FlaskSimpleAuth 4.0.0

Simple authentication, authorization and parameters for Flask, emphasizing configurability

Flask Simple Auth

Simple authentication, authorization, parameter checks and utils for Flask, controled from Flask configuration and the extended route decorator.

Example

The application code below performs authentication, authorization and parameter checks triggered by the extended route decorator. There is no clue in the source about what kind of authentication is used, which is the whole point: authentication schemes are managed in the configuration, not explicitely in the application code. The authorization rule is declared explicitely on each function with the authorize parameter. Path and HTTP/JSON parameters are type checked and converted automatically based on type annotations. Basically, you just have to implement a type-annotated Python function and most of the crust is managed by Flask and FlaskSimpleAuth.

from FlaskSimpleAuth import Flask
app = Flask("demo")
app.config.from_envvar("DEMO_CONFIG")

# users belonging to the "patcher" group can patch "whatever/*"
# the function gets 3 typed parameters: one int coming from the path (id)
# and the remaining two coming from request parameters (some, stuff).
# "some" is mandatory, stuff is optional because it has a default.
@app.route("/whatever/<id>", methods=["PATCH"], authorize="patcher")
def patch_whatever(id: int, some: int, stuff: str = "wow"):
    # ok to do it, with parameters id, some & stuff
    return "", 204

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

FSA_AUTH = "httpd"     # inherit web-serveur authentication
# or others schemes such as: basic, digest, token (eg jwt), param…
# hooks must be provided for retrieving user's passwords and
# checking whether a user belongs to a group, if these features are used.

If the authorize argument is not supplied, the security first approach results in the route to be forbidden (403).

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.

Documentation

This module helps managing authentication, authorizations and parameters in a Flask REST application.

Features

The module provides a wrapper around the Flask class which extends its capabilities for managing authentication, authorization and parameters.

This is intended for a REST API implementation serving a remote client 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 to 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.

Authentication is available through the get_user function. It is performed on demand when the function is called, automatically when checking for permissions in a per-role authorization model, or possibly forced for all/most paths. The module implements inheriting the web-server authentication, password authentication (HTTP Basic, or HTTP/JSON parameters), authentication tokens (custom or JWT passed in headers or as a parameter), and a fake authentication scheme useful for local application testing. It allows to have a login route to generate authentication tokens. For registration, support functions allow to hash new passwords consistently with password checks.

Authorizations are managed by declaring permissions on a route (eg a role name), and relies on a supplied function to check whether a user has this role. This approach is enough for simple authorization management, but would be insufficient for realistic applications where users can edit their own data but not those of others. An additional feature is that the application aborts requests on routes for which there is no explicit authorization declarations, allowing to catch forgotten requirements.

Parameters expected in the request can be declared, their presence and type checked, and they are added automatically as named parameters to route functions, skipping the burden of checking them in typical REST functions. In practice, importing Flask's request global variable is not necessary.

Utils include the convenient Reference class which allows to share for import an unitialized variable, and the CacheOK decorator to memoize true answers (eg for user/group checks).

Install

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

Depending on options, the following modules should be installed:

• passlib for password management.
• bcrypt for password hashing (default algorithm).
• PyJWT for JSON Web Token (JWT).
• cryptography for pubkey-signed JWT.
• Flask HTTPAuth for http-* authentication options.

Initialization

The module is simply initialize by calling its Flask constructor and providing a configuration through FSA_* directives, or possibly by calling some methods to register helper functions.

• a function to retrieve the password hash from the user name.
• a function which tells whether a user is in a group or role.

from FlaskSimpleAuth import Flask
app = Flask("test")
app.config.from_envvar("TEST_CONFIG")

# register hooks

# return password hash if any, or None
@app.get_user_pass
def get_user_pass(user):
    return …

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

# they can also be provided in the Flask configuration with
# - FSA_GET_USER_PASS
# - FSA_USER_IN_GROUP

Once initialized app is a standard Flask object with some additions:

• route decorator, an extended version of Flask's own.
• user_in_group and get_user_pass methods/decorator to register helper functions.
• get_user to extract the authenticated user or raise an AuthException.
• current_user to get the authenticated user if any, or None.
• hash_password and check_password to hash or check a password.
• create_token to compute a new authentication token for the current user.
• clear_caches to clear internal caches.

