FlaskSimpleAuth 1.2.0

Simple authentication for Flask, emphasizing configurability

Flask Simple Auth

Simple authentication for Flask, which is controled from Flask configuration.

Description

Help to manage authentication and authorizations in a Flask application.

For authentication, the idea is that the authentication is checked in a before_request hook, and can be made available through some global à-la-Flask variable.

The module implements inheriting the web-server authentication, password authentication (HTTP Basic, or HTTP/JSON parameters), simple time-limited authentication tokens, and a fake authentication scheme useful for application testing.

It allows to have a login route to generate authentication tokens. Support functions allow to hash new passwords consistently with password checks performed by the module.

For authorization, a simple decorator allows to declare required permissions on a route (eg a role name), and relies on a supplied function to check whether a user is in a given group. This is approach is enough for basic authorization management, but would be insufficient for most application where user can edit their own data but not those of others.

Compared to Flask HTTPAuth, there is one code in the app which does not need to know about which scheme is being used, so switching between schemes only impacts the configuration, not the application code.

Simple Example

The application code extract below maintains a LOGIN global variable which holds the authenticated user name for the current request. There is no clue in the source about what kind of authentication is used, which is the whole point: authentication schemes are managed elsewhere, not explicitely in the application code.

# app is the Flask application…
# user_to_password_fun is a function returning the hashed password for a user.
# user_in_group_fun is a function telling whether a user is in a group.

# initialize module
import FlaskSimpleAuth as auth
auth.setConfig(app, user_to_password_fun, user_in_group_fun)

# mandatory authentication
LOGIN = None

def set_login():
    global LOGIN
    LOGIN = None                  # remove previous value, just in case
    try:
        LOGIN = auth.get_user()    
    except auth.AuthException as e:
        return Response(e.message, e.status)
    assert LOGIN is not None      # defensive check

app.before_request(set_login)

# elsewhere use the authentication
# here implicitely by the authorize decorator
@app.route("/whatever", methods=["PATCH"])
@auth.authorize("patcher")
def patch_whatever():
    # ok to do it
    return "", 204

# here explicitely at the beginning of the function
@app.route("/something", methods=["PUT"])
def put_something():
    if not can_put_something(LOGIN):
        return "", 403
    # else ok to do it
    return "", 204

Authentication is manage from the application flask configuration with FSA_* (Flask simple authentication) directives:

FSA_TYPE = 'httpd'     # inherit web-serveur authentication
# OR others such as:
FSA_TYPE = 'basic'     # HTTP Basic auth
FSA_TYPE = 'param'     # HTTP parameter auth

Various aspects of the implemented schemes can be configured with other directives, with reasonable defaults provided so that not much is really needed beyond choosing the authentication scheme. See below for details.

Documentation

Install

Use pip install FlaskSimpleAuth to install the module, or whatever other installation method you prefer.

Features

This simple module allows configurable authentication (FSA_TYPE):

• httpd web-server checked authentication passed in the request.

• basic http basic auth with a function hook for getting the password hash.

• param same with http parameter-provided login/password.

• password tries basic then param.

• token auth uses a signed parameter to authenticate a user in a realm for some limited time. The token can be obtained by actually authenticating with previous methods.

• fake parameter-based auth for fast and simple testing the claimed login is coldly trusted…

I have considered Flask HTTPAuth, obviously, which provides many options, but I do not want to force their per-route model and explicit classes but rather rely on mandatory request hooks and have everything managed from the configuration file to easily switch between schemes.

Note that this is intended for a REST API implementation serving a remote application. It does not make much sense to "login" and "logout" to/from a REST API because the point of the API is to serve and collect data to all who deserve it, i.e. are authorized, unlike a web application which is served while the client is on the page and should disappear when disconnected as the web browser page is wiped out. However, there is still a "login" concept which is only dedicated at obtaining an auth token, that the application client needs to update from time to time.

Note that web-oriented flask authentication modules are not really relevant in the REST API context, where the server does not care about presenting login forms for instance.

Initialisation

The module is initialized by calling setConfig with three arguments:

• the Flask application object.
• a function to retrieve the password hash from the user name.
• a function which tells whether a user is in a group.

# app is already initialized and configured the Flask application

# return password hash if any, or None
def get_user_password(user):
    return …

# return whether user is in group
def user_in_group(user, group):
    return …

import FlaskSimpleAuth as auth
auth.setConfig(app, get_user_password, user_in_group)

Then the module can be used to retrieve the authenticated user with get_user. This functions raises AuthException on failures.

A good practice (IMHO) is to use a before request hook to set a global variable with the value and warrant that the authentication is always checked.

Some path may require to skip authentication, for instance registering a new user. This can be achieved simply by checking request.path.

LOGIN: Optional[str] = None

def set_login():
    global LOGIN
    LOGIN = None      # not really needed, but this is safe
    if request.path == "/register":
        return
    try:
        LOGIN = auth.get_user()
    except auth.AuthException as e:
        # before request hooks can return an alternate response
        return Response(e.message, e.status)

app.before_request(set_login)

Using Authentication and Authorization

Then all route functions can take advantage of this information to check for authorizations with a decorator:

@app.route("/somewhere", methods=["POST"])
@auth.authorize("posters")
def post_somewhere():
    …

Note that more advanced permissions (eg users can edit themselves) will still require manual permission checks at the beginning of the function.

