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Approov QuickStart - Python Flask Token Check

Approov is an API security solution used to verify that requests received by your backend services originate from trusted versions of your mobile apps.

This repo implements the Approov server-side request verification code with the Python Flask framework in a simple Hello API server, which performs the verification check before allowing valid traffic to be processed by the API endpoint.

Originally this repo was just to show the Approov token integration example on a Python 3 Flask API as described in the article: Approov Integration in a Python Flask API, that you can still find at /servers/shapes-api.

Approov Integration Quickstart

The quickstart was tested with the following Operating Systems:

  • Ubuntu 20.04
  • MacOS Big Sur
  • Windows 10 WSL2 - Ubuntu 20.04

First, setup the Approov CLI.

Now, register the API domain for which Approov will issues tokens:

approov api -add api.example.com

NOTE: By default a symmetric key (HS256) is used to sign the Approov token on a valid attestation of the mobile app for each API domain it's added with the Approov CLI, so that all APIs will share the same secret and the backend needs to take care to keep this secret secure.

A more secure alternative is to use asymmetric keys (RS256 or others) that allows for a different keyset to be used on each API domain and for the Approov token to be verified with a public key that can only verify, but not sign, Approov tokens.

To implement the asymmetric key you need to change from using the symmetric HS256 algorithm to an asymmetric algorithm, for example RS256, that requires you to first add a new key, and then specify it when adding each API domain. Please visit Managing Key Sets on the Approov documentation for more details.

Next, enable your Approov admin role with:

eval `approov role admin`

For the Windows powershell:

set APPROOV_ROLE=admin:___YOUR_APPROOV_ACCOUNT_NAME_HERE___

Now, get your Approov Secret with the Approov CLI:

approov secret -get base64

Next, add the Approov secret to your project .env file:

APPROOV_BASE64_SECRET=approov_base64_secret_here

Now, add to your requirements.txt file the JWT dependency:

PyJWT==1.7.1 # update the version to the latest one

Next, you need to install the dependencies:

pip3 install -r requirements.txt

Now, add this code to your project, just before your first API endpoint:

from flask import Flask, jsonify, request, abort, g, make_response

# @link https://github.com/jpadilla/pyjwt/
import jwt
import base64
import hashlib

# @link https://github.com/theskumar/python-dotenv
from dotenv import load_dotenv, find_dotenv
load_dotenv(find_dotenv(), override=True)
from os import getenv

api = Flask(__name__)

# Token secret value obtained with the Approov CLI tool:
#  - approov secret -get
approov_base64_secret = getenv('APPROOV_BASE64_SECRET')

if approov_base64_secret == None:
    raise ValueError("Missing the value for environment variable: APPROOV_BASE64_SECRET")

APPROOV_SECRET = base64.b64decode(approov_base64_secret)

@api.before_request
def _verifyApproovToken():
    approov_token = request.headers.get("Approov-Token")

    # If we didn't find a token, then reject the request.
    if approov_token is None or approov_token == "":
        # You may want to add some logging here.
        return abort(make_response({}, 401))

    try:
        # Decode the Approov token explicitly with the HS256 algorithm to
        # avoid the algorithm None attack.
        g.approov_token_claims = jwt.decode(approov_token, APPROOV_SECRET, algorithms=['HS256'])

        # When doesn't occur an exception we have a valid Aproov Token

    except jwt.ExpiredSignatureError as e:
        # You may want to add some logging here.
        return abort(make_response({}, 401))

    except jwt.InvalidTokenError as e:
        # You may want to add some logging here.
        return abort(make_response({}, 401))

    except:
        return abort(make_response({}, 401))


#@api.route("/")
#def hello():
#    return jsonify({"message": "Hello World"})

NOTE: When the Approov token validation fails we return a 401 with an empty body, because we don't want to give clues to an attacker about the reason the request failed, and you can go even further by returning a 400.

Using the before_request decorator approach will ensure that all endpoints in your API will be protected by Approov.

Not enough details in the bare bones quickstart? No worries, check the detailed quickstarts that contain a more comprehensive set of instructions, including how to test the Approov integration.

More Information

System Clock

In order to correctly check for the expiration times of the Approov tokens is very important that the backend server is synchronizing automatically the system clock over the network with an authoritative time source. In Linux this is usually done with a NTP server.

Issues

If you find any issue while following our instructions then just report it here, with the steps to reproduce it, and we will sort it out and/or guide you to the correct path.

Useful Links

If you wish to explore the Approov solution in more depth, then why not try one of the following links as a jumping off point: