Question

I have a CherryPy web application that requires authentication. I have working HTTP Basic Authentication with a configuration that looks like this:

app_config = {
    '/' : {
        'tools.sessions.on': True,
        'tools.sessions.name': 'zknsrv',
        'tools.auth_basic.on': True,
        'tools.auth_basic.realm': 'zknsrv',
        'tools.auth_basic.checkpassword': checkpassword,
        }
    }

HTTP auth works great at this point. For example, this will give me the successful login message that I defined inside AuthTest:

curl http://realuser:realpass@localhost/AuthTest/

Since sessions are on, I can save cookies and examine the one that CherryPy sets:

curl --cookie-jar cookie.jar http://realuser:realpass@localhost/AuthTest/

The cookie.jar file will end up looking like this:

# Netscape HTTP Cookie File
# http://curl.haxx.se/rfc/cookie_spec.html
# This file was generated by libcurl! Edit at your own risk.

localhost       FALSE   /       FALSE   1348640978      zknsrv  821aaad0ba34fd51f77b2452c7ae3c182237deb3

However, I’ll get an HTTP 401 Not Authorized failure if I provide this session ID without the username and password, like this:

curl --cookie 'zknsrv=821aaad0ba34fd51f77b2452c7ae3c182237deb3' http://localhost/AuthTest

What am I missing?

Thanks very much for any help.

Answer 1

So, the short answer is you can do this, but you have to write your own CherryPy tool (a before_handler), and you must not enable Basic Authentication in the CherryPy config (that is, you shouldn’t do anything like tools.auth.on or tools.auth.basic... etc) – you have to handle HTTP Basic Authentication yourself. The reason for this is that the built-in Basic Authentication stuff is apparently pretty primitive. If you protect something by enabling Basic Authentication like I did above, it will do that authentication check before it checks the session, and your cookies will do nothing.

My solution, in prose

Fortunately, even though CherryPy doesn’t have a way to do both built-in, you can still use its built-in session code. You still have to write your own code for handling the Basic Authentication part, but in total this is not so bad, and using the session code is a big win because writing a custom session manager is a good way to introduce security bugs into your webapp.

I ended up being able to take a lot of things from a page on the CherryPy wiki called Simple authentication and access restrictions helpers. That code uses CP sessions, but rather than Basic Auth it uses a special page with a login form that submits ?username=USERNAME&password=PASSWORD. What I did is basically nothing more than changing the provided check_auth function from using the special login page to using the HTTP auth headers.

In general, you need a function you can add as a CherryPy tool – specifically a before_handler. (In the original code, this function was called check_auth(), but I renamed it to protect().) This function first tries to see if the cookies contain a (valid) session ID, and if that fails, it tries to see if there is HTTP auth information in the headers.

You then need a way to require authentication for a given page; I do this with require(), plus some conditions, which are just callables that return True. In my case, those conditions are zkn_admin(), and user_is() functions; if you have more complex needs, you might want to also look at member_of(), any_of(), and all_of() from the original code.

If you do it like that, you already have a way to log in – you just submit a valid session cookie or HTTPBA credentials to any URL you protect with the @require() decorator. All you need now is a way to log out.

(The original code instead has an AuthController class which contains login() and logout(), and you can use the whole AuthController object in your HTTP document tree by just putting auth = AuthController() inside your CherryPy root class, and get to it with a URL of e.g. http://example.com/auth/login and http://example.com/auth/logout. My code doesn’t use an authcontroller object, just a few functions.)

Some notes about my code

• Caveat: Because I wrote my own parser for HTTP auth headers, it only parses what I told it about, which means just HTTP Basic Auth – not, for example, Digest Auth or anything else. For my application that’s fine; for yours, it may not be.
• It assumes a few functions defined elsewhere in my code: user_verify() and user_is_admin()
• I also use a debugprint() function which only prints output when a DEBUG variable is set, and I’ve left these calls in for clarity.
• You can call it cherrypy.tools.WHATEVER (see the last line); I called it zkauth based on the name of my app. Take care NOT to call it auth, or the name of any other built-in tool, though .
• You then have to enable cherrypy.tools.WHATEVER in your CherryPy configuration.
• As you can see by all the TODO: messages, this code is still in a state of flux and not 100% tested against edge cases – sorry about that! It will still give you enough of an idea to go on, though, I hope.

