Simple authentication for Flask, emphasizing configurability
Project description
Flask Simple Auth
Simple authentication and authorization 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 has this role. 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 authentication scheme is being used, so switching between schemes only impacts the configuration, not the application code.
Very Simple Example
The application code below performs authentication on authorization checks
triggered by the authorize
decorator.
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)
# users belonging to the patcher group can patch whatever:
@app.route("/whatever", methods=["PATCH"])
@auth.authorize("patcher")
def patch_whatever():
# ok to do it, then
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.
Simple Example
The application code extract below maintains a LOGIN
global variable which
holds the authenticated user name for the current request.
This allows more complex application-level permission management.
# app and auth are initialized as in the previous example, then:
# mandatory authentication for all path
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)
# authorization is checked 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, then
return "", 204
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
triesbasic
thenparam
. -
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 other 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, without impact on the application code.
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 which maintains a session 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 or role.
# 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
add_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, which rely on the Authorization
HTTP header in the request.
See also Password Authentication below for how the password is retrieved and checked.
param
Authentication
HTTP parameter or JSON password authentication. User name and password are passed as request parameters.
The following configuration directives are available:
FSA_PARAM_USER
parameter name for the user name. Default isUSER
.FSA_PARAM_PASS
parameter name for the password. Default isPASS
.
See also Password Authentication below for how the password is retrieved and checked.
password
Authentication
Tries basic
then param
authentication.
token
Authentication
Only rely on signed tokens for authentication.
A token certifies that a user is authenticated in a realm up to some
time limit.
The token is authenticated by a signature which is the hash of the payload
(realm, user and limit) and a secret hold by the server.
The token syntax is: <realm>:<user>:<limit>:<signature>
,
for instance: kiva:calvin:20210221160258:4ee89cd4cc7afe0a86b26bdce6d11126
.
The time limit is an easily parsable UTC timestamp YYYYMMDDHHmmSS so that
it can be checked easily by the application client.
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 isauth
.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 is not shared across server instances or processes. Set toNone
to disable tokens.FSA_TOKEN_DELAY
number of minutes of 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 isblake2s
.FSA_TOKEN_LENGTH
number of hash bytes kept for token signature. Default is 16.
Function create_token(user)
creates a token for the user.
Note that token authentication is always attempted unless the secret is empty.
Setting FSA_TYPE
to token
results in only token authentication to be used.
Also note that token authentication is usually much faster than password verification because password checks are designed to be slow so as to hinder password cracking. Another benefit of token is that it avoids sending passwords over and over. The rational option is to use a password scheme to retrieve a token and then to use it till it expires.
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 isLOGIN
.
Password Authentication (param
or basic
)
For checking passwords the password (salted 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 isbcrypt
. See passlib documentation for available options.FSA_PASSWORD_OPTIONS
relevant options (forpasslib.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). The above defaults result in manageable password checks of a few milliseconds. Consider enabling tokens to reduce the authentication load on each request.
Function hash_password(pass)
computes the password salted digest compatible
with the current 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, WRITE, READ = 1, 2, 3
@app.route("/some/place", methods=["POST"])
@auth.authorize([ ADMIN, WRITE ])
def post_some_place():
…
The check will call user_in_group(user, group)
function to check whether the
authenticated user belongs to any of the authorized groups.
The following configuration directive is available:
FSA_LAZY
allows theauthorize
decorator to perform the authentication when needed, which mean that the before request hook can be skipped. Default is True.
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 and packaged on PyPI.
1.4.0
Add FSA_LAZY
configuration directive.
Simplify code.
Improve warning on short secrets.
Repackage…
1.3.0
Improved documentation. Reduce default token signature length and default token secret. Warn on random or short token secrets.
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
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