Alternatively, it is possible to use the flask extension model, in which case the FlaskSimpleAuth object must be instanciated and routes must be created using this object:

from flask import Flask
app = Flask("demo")
app.config.from_envvar("DEMO_CONFIG")

from FlaskSimpleAuth import FlaskSimpleAuth
fsa = FlaskSimpleAuth(app)

# imaginary blueprint registration on the fsa object:
from DemoAdmin import abp
fsa.register_blueprint(abp, url_path="/admin")

# define a route with an optional paramater "flt"
@fsa.route("/users", methods=["GET"], authorize="ALL")
def get_what(flt: str = None):
    …

Authentication

Three directives impact how and when authentication is performed. The main configuration directive is FSA_AUTH which governs authentication methods used by the get_user function, as described in the following sections.

• FSA_AUTH governs the how: none, httpd, basic, param, password, token… as described in details in the next sections. Default is httpd.

  If a non-token single scheme is provided, authentication will be token followed by the provided scheme, i.e. token are tried first anyway.

  To take full control of authentication scheme, provide an ordered list. Note that it does not always make much sense to mix some schemes, e.g. basic and digest password storage assumptions are distinct and should not be merged. Also, only one HTTPAuth-based scheme can be active at a time.

• FSA_MODE tells when to attempt authentication.

  • With always, authentication is performed in a before request hook. Once in a route function, get_user will always return the authenticated user and cannot fail.

  • With lazy, it is performed lazily when needed by an authorization.

  • With all, it is always performed in the hook, which may skip some path because of FSA_SKIP_PATH, and may be re-attempted lazily for path that were skipped.

  On authentication failures 401 is returned. Default is lazy.

• FSA_SKIP_PATH is a list of regular expression patterns which are matched against the request path for skipping systematic authentication when in always mode. Default is empty, i.e. authentication is applied for all paths.

• FSA_CHECK tells whether to generate a 500 internal error if a route is missing an explicit authorization check. Default is True.

• FSA_CACHE_SIZE control size of internal lru caches. Default is 1024. None means unbounded. Disable with 0.

• FSA_401_REDIRECT url to redirect to on 401. Default is None. This can be used for the web application login page.

• FSA_URL_NAME name of parameter for the target URL after a successful login. Default is URL if redirect is activated, else None. Currently, the login page should use this parameter to redirect to when ok.

The authentication scheme attempted on a route can be altered with the auth parameter added to the route decorator. This may be used to restrict the authentication scheme to a subset if those configured globally, and may or may not work otherwise depending on module internals. This feature is best avoided but in very particular cases because it counters a goal of this module which is to remove authentication considerations from the code and put them in the configuration only.

none Authentication

Use to disactivate authentication.

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 Management below for how the password is retrieved and checked.

http-basic Authentication

Same as previous based on flask-HTTPAuth.

Directive FSA_HTTP_AUTH_OPTS allow to pass additional options to the HTTPAuth authentication class.

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

See also Password Management below for how the password is retrieved and checked.

password Authentication

Tries basic then param authentication.

http-digest or digest Authentication

HTTP Digest authentication based on flask-HTTPAuth.

Note that the implementation relies on sessions, which may require the SECRET_KEY option to be set to something. The documentation states that server-side sessions are needed because otherwise the nonce and opaque parameters could be reused, which may be a security issue under some conditions. I'm unsure about that, but I agree that client-side cookie sessions are strange things best avoided if possible.

Directive FSA_HTTP_AUTH_OPTS allow to pass additional options to the HTTPAuth authentication class, such as use_ha1_pw, as a dictionnary.

See also Password Management below for how the password is retrieved and checked. Note that password management is different for digest authentication because the simple hash of the password or the password itself is needed for the verification.

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.

