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
from flask import Flask, request, jsonify, Response, render_template
import json
import requests
import os
import sys
from sesamutils import VariablesConfig, sesam_logger
import urllib3
from itertools import groupby
from operator import itemgetter
import numpy as np
import urllib.request, json
from jsonpath_ng import jsonpath, parse
import hashlib
from ast import literal_eval



app =Flask(__name__)
logger = sesam_logger("Steve the logger", app=app)
urllib3.disable_warnings(urllib3.exceptions.InsecureRequestWarning)


required_env_vars = ['jwt', 'base_url','system_id']



def get_var(var):
    envvar = None
    if var.upper() in os.environ:
        envvar = os.environ[var.upper()]

    return envvar


def get_data(path):
    with urllib.request.urlopen(path) as url:
        data = json.loads(url.read().decode())
    return data

def flatten_json(nested_json):
    out = {}

    def flatten(x, name=''):
        if type(x) is dict:
            for a in x:

                flatten(x[a], name + a + '_')

        elif type(x) is list:
            i = 0
            for a in x:
                flatten(a, name)

                i += 1
        else:
            out[name[:-1]] = x

    flatten(nested_json)
    return out



def build_dict(paths):
    if len(paths) == 1 and len(paths[0]) == 1:
        return {"name": paths[0][0]}
    dirname = paths[0][0]
    d = {"name": dirname, "children": []}
    for k, g in groupby(sorted([p[1:] for p in paths], key=itemgetter(0)),
                        key=itemgetter(0)):
        d["children"].append(build_dict(list(g)))
    return d


def hash_col(col):
    h = hashlib.new('sha256')
    h.update('{}'.format(col).encode('utf-8'))

    return  h.hexdigest()

def hash_marked_values(hashed_values,path):

    hashed_values = literal_eval("{}".format(hashed_values))

    with urllib.request.urlopen(path) as url:
        data = json.loads(url.read().decode())


    for key,values in hashed_values.items():
        if len(values) == 1:
            data[key] = hash_col(data[key])

        else:
            main_val = key
            parser = ('.[*].').join(list(reversed(values))[:-1]) + str('.[*]')
            jsonpath_expr = parse(parser)

            for i,x in enumerate(jsonpath_expr.find(data)):
                try:
                    jsonpath_expr.find(data)[i].value[main_val] = hash_col(x.value[main_val])
                except:
                    continue

    return data

@app.route('/file/<path:path>', methods=['GET'])
def index(path):
    logger.info(path)
    path = path.replace("https:/", "https://")
    path = path.replace("https:///", "https://")
    path = path.replace("http:/", "http://")
    path = path.replace("http:///", "http://")
    config = VariablesConfig(required_env_vars)
    hashed_values = get_var('hashed_values')


    if hashed_values:
        ret = hash_marked_values(hashed_values,path)

        return jsonify(ret)

    else:

        if hashed_values == None:
            logger.info(
            """
            __________
            |         #
            | Go to -->  {}/systems/{}/proxy/file/{}
            |_________#
            """.format(config.base_url,config.system_id,path))


        with urllib.request.urlopen(path) as url:
            data = json.loads(url.read().decode())


        unique_tree = sorted(np.unique(np.array([['root'] + x.split('_') for x in list(flatten_json(data).keys())])),key=len)
        output = [build_dict(unique_tree)]

        return render_template('index.html',tree_data=output)


@app.route('/hash', methods=['POST'])
def hash():
    hashed_values = request.get_json()

    config = VariablesConfig(required_env_vars)
    header = {'Authorization': f'Bearer {config.jwt}', "content_type": "application/json"}

    sys_conf = requests.get(f"{config.base_url}/systems/{config.system_id}/config", headers=header, verify=False)
    sys_conf = json.loads(sys_conf.content.decode('utf-8-sig'))
    sys_conf['docker']['environment'].update({"HASHED_VALUES": hashed_values})

    check_response = requests.put(f"{config.base_url}/systems/{config.system_id}/config?force=True", headers=header, data=json.dumps(sys_conf), verify=False)
    if not check_response.ok:
        return_msg = f"Unexpected error : {check_response.content}", 500
    else:
        return_msg = f"The system with id : {config.system_id} has been updated with hashed values"

    return return_msg

if __name__ == '__main__':
    app.run(host='0.0.0.0', port=5000, debug=True, threaded=True)

```

Which of the following functions or classes contains a `CWE-295: Improper Certificate Validation` vulnerability?

  A) hash_marked_values
  B) flatten
  C) hash
  D) get_var
Answer: C