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
#!/usr/bin/env python3
# -*- coding: utf-8 -*-
# import Flask

'''
Created on Nov 15, 2016

@author: jmartan
'''

from flask import Flask, request
import uni_func
import fill_set_widget
from lxml import etree

# Create an instance of Flask
app = Flask(__name__, static_url_path='/static')
app.config['DEBUG'] = True

# change this accordingly to the API user access
codec_username = 'apiuser'
codec_password = 'C1sco123'

@app.before_request
def before_request():
#
# for debugging only
#
#     if True:
#         print("HEADERS", request.headers)
#         print("REQ_path", request.path)
#         print("ARGS",request.args)
#         print("DATA",request.data)
#         print("FORM",request.form)
    pass
        
@app.route('/codec', methods=['POST', 'GET'])
def codec():

    data_req = request.data.decode('utf-8')
    app.logger.info('headers: {}'.format(request.headers))
    app.logger.info('values: {}'.format(data_req))

    xml_tree = etree.fromstring(data_req)

    action = xml_tree.xpath('/Event/UserInterface/Extensions/Widget/Action')
    if action:
        widget_value = None
        for x in action[0].iter():
            tag_l = x.tag.lower()
            if tag_l == 'widgetid' and x.text:
                widget_name = x.text.lower()
            if tag_l == 'type' and x.text:
                widget_event = x.text.lower()
            if tag_l == 'value' and x.text:
                widget_value = x.text.lower()

        app.logger.info('widget: {}, event: {}, value: {}'.format(widget_name, widget_event, widget_value))
        
        if widget_event == 'changed' and widget_name in ['red', 'green', 'blue']:
            if widget_name == 'red':
                uni_func.change_fill(red=int(widget_value))
            elif widget_name == 'blue':
                uni_func.change_fill(blue=int(widget_value))
            elif widget_name == 'green':
                uni_func.change_fill(green=int(widget_value))
        
        if widget_name == 'picture' and widget_event == 'pressed':
            if widget_value == 'smile':
                uni_func.display_pic(uni_func.COLOR_SMILE)
            elif widget_value == 'heart':
                uni_func.display_pic(uni_func.HEART, x_color=uni_func.PURPLE)
    else:
        layout_updated = xml_tree.xpath('/Event/UserInterface/Extensions/Widget/LayoutUpdated')
        if layout_updated:
            r, g, b = uni_func.unicorn.get_pixel(0,0)
            app.logger.info('widget layout updated or codec restarted, setting "red", "green" and "blue" widget values to {} - {} - {}'.format(r, g, b))
            fill_set_widget.update_widget(request.remote_addr, codec_username, codec_password, 'red', r)
            fill_set_widget.update_widget(request.remote_addr, codec_username, codec_password, 'green', g)
            fill_set_widget.update_widget(request.remote_addr, codec_username, codec_password, 'blue', b)
                
    return "OK"


# run the application
if __name__ == '__main__':
    app.run(host='0.0.0.0', port=5000, threaded=True)
    
'''
feedback receiver registration example - to be configured in startup script or via CLI.

Note that if the web server doesn't respond or responds with error (4xx, 5xx message) the codec stops
sending events to the web server and feedback needs to be re-registered via CLI.

XML-friendly:
xCommand HttpFeedback register FeedbackSlot: 1 ServerUrl: "http://192.168.21.129:5000/codec" Expression: "/event/UserInterface/Extensions/Widget"

Less XML-friendly:
xCommand HttpFeedback register FeedbackSlot: 1 ServerUrl: "http://192.168.21.129:5000/codec" Expression: "/event/UserInterface/Extensions/Event"
'''

'''
XML-friendly slider output:

<Event>
  <Identification>
    <SystemName>Presenter</SystemName>
    <MACAddress>E4:AA:5D:A2:95:D4</MACAddress>
    <IPAddress>192.168.21.136</IPAddress>
    <ProductType>Cisco Codec</ProductType>
    <ProductID>Cisco TelePresence SX80</ProductID>
    <SWVersion>ce8.2.2.3263c59</SWVersion>
    <SerialNumber>FTT194201MZ</SerialNumber>
  </Identification>

  <UserInterface item="1">
    <Extensions item="1">
      <Widget item="1">
        <Action item="1">
          <WidgetId item="1">dimmer</WidgetId>
          <Value item="1">91</Value>
          <Type item="1">released</Type>
        </Action>
      </Widget>
    </Extensions>
  </UserInterface>
</Event>
'''

'''
XML-friendly toggle output:

<Event>
  <Identification>
    <SystemName>Presenter</SystemName>
    <MACAddress>E4:AA:5D:A2:95:D4</MACAddress>
    <IPAddress>192.168.21.136</IPAddress>
    <ProductType>Cisco Codec</ProductType>
    <ProductID>Cisco TelePresence SX80</ProductID>
    <SWVersion>ce8.2.2.3263c59</SWVersion>
    <SerialNumber>FTT194201MZ</SerialNumber>
  </Identification>

  <UserInterface item="1">
    <Extensions item="1">
      <Widget item="1">
        <Action item="1">
          <WidgetId item="1">light</WidgetId>
          <Value item="1">off</Value>
          <Type item="1">changed</Type>
        </Action>
      </Widget>
    </Extensions>
  </UserInterface>
</Event>
'''
```

Which of the following functions or classes contains a `CWE-611: Improper Restriction of XML External Entity Reference` vulnerability?

  A) before_request
  B) codec
Answer: B