There are two token types chosen with the FSA_TOKEN_TYPE configuration directive: fsa is a compact custom format, and jwt RFC 7519 standard based on PyJWT implementation.

The fsa token syntax is: <realm>:<user>:<limit>:<signature>, for instance: kiva:calvin:20380119031407:4ee89cd4cc7afe0a86b26bdce6d11126. The time limit is an easily parsable UTC timestamp YYYYMMDDHHmmSS so that it can be checked easily by the application client. Compared to jwt tokens, they are easy to interpret and compare manually, no decoding is involved.

The following configuration directives are available:

• FSA_TOKEN_TYPE type of token, either fsa, jwt or None to disable. Default is fsa.
• FSA_TOKEN_CARRIER how to transport the token: bearer (Authentication HTTP header), param, cookie or header.
• FKA_TOKEN_NAME name of parameter or cookie holding the token, or bearer scheme, or header name. Default is auth for param and cookie carrier, Bearer for HTTP Authentication header (bearer carrier), Auth for header carrier.
• FSA_TOKEN_REALM realm of token. Default is the simplified lower case application name. For jwt, this is translated as the audience.
• FSA_TOKEN_SECRET secret string used for validating tokens. Default is a system-generated random string containing 256 bits. This default will only work with itself, as it is not shared across server instances or processes.
• FSA_TOKEN_SIGN secret string used for signing tokens, if different from previous secret. This is only relevant for public-key jwt schemes (R…, E…, P…). Default is to use the previous secret.
• 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_ALGO algorithm used to sign the token. Default is blake2s for fsa and HS256 for jwt.
• FSA_TOKEN_LENGTH number of hash bytes kept for token signature. Default is 16 for fsa. The directive is ignored for jwt.

Function create_token(user) creates a token for the user depending on the current scheme.

Token authentication is always attempted unless the secret is empty. Setting FSA_AUTH to token results in only token authentication to be used.

Token authentication is usually much faster than password verification because password checks are designed to be slow so as to hinder password cracking, whereas token authentication relies on simple hashing for its security. 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.

Token expiration can be understood as a kind of automatic logout, which suggests to choose the delay with some care depending on the use case.

When the token is carried as a cookie, it is automatically updated when 25% of the delay remains, if possible.

Internally jwt token checks are cached so that even with slow public-key schemes the performance impact should be low.

http-token Authentication

Token scheme based on flask-HTTPAuth. Carrier is bearer or header.

Directive FSA_HTTP_AUTH_OPTS allow to pass additional options to the HTTPAuth authentication class, such as header, as a dictionnary.

fake Authentication

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

The following configuration directive is available:

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

Password Management

Password authentication is performed for the following authentication schemes: param, basic, http-basic, http-digest, digest, password.

For checking passwords the password (salted hash) must be retrieved through get_user_pass(user). This function must be provided by the application when the module is initialized.

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

• FSA_PASSWORD_SCHEME password scheme to use for passwords. Default is bcrypt. See passlib documentation for available options. Set to None to disable password checking.
• FSA_PASSWORD_OPTIONS relevant options (for passlib.CryptContext). Default is {'bcrypt__default_rounds': 4, 'bcrypt__default_ident': '2y'}.

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 using tokens to reduce the authentication load on each request.

For digest authentication, the password must be either in plaintext or a simple MD5 hash (RFC 2617), and the authentication setup must be consistent (set use_ha1_pw as True for the later). As retrieving the stored information is enough to steal the password (plaintext) or at least impersonate a user, consider avoiding digest altogether. HTTP Digest Authentication only makes sense for unencrypted connexions, which are a bad practice anyway. It is just provided here for completeness.

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

An opened route for user registration with mandatory parameters could look like that:

# with FSA_SKIP_PATH = (r"/register", …)
@app.route("/register", methods=["POST"], authorize="ANY")
def post_register(user: str, password: str):
    if user_already_exists_somewhere(user):
        return f"cannot create {user}", 409
    add_new_user_with_hashed_pass(user, app.hash_password(password))
    return "", 201

Because password checks are usually expensive, it is advisable to switch to token authentication. A token can be created on a path authenticated by a password method:

# token creation route for all registered users
@app.route("/login", methods=["GET"], authorize="ALL")
def get_login():
    return jsonify(app.create_token(app.get_user())), 200

The client application will return the token as a parameter or in headers for authenticating later requests, till it expires.