My solution, in code

import base64
import re
import cherrypy 

SESSION_KEY = '_zkn_username'

def protect(*args, **kwargs):
    debugprint("Inside protect()...")

    authenticated = False
    conditions = cherrypy.request.config.get('auth.require', None)
    debugprint("conditions: {}".format(conditions))
    if conditions is not None:
        # A condition is just a callable that returns true or false
        try:
            # TODO: I'm not sure if this is actually checking for a valid session?
            # or if just any data here would work? 
            this_session = cherrypy.session[SESSION_KEY]

            # check if there is an active session
            # sessions are turned on so we just have to know if there is
            # something inside of cherrypy.session[SESSION_KEY]:
            cherrypy.session.regenerate()
            # I can't actually tell if I need to do this myself or what
            email = cherrypy.request.login = cherrypy.session[SESSION_KEY]
            authenticated = True
            debugprint("Authenticated with session: {}, for user: {}".format(
                    this_session, email))

        except KeyError:
            # If the session isn't set, it either wasn't present or wasn't valid. 
            # Now check if the request includes HTTPBA?
            # FFR The auth header looks like: "AUTHORIZATION: Basic <base64shit>"
            # TODO: cherrypy has got to handle this for me, right? 

            authheader = cherrypy.request.headers.get('AUTHORIZATION')
            debugprint("Authheader: {}".format(authheader))
            if authheader:
                #b64data = re.sub("Basic ", "", cherrypy.request.headers.get('AUTHORIZATION'))
                # TODO: what happens if you get an auth header that doesn't use basic auth?
                b64data = re.sub("Basic ", "", authheader)

                decodeddata = base64.b64decode(b64data.encode("ASCII"))
                # TODO: test how this handles ':' characters in username/passphrase.
                email,passphrase = decodeddata.decode().split(":", 1)

                if user_verify(email, passphrase):

                    cherrypy.session.regenerate()

                    # This line of code is discussed in doc/sessions-and-auth.markdown
                    cherrypy.session[SESSION_KEY] = cherrypy.request.login = email
                    authenticated = True
                else:
                    debugprint ("Attempted to log in with HTTBA username {} but failed.".format(
                            email))
            else:
                debugprint ("Auth header was not present.")

        except:
            debugprint ("Client has no valid session and did not provide HTTPBA credentials.")
            debugprint ("TODO: ensure that if I have a failure inside the 'except KeyError'"
                        + " section above, it doesn't get to this section... I'd want to"
                        + " show a different error message if that happened.")

        if authenticated:
            for condition in conditions:
                if not condition():
                    debugprint ("Authentication succeeded but authorization failed.")
                    raise cherrypy.HTTPError("403 Forbidden")
        else:
            raise cherrypy.HTTPError("401 Unauthorized")

cherrypy.tools.zkauth = cherrypy.Tool('before_handler', protect)

def require(*conditions):
    """A decorator that appends conditions to the auth.require config
    variable."""
    def decorate(f):
        if not hasattr(f, '_cp_config'):
            f._cp_config = dict()
        if 'auth.require' not in f._cp_config:
            f._cp_config['auth.require'] = []
        f._cp_config['auth.require'].extend(conditions)
        return f
    return decorate

#### CONDITIONS
#
# Conditions are callables that return True
# if the user fulfills the conditions they define, False otherwise
#
# They can access the current user as cherrypy.request.login

# TODO: test this function with cookies, I want to make sure that cherrypy.request.login is 
#       set properly so that this function can use it. 
def zkn_admin():
    return lambda: user_is_admin(cherrypy.request.login)

def user_is(reqd_email):
    return lambda: reqd_email == cherrypy.request.login

#### END CONDITIONS

def logout():
    email = cherrypy.session.get(SESSION_KEY, None)
    cherrypy.session[SESSION_KEY] = cherrypy.request.login = None
    return "Logout successful"

Now all you have to do is enable both builtin sessions and your own cherrypy.tools.WHATEVER in your CherryPy configuration. Again, take care not to enable cherrypy.tools.auth. My configuration ended up looking like this:

config_root = {
    '/' : {
        'tools.zkauth.on': True, 
        'tools.sessions.on': True,
        'tools.sessions.name': 'zknsrv',
        }
    }