" }); v("toString", {}.toString)? \$\endgroup\$
    – Wezl
    Apr 26, 2021 at 18:47
  • 1
    \$\begingroup\$ valueOf is a shorter crack :-p \$\endgroup\$
    – Arnauld
    Apr 26, 2021 at 19:00
  • 1
    \$\begingroup\$ "this answer is not valid for the current challenge" -- Isn't this the one that was intended by the OP? \$\endgroup\$
    – Jonah
    Apr 26, 2021 at 19:20
  • 1
    \$\begingroup\$ @Wezl You may still define your own Object.prototype methods using a newly created Symbol to make sure no such username may ever exist. But I'm not sure if define some newly created attributes is allowed here. \$\endgroup\$
    – tsh
    Apr 27, 2021 at 2:15

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