Authorization

Role-oriented authorizations are managed through the authorize parameter to the route decorator, which provides a just one or possibly a list of roles authorized to call a route. A role is identified as an integer or a string. The check calls user_in_group(user, group) function to check whether the authenticated user belongs to any of the authorized roles.

There are three special values that can be passed to the authorize decorator:

• ANY declares that no authentication is needed on that route.
• ALL declares that all authenticated user can access this route.
• NONE returns a 403 on all access. It can be used to close a route temporarily. This is the default.

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.

Parameters

Request parameters (HTTP or JSON) are translated automatically to function parameters, by relying on function type annotations. By default, the decorator guesses whether parameters are mandatory based on provided default values, i.e. they are optional when a default is provided.

@app.route("/something/<id>", methods=…, authorize=…)
def do_some_id(id: int, when: date, what: str = "nothing):
    # `id` is an integer path-parameter
    # `when` is a mandatory date HTTP or JSON parameter
    # `what` is an optional string HTTP or JSON parameter
    return …

Request parameter string values are actually converted to the target type. For int, base syntax is accepted for HTTP/JSON parameters, i.e. 0x11, 0o21, 0b10001 and 17 all mean decimal 17. For bool, False is an empty string, 0, False or F, otherwise the value is True. Type path is a special str type which allow to trigger accepting any path on a route.

The required parameter allows to declare whether all parameters must be set (when True), or whether they are optional (False) in which case None values are passed if no defaults are given, or if this is guessed (when None, the default).

The allparams parameter makes all request parameters be translated to named function parameters that can be manipulated as such, as shown below:

@app.route("/awesome", methods=["PUT"], authorize="ALL", allparams=True)
def put_awesome(**kwargs):
    …

A side-effect of passing of request parameters as named function parameters is that request parameter names must be valid python identifiers, which excludes keywords such as pass, def or for, unless passed as keyword arguments.

Custom classes can be used as path and HTTP parameter types, provided that the constructor accepts a string to convert the parameter value to the expected type.

class EmailAddr:
    def __init__(self, addr: str):
        self._addr = addr

@app.route("/mail/<addr>", methods=["GET"], authorize="ALL")
def get_mail_addr(addr: EmailAddr):
    …

Utils

Utilities include the Reference generic object wrapper class and the CacheOK decorator.

Reference Object Wrapper

This class implements a generic share-able global variable which can be used by modules (eg app, blueprints…) with its initialization differed. Under the hood, most methods calls are forwarded to the object stored inside the wrapper, so that the Reference object mostly behaves like the wrapped object. The wrapped object can be reset at will with set. The set method name can be changed with the set_name initialization parameter.

# file Shared.py
from FlaskSimpleAuth import Reference
stuff = Reference()
def init_app(**conf):
    stuff.set(…)

Then in a blueprint:

# file SubStuff.py
from FlaskSimpleAuth import Blueprint
from Shared import stuff

sub = Blueprint(…)

@sub.add("/stuff", authorize="ALL"):
def get_stuff():
    return str(stuff), 200

Then in the app itself:

# file App.py
from FlaskSimpleAuth import Flask
app = Flask(__name__)

from SubStuff import sub
app.register_blueprint(sub, url_prefix="/sub")

# deferred "stuff" initialization
import Shared
Shared.init_app(…)

…

CacheOK Decorator

This decorator memorize the underlying function true answers, but keep trying on false answers. Call cache_clear to reset cache.

@CacheOK
def user_in_group(user, group):
    return …

Versions

Sources are available on GitHub and packaged on PyPI. Software license is public domain.

4.0.0

Port to Flask 2.0, working around a regression on request.values handling. Add support for Flask 2.0 per-method decorator shortcuts get, post, put, delete and patch. Rework documentation. Minor style improvements. Fix all authentication mode.

