https://github.com/cherrypy/cherrypy/pull/1683 with some hexcode(...) sprinkled to debug the intermediate results. I've added the relevant output testing with the different browsers available

auth_basic.py

# This file is part of CherryPy <http://www.cherrypy.org/>
# -*- coding: utf-8 -*-
# vim:ts=4:sw=4:expandtab:fileencoding=utf-8

import binascii
import unicodedata

import cherrypy
from cherrypy._cpcompat import base64_decode, ntob, ntou, tonative


__doc__ = """This module provides a CherryPy 3.x tool which implements
the server-side of HTTP Basic Access Authentication, as described in
:rfc:`2617`.

Example usage, using the built-in checkpassword_dict function which uses a dict
as the credentials store::

    userpassdict = {'bird' : 'bebop', 'ornette' : 'wayout'}
    checkpassword = cherrypy.lib.auth_basic.checkpassword_dict(userpassdict)
    basic_auth = {'tools.auth_basic.on': True,
                  'tools.auth_basic.realm': 'earth',
                  'tools.auth_basic.checkpassword': checkpassword,
    }
    app_config = { '/' : basic_auth }

"""

__author__ = 'visteya'
__date__ = 'April 2009'


def checkpassword_dict(user_password_dict):
    """Returns a checkpassword function which checks credentials
    against a dictionary of the form: {username : password}.

    If you want a simple dictionary-based authentication scheme, use
    checkpassword_dict(my_credentials_dict) as the value for the
    checkpassword argument to basic_auth().
    """
    def checkpassword(realm, user, password):
        p = user_password_dict.get(user)
        return p and p == password or False

    return checkpassword


def to_ord(n):
    if type(n) is not type(0):
        return ord(n)
    else:
        return n

def hexcode(s):
    return ' '.join(hex(to_ord(x))[2:] for x in s)

def basic_auth(realm, checkpassword, debug=False, accept_charset='utf-8'):
    """A CherryPy tool which hooks at before_handler to perform
    HTTP Basic Access Authentication, as specified in :rfc:`2617`
    and :rfc:`7617`.

    If the request has an 'authorization' header with a 'Basic' scheme, this
    tool attempts to authenticate the credentials supplied in that header.  If
    the request has no 'authorization' header, or if it does but the scheme is
    not 'Basic', or if authentication fails, the tool sends a 401 response with
    a 'WWW-Authenticate' Basic header.

    realm
        A string containing the authentication realm.

    checkpassword
        A callable which checks the authentication credentials.
        Its signature is checkpassword(realm, username, password). where
        username and password are the values obtained from the request's
        'authorization' header.  If authentication succeeds, checkpassword
        returns True, else it returns False.

    """

    fallback_charset = 'ISO-8859-1'

    if '"' in realm:
        raise ValueError('Realm cannot contain the " (quote) character.')
    request = cherrypy.serving.request

    auth_header = request.headers.get('authorization')
    if auth_header is not None:
        # split() error, base64.decodestring() error
        msg = 'Bad Request'
        with cherrypy.HTTPError.handle((ValueError, binascii.Error), 400, msg):
            scheme, params = auth_header.split(' ', 1)
            if scheme.lower() == 'basic':
                decoded_params = base64_decode(params)
                hex_decoded_params = hexcode(decoded_params)
                print("1, decoded_params: {decoded_params!r}, hex_decoded_params: {hex_decoded_params}".format(**locals()))
                decoded_params = ntob(decoded_params)
                hex_decoded_params = hexcode(decoded_params)
                print("2, decoded_params: {decoded_params!r}, hex_decoded_params: {hex_decoded_params}".format(**locals()))

                last_err = None
                for charset in (accept_charset, fallback_charset):
                    try:
                        decoded_params = tonative(decoded_params, charset)
                        hex_decoded_params = hexcode(decoded_params)
                        print("3, decoded_params: {decoded_params!r}, hex_decoded_params: {hex_decoded_params}".format(**locals()))
                        break
                    except ValueError as ve:
                        last_err = ve
                else:
                    raise last_err

