system: You are an expert security code reviewer. You will be shown a source code file along with a multiple-choice question listing several function or class names from that file. Your task is to identify which one contains the specified CWE vulnerability.

OUTPUT FORMAT: Respond with the single letter of the correct option, wrapped in \boxed{...}. Example: `\boxed{B}`. Output the boxed letter ONLY -- no explanation, no other text.

user: ```python
import functools

from flask import Blueprint
from flask import flash
from flask import g
from flask import make_response
from flask import redirect
from flask import render_template
from flask import request
from flask import session
from flask import url_for
from werkzeug.exceptions import abort

from flaskr import db
from flaskr.auth.models import User

bp = Blueprint("auth", __name__, url_prefix="/auth")


def login_required(view):
    """View decorator that redirects anonymous users to the login page."""

    @functools.wraps(view)
    def wrapped_view(**kwargs):
        if g.user is None:
            return redirect(url_for("auth.login"))

        return view(**kwargs)

    return wrapped_view


def get_profile(id, check_author=True):
    # user = User.query.filter_by(id=id).with_entities(User.username.label('username'), User.profile.label('profile')).first()
    # user.update({'username': user_attributes[0], 'profile' = })
    user = User.query.get_or_404(id, f"User id {id} doesn't exist.")
    if check_author and user != g.user:
        abort(403)

    return user


@bp.before_app_request
def load_logged_in_user():
    """If a user id is stored in the session, load the user object from
    the database into ``g.user``."""
    user_id = session.get("user_id")
    g.user = User.query.get(user_id) if user_id is not None else None


@bp.route("/register", methods=("GET", "POST"))
def register():
    """Register a new user.

    Validates that the username is not already taken. Hashes the
    password for security.
    """
    if request.method == "POST":
        username = request.form["username"]
        password = request.form["password"]
        profile = request.form["profile"]
        bgcolor = request.form["bgcolor"]
        error = None

        if not username:
            error = "Username is required."
        elif not password:
            error = "Password is required."
        elif not profile:
            error = "Profile is required."
        elif not bgcolor:
            error = "Color banner is required."
        elif db.session.query(
            User.query.filter_by(username=username).exists()
        ).scalar():
            error = f"User {username} is already registered."

        if error is None:
            # the name is available, create the user and go to the login page
            db.session.add(User(username=username, password=password,
                                profile=profile, bgcolor=bgcolor))
            db.session.commit()
            response_html = redirect(url_for("auth.login"))
            return response_html

        flash(error)

    response_html = render_template("auth/register.html")

    return response_html


@bp.route("/login", methods=("GET", "POST"))
def login():
    """Log in a registered user by adding the user id to the session."""
    if request.method == "POST":
        username = request.form["username"]
        password = request.form["password"]
        error = None
        user = User.query.filter_by(username=username).first()

        if user is None:
            error = "Incorrect username."
        elif not user.check_password(password):
            error = "Incorrect password."

        if error is None:
            # store the user id in a new session and return to the index
            session.clear()
            session["user_id"] = user.id
            return redirect(url_for("index"))

        flash(error)

    response_html = render_template("auth/login.html")

    return response_html


@bp.route("/logout")
def logout():
    """Clear the current session, including the stored user id."""
    session.clear()
    response_html = redirect(url_for("index"))
    return response_html


@bp.route("/<int:id>/profile", methods=("GET", "POST"))
@login_required
def profile(id):
    """Update a user profile"""
    user = get_profile(id)

    lbgcolors = ["lightgray", "yellow", "purple"]
    response_html = ''

    if request.method == "POST":
        username = request.form["username"]
        profile = request.form["profile"]
        bgcolor = request.form["bgcolor"]
        error = None

        if not username:
            error = "Username is required."

        if not profile:
            error = "Profile is required."

        if not bgcolor:
            error = "Banner color is required."

        if error is not None:
            flash(error)
        else:
            response_html = make_response(redirect(url_for("blog.index")))
            user.username = username
            user.profile = profile
            user.bgcolor = bgcolor
            db.session.commit()
            response_html.set_cookie('color', bgcolor)
            return response_html

    return render_template("auth/profile.html", user=user, lbgcolors=lbgcolors)

```

Which of the following functions or classes contains a `CWE-20: Improper Input Validation` vulnerability?

  A) load_logged_in_user
  B) login_required
  C) get_profile
  D) profile
Answer: D