3.1.1

Tell setup that Flask 2.0 is not yet supported.

3.1.0

Defer password manager setup till it is actually needed, so as to avoid importing passlib for nothing. Do not attempt to re-create a token if it is not possible, i.e. when relying on a third party token provider. Allow to fully control the list of authentication schemes. Allow to control the authentication scheme on a route. Improve test code coverage.

3.0.0

Add FSA_CACHE_SIZE to control caches. Merge FSA_ALWAYS and FSA_LAZY in a single FSA_MODE directive with 3 values: always, lazy and all. Make ANY, ALL and NONE special groups simple strings as well. Package as a one file module (again), and add more files to packaging.

2.5.0

Add header carrier for authentication tokens. Make it work both with internal and HTTPAuth implementations. Force HTTPAuth implementation on http-token.

2.4.1

Fix packaging issue… the python file was missing. Add digest as a synonymous for http-digest. Improve documentation.

2.4.0

Add http-basic, http-digest and http-token authentication schemes based on flask-HTTPAuth. Add coverage report on tests. Distribute as a one file python module. Only simplify realm for fsa tokens. Renew cookies when they are closing expiration.

2.3.0

Use a fully dynamic method for set in Reference. Add a string type. Add caching of get_user_pass and user_in_group helpers. Add clear_caches method. Warn on missing authorize on a route declaration. Add FSA_TOKEN_CARRIER to specify how token auth is transfered, including a new cookie option. Rename FSA_TYPE to FSA_AUTH. Make create_token argument optional. Add WWW-Authenticate headers when appropriate. Set Content-Type to text/plain on generated responses.

2.2.1

Partial fix for method renaming in Reference.

2.2.0

Rename _setobj to set in Reference, with an option to rename the method if needed. Shorten Reference class implementation. Add current_user to FlaskSimpleAuth as well. Add python documentation on class and methods. Fix Reference issue when using several references.

2.1.0

Add Reference any object wrapper class. Add CacheOK positive caching decorator. Add current_user function. Add none authentication type. Add path parameter type. Add more tests.

2.0.0

Make the module as an extension and a full Flask wrapper. Advertise only the extended route decorator in the documentation (though others are still used internally). Change passlib bcrypt version to be compatible with Apache httpd. Allow disabling password checking. Rename FSA_TOKEN_HASH as FSA_TOKEN_ALGO. Disable tokens by setting their type to None. Import Flask session, redirect, url_for, make_response, abort, render_template, current_app objects. Add parameter support for date, time and datetime in iso format. Allow to use any type as path parameters, not just Flask predefined ones. Make blueprints work. Add special path type for parameters taken from the path.

1.9.0

Add bearer authorization for tokens and make it the default. Add JWT tokens, both hmac and pubkey variants. Add 500 generation if a route is missing an authorization declaration. Add convenient route decorator. Add type inference for HTTP/JSON parameters based on default value, when provided. Add type inference for root path parameters based on function declaration.

1.8.1

Fix typo in distribution configuration file.

1.8.0

Merge autoparams and parameters decorators into a single parameters decorator. Make it guess optional parameters based on default values. Fix conversion issues with boolean type parameters. Enhance integer type to accept other base syntaxes. Improve documentation to advertise the simple and elegant approach. Implement decorator with functions instead of a class.

1.7.0

Simplify code. Add FSA_ALWAYS configuration directive and move the authentication before request hook logic inside the module. Add FSA_SKIP_PATH to skip authentication for some paths. Update documentation to reflect this simplified model. Switch all decorators to functions.

1.6.0

Add autoparams decorator with required or optional parameters. Add typed parameters to parameters decorator. Make parameters pass request parameters as named function parameters. Simplify authorize decorator syntax and implementation. Advise authorize then parameters or autoparams decorator order. Improved documentation.

1.5.0

Flask internal tests with a good coverage. Switch to setup.cfg configuration. Add convenient parameters decorator.

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

• test FSA_HTTP_AUTH_OPTS?
• do test digest?
• add any token scheme?
• automate URL-parameter redirect?