                decoded_params = ntou(decoded_params)
                hex_decoded_params = hexcode(decoded_params)
                print("4, decoded_params: {decoded_params!r}, hex_decoded_params: {hex_decoded_params}".format(**locals()))
                decoded_params = unicodedata.normalize('NFC', decoded_params)
                hex_decoded_params = hexcode(decoded_params)
                print("5, decoded_params: {decoded_params!r}, hex_decoded_params: {hex_decoded_params}".format(**locals()))
                decoded_params = tonative(decoded_params)
                hex_decoded_params = hexcode(decoded_params)
                print("6, decoded_params: {decoded_params!r}, hex_decoded_params: {hex_decoded_params}".format(**locals()))
                username, password = decoded_params.split(':', 1)
                if checkpassword(realm, username, password):
                    if debug:
                        cherrypy.log('Auth succeeded', 'TOOLS.AUTH_BASIC')
                    request.login = username
                    return  # successful authentication

    charset = accept_charset.upper()
    charset_declaration = (
        (', charset="%s"' % charset)
        if charset != fallback_charset
        else ''
    )
    # Respond with 401 status and a WWW-Authenticate header
    cherrypy.serving.response.headers['www-authenticate'] = (
        'Basic realm="%s"%s' % (realm, charset_declaration)
    )
    raise cherrypy.HTTPError(
        401, 'You are not authorized to access that resource')