An opened route for user registration could look like that:

@app.route("/register", methods=["POST"])
def post_register():
    assert LOGIN is None
    params = request.values if request.json is None else request.json
    if "user" not in params or "pass" not in params:
        return "missing parameter", 404
    # FIXME should handle an existing user and respond appropriately
    insert_new_user_with_hashed_pass(params["user"], auth.hash_password(params["pass"]))
    return "", 201

For token authentication, a token can be created on a path authenticated by one of the other methods. The code for that would be as simple as:

# token creation route
@app.route("/login", methods=["GET"])
def get_login():
    return jsonify(auth.create_token(LOGIN)), 200

The client application will return the token as a parameter for authenticatiing later requests, till it expires.

The main configuration directive is FSA_TYPE which governs authentication methods used by the get_user function, as described in the following sections:

httpd Authentication

Inherit web server supplied authentication through request.remote_user. This is the default.

There are plenty authentication schemes available in a web server such as Apache or Nginx, all of which probably more efficiently implemented than python code, so this should be the preferred option. However, it could require significant configuration effort compared to the application-side approach.

basic Authentication

HTTP Basic password authentication.

See also Password Authentication below for how the password is retrieved.

param Authentication

HTTP parameter or JSON password authentication.

The following configuration directives are available:

• FSA_PARAM_USER parameter name for the user name. Default is USER.
• FSA_PARAM_PASS parameter name for the password. Default is PASS.

See also Password Authentication below for how the password is retrieved.

password Authentication

Tries basic then param authentication.

token Authentication

Only rely on signed tokens for authentication. A token certifies that a user is authenticated up to some time limit. The token syntax is: <realm>:<user>:<limit>:<signature>, for instance: kiva:calvin:20210221160258:4ee89cd4cc7afe0a86b26bdce6d11126. The limit is an easily parsable UTC timestamp YYYYMMDDHHmmSS.

The following configuration directives are available:

• FSA_TOKEN_REALM realm of token. Default is the simplified lower case application name.
• FKA_TOKEN_NAME name of parameter holding the auth token. Default is auth.
• FSA_TOKEN_SECRET secret string used for signing tokens. Default is a system-generated random string containing 128 bits. This default with only work with itself, as it cannot be shared across server instances.
• FSA_TOKEN_DELAY number of minutes a token validity. Default is 60 minutes.
• FSA_TOKEN_GRACE number of minutes of grace time for token validity. Default is 0 minutes.
• FSA_TOKEN_HASH hash algorithm used to sign the token. Default is blake2s.
• FSA_TOKEN_LENGTH number of hash bytes kept for token signature. Default is 32.

Function create_token(user) creates a token for the user.

Note: token authentication is always attempted unless the secret is empty. Setting FSA_TYPE to token results in only token auth to be used.

fake Authentication

Trust a parameter for authentication claims. Only for local tests, obviously. This is inforced.

The following configuration directive is available:

• FSA_FAKE_LOGIN name of parameter holding the user name. Default is LOGIN.

Password Authentication (param or basic)

For checking passwords the password (hash) must be retrieved through get_user_password(user). This function must be provided by the application.

The following configuration directives are available to configure passlib password checks:

• FSA_PASSWORD_SCHEME password scheme to use for passwords. Default is bcrypt.
• FSA_PASSWORD_OPTIONS relevant options (for passlib.CryptContext). Default is {'bcrypt__default_rounds': 4}.

Beware that modern password checking is often pretty expensive in order to thwart password cracking if the hashed passwords are leaked, so that you do not want to have to use that on every request in real life (eg hundreds milliseconds for passlib bcrypt 12 rounds). These defaults result in manageable password checks of a few milliseconds.

Function hash_password(pass) computes the password salted digest compatible with the configuration.

authorize Decorator

The decorator expects a list or possibly one group identifier. A group identifier can be either a name or a number. When several groups are specified, any will allow the operation to proceed.

# group ids
ADMIN, READ, WRITE = 1, 2, 3

@app.route("/some/place", methods=["POST"])
@auth.authorize([ ADMIN, WRITE ])
def post_some_place():
    …

The check will call user_in_group function to check whether the authenticated user belongs to any of the authorized groups.

Note that this simplistic model does is not enough for non-trivial applications, where permissions on objects often depend on the object owner. For those, careful per-operation authorization will still be needed.

Versions

Sources are available on GitHub.

1.2.0

Add grace time for auth token validity. Some code refactoring.

1.1.0

Add after request module cleanup.

1.0.0

Add authorize decorator. Add password authentication scheme. Improved documentation.

0.9.0

Initial release in beta.

TODO

Features

• better control which schemes are attempted?
• add support for JWT?

Implementation

• should it be an object instead of a flat module?
• expand tests

How not to forget autorizations?

• set a autorization_checked variable to False before the request
• reset it to True when autorization is checked
• check whether it was done and possibly abort after the request