# C:\Users\jarop\PycharmProjects\cherrypy\venv\Scripts\python.exe C:/Users/jarop/PycharmProjects/cherrypy/Test_code.py
# [05/Jan/2018:09:31:49] ENGINE Bus STARTING
# [05/Jan/2018:09:31:49] ENGINE Serving on http://0.0.0.0:8080
# [05/Jan/2018:09:31:49] ENGINE Bus STARTED
#
# --- curl 7.56.1 (i686-pc-cygwin) libcurl/7.56.1 OpenSSL/1.0.2m zlib/1.2.11 libidn2/2.0.4 libpsl/0.18.0 (+libidn2/2.0.2) libssh2/1.7.0 nghttp2/1.23.1
# --- Release-Date: 2017-10-23 on Windows 10
# --- from cygwin-shell $ curl -u 'curl:€öäü' -i -X GET http://127.0.0.1:8080/
#
# 1, decoded_params: 'curl:â\x82¬Ã¶Ã¤Ã¼', hex_decoded_params: 63 75 72 6c 3a e2 82 ac c3 b6 c3 a4 c3 bc
# 2, decoded_params: b'curl:\xe2\x82\xac\xc3\xb6\xc3\xa4\xc3\xbc', hex_decoded_params: 63 75 72 6c 3a e2 82 ac c3 b6 c3 a4 c3 bc
# 3, decoded_params: 'curl:€öäü', hex_decoded_params: 63 75 72 6c 3a 20ac f6 e4 fc
# 4, decoded_params: 'curl:€öäü', hex_decoded_params: 63 75 72 6c 3a 20ac f6 e4 fc
# 5, decoded_params: 'curl:€öäü', hex_decoded_params: 63 75 72 6c 3a 20ac f6 e4 fc
# 6, decoded_params: 'curl:€öäü', hex_decoded_params: 63 75 72 6c 3a 20ac f6 e4 fc
# realm: 'MY_REALM', username: 'curl'-'63 75 72 6c', password: '€öäü'-'20ac f6 e4 fc'
# ...
# --- Conlusion: curl is sending a UTF-8 encoded String. Those are secessfully decoded with step 3
#
# --- Firefox 57.0.3 (32-Bit) on Windows 10
# 1, decoded_params: 'firefox:¬öäü', hex_decoded_params: 66 69 72 65 66 6f 78 3a ac f6 e4 fc
# 2, decoded_params: b'firefox:\xac\xf6\xe4\xfc', hex_decoded_params: 66 69 72 65 66 6f 78 3a ac f6 e4 fc
# 3, decoded_params: 'firefox:¬öäü', hex_decoded_params: 66 69 72 65 66 6f 78 3a ac f6 e4 fc
# 4, decoded_params: 'firefox:¬öäü', hex_decoded_params: 66 69 72 65 66 6f 78 3a ac f6 e4 fc
# 5, decoded_params: 'firefox:¬öäü', hex_decoded_params: 66 69 72 65 66 6f 78 3a ac f6 e4 fc
# 6, decoded_params: 'firefox:¬öäü', hex_decoded_params: 66 69 72 65 66 6f 78 3a ac f6 e4 fc
# realm: 'MY_REALM', username: 'firefox'-'66 69 72 65 66 6f 78', password: '¬öäü'-'ac f6 e4 fc'
# ...
# --- Conlusion: Firefox is probably sending a "ISO 8859-1" encoded String. The € (Euro) sign
# isn't part of that charset and Firefox decided to encode it as AC which is a ¬ (Not) sign in
# 8859-1, windows-1252, latin-1, cp850 and probably some more.
#
# Appart from the €-Sign, the rest is succesfully decoded with step 3
#
# --- Google Chrome 63.0.3239.108 (64-Bit) on Windows 10
# 1, decoded_params: 'chrome:â\x82¬Ã¶Ã¤Ã¼', hex_decoded_params: 63 68 72 6f 6d 65 3a e2 82 ac c3
#  b6 c3 a4 c3 bc
# 2, decoded_params: b'chrome:\xe2\x82\xac\xc3\xb6\xc3\xa4\xc3\xbc', hex_decoded_params: 63 68 72 6f 6d 65 3a e2 82 ac c3 b6 c3 a4 c3 bc
# 3, decoded_params: 'chrome:€öäü', hex_decoded_params: 63 68 72 6f 6d 65 3a 20ac f6 e4 fc
# 4, decoded_params: 'chrome:€öäü', hex_decoded_params: 63 68 72 6f 6d 65 3a 20ac f6 e4 fc
# 5, decoded_params: 'chrome:€öäü', hex_decoded_params: 63 68 72 6f 6d 65 3a 20ac f6 e4 fc
# 6, decoded_params: 'chrome:€öäü', hex_decoded_params: 63 68 72 6f 6d 65 3a 20ac f6 e4 fc
# realm: 'MY_REALM', username: 'chrome'-'63 68 72 6f 6d 65', password: '€öäü'-'20ac f6 e4 fc'
# ...
# --- Conclusion: Same as with curl -> utf-8 decoding sucessfull after step 3
#
# --- Microsoft Edge 40.15063.674.0 on Windows 10
# 1, decoded_params: 'edge:\x80öäü', hex_decoded_params: 65 64 67 65 3a 80 f6 e4 fc
# 2, decoded_params: b'edge:\x80\xf6\xe4\xfc', hex_decoded_params: 65 64 67 65 3a 80 f6 e4 fc
# 3, decoded_params: 'edge:\x80öäü', hex_decoded_params: 65 64 67 65 3a 80 f6 e4 fc
# 4, decoded_params: 'edge:\x80öäü', hex_decoded_params: 65 64 67 65 3a 80 f6 e4 fc
# 5, decoded_params: 'edge:\x80öäü', hex_decoded_params: 65 64 67 65 3a 80 f6 e4 fc
# 6, decoded_params: 'edge:\x80öäü', hex_decoded_params: 65 64 67 65 3a 80 f6 e4 fc
# realm: 'MY_REALM', username: 'edge'-'65 64 67 65', password: '\x80öäü'-'80 f6 e4 fc'
# ...
# --- Conclusion: Same as with Firefox-> Some sort of of ISO 8859-1 send have problems to
# encode € sign. Appart of that successfull decoed auth string after step 3
#
# --- Microsoft Internet Explorer 11.786.15063.0 on Windows 10
# 1, decoded_params: 'IE-11:\x80öäü', hex_decoded_params: 49 45 2d 31 31 3a 80 f6 e4 fc
# 2, decoded_params: b'IE-11:\x80\xf6\xe4\xfc', hex_decoded_params: 49 45 2d 31 31 3a 80 f6 e4 fc
# 3, decoded_params: 'IE-11:\x80öäü', hex_decoded_params: 49 45 2d 31 31 3a 80 f6 e4 fc
# 4, decoded_params: 'IE-11:\x80öäü', hex_decoded_params: 49 45 2d 31 31 3a 80 f6 e4 fc
# 5, decoded_params: 'IE-11:\x80öäü', hex_decoded_params: 49 45 2d 31 31 3a 80 f6 e4 fc
# 6, decoded_params: 'IE-11:\x80öäü', hex_decoded_params: 49 45 2d 31 31 3a 80 f6 e4 fc
# realm: 'MY_REALM', username: 'IE-11'-'49 45 2d 31 31', password: '\x80öäü'-'80 f6 e4 fc'
# ...
# --- Conclusion: Same as with Firefox-> Some sort of of ISO 8859-1 send have problems to
# encode € sign. Appart of that successfull decoed auth string after step 3
#
# Judgeing from these results everthing works as expected. Steps 4, 5 and 6 can safely be removed.