flask-Vouch 1.2.4

Bot-challenge Python middleware issuing brief challenges, granting solvers signed access cookies.

𐌅𐌋𐌀𐌔𐌊-ᕓꝊ𐌵𐌂𐋅

Bot-challenge middleware for Flask intercepts unrecognized visitors, issues proof-of-work or CAPTCHA challenges, and grants HMAC-signed JWT access cookies to solvers.

from flask import Flask
from flask_vouch import Vouch

app = Flask(__name__)
vouch = Vouch(app, secret="change-me")

Bots get a browser challenge page. Humans solve it once, get a cookie, browse freely.

Install

pip install flask-Vouch

Optional extras:

pip install flask-Vouch[image]      # image-based captchas (Pillow, numpy)
pip install flask-Vouch[audio]      # audio captcha (numpy, scipy)

How it works

1. Every suspicious unauthenticated request matching the configured rules is redirected to a challenge page.
2. A proof-of-work challenge (SHA-256 Balloon by default) is issued.
3. The browser solves it in JavaScript and POSTs to /.tollbooth/verify.
4. A valid solution sets a signed JWT cookie subsequent requests pass through.

Quick start

from flask import Flask
from flask_vouch import Vouch

app = Flask(__name__)
vouch = Vouch(app, secret="change-me")

@app.route("/")
def index():
    return "You passed the challenge!"

@app.route("/internal")
@vouch.exempt
def internal():
    return "ok"

Application factory:

vouch = Vouch(secret="change-me")

def create_app():
    app = Flask(__name__)
    vouch.init_app(app)
    return app

SECRET_KEY fallback if no secret= is passed, app.config["SECRET_KEY"] is used automatically:

app.config["SECRET_KEY"] = "change-me"
vouch = Vouch()
vouch.init_app(app)

Configuration

Pass as kwargs or via app.config with the VOUCH_ prefix:

Parameter	Default	Description
secret	SECRET_KEY	HMAC/JWT signing key
policy	default rules	Policy instance
exclude	[]	Path regexes to skip entirely
json_mode	False	Return JSON challenge instead of HTML
cookie_name	_tollbooth	Access cookie name
cookie_ttl	604800	Cookie lifetime in seconds (7 days)
cookie_secure	True	Secure flag; only set over HTTPS
verify_path	/.tollbooth/verify	Challenge verification endpoint
challenge_handler	SHA256Balloon	Challenge implementation
blocklist	None	NetSet instance or list of them

app.config["VOUCH_COOKIE_NAME"] = "_v"
app.config["VOUCH_COOKIE_TTL"] = 3600

Route decorators

Decorator	Behavior
@vouch.exempt	Skip challenge entirely for this route
@vouch.protect	Always run challenge check (overrides global allow)
@vouch.challenge	Always issue a challenge regardless of policy
@vouch.block	Deny detected crawlers outright; challenge or pass others

Custom rules

from flask_vouch import Vouch, Policy, Rule

policy = Policy(
    rules=[
        Rule(name="allow-google", action="allow", user_agent="Googlebot"),
        Rule(name="block-scrapers", action="deny", user_agent="AhrefsBot|SemrushBot"),
        Rule(name="challenge-curl", action="challenge", difficulty=8, user_agent="curl"),
    ]
)

vouch = Vouch(app, secret="s", policy=policy)

Load the built-in ruleset from rules.json:

from flask_vouch import load_policy

vouch = Vouch(app, secret="s", policy=load_policy())

Rule fields:

Field	Type	Description
name	str	Identifier
action	str	allow · deny · challenge · weigh
user_agent	str (regex)	Match on User-Agent header
path	str (regex)	Match on request path
headers	dict	Match on arbitrary headers (regex values)
remote_addresses	list[str]	CIDR ranges to match
difficulty	int	Challenge difficulty (default: policy)
weight	int	Score added when action=weigh
blocklist	bool	Match IPs in the loaded netset
bogon_ip	bool	Match non-global / bogon IPs
crawler	bool	Match detected crawler user agents

Challenge types

from flask_vouch import (
    SHA256Balloon,              # default, proof of work    (SHA-256 balloon hashing)
    SHA256,                     # lightweight SHA-256 PoW
    ChainCaptcha,               # no-interaction iterated-SHA-256 PoW
    CharacterCaptcha,           # text CAPTCHA
    ImageCaptcha,               # image CAPTCHA             (requires [image])
    RotationCaptcha,            # rotation CAPTCHA          (requires [image])
    CupCaptcha,                 # cup fill CAPTCHA          (requires [image])
    SlidingCaptcha,             # sliding puzzle            (requires [image])
    CircleCaptcha,              # circle select CAPTCHA     (requires [image])
    TraceCaptcha,               # curve-trace CAPTCHA       (pure Python, kinematics)
    ImageGridCaptcha,           # image grid CAPTCHA        (requires [image])
    AudioCaptcha,               # audio CAPTCHA             (requires [audio])
    NavigatorAttestation,       # browser signal attestation
    QuirkProbe,                 # browser-engine quirk verification
    ThirdPartyCaptchaChallenge, # embed external CAPTCHAs
)

vouch = Vouch(app, secret="s", challenge_handler=CharacterCaptcha())

IP netset

A netset is a newline-delimited list of IPs, CIDR ranges, or start-end ranges (# comments ignored) the FireHOL ipset/netset format. NetSet loads one from a file path or URL, merges overlapping ranges, and answers membership in O(log n).

from flask_vouch import Vouch, NetSet

ns = NetSet()        # defaults to bundled blocklist.netset URL
ns.load()
ns.start_updates()   # auto-refresh daily in a daemon thread

vouch = Vouch(app, secret="s", blocklist=ns)

Custom source(s) path or URL:

ns = NetSet("https://example.com/bad-ips.netset")
many = NetSet.from_sources(["a.netset", "b.netset"])

vouch = Vouch(app, secret="s", blocklist=[ns1, ns2])

Redis backend

For multi-process / multi-worker deployments:

import redis
from flask_vouch.redis import RedisEngine
from flask_vouch import Vouch

r = redis.Redis()
engine = RedisEngine(r, secret="s")
vouch = Vouch(app, engine=engine)

Extras

ErrorHandler

from flask_vouch.extras import ErrorHandler

eh = ErrorHandler(bouncer=vouch)
eh.init_flask(app)

RateLimiter

from flask_vouch.extras import RateLimiter

rl = RateLimiter(default="100/minute")
rl.init_flask(app)

@app.route("/login")
@rl.limit("5/minute")
def login(): ...

ThirdPartyCaptcha

from flask_vouch.extras import ThirdPartyCaptcha

tpc = ThirdPartyCaptcha(turnstile_site_key="...", turnstile_secret="...")
tpc.init_flask(app)

@app.route("/submit", methods=["POST"])
def submit():
    if not tpc.is_turnstile_valid():
        abort(403)
    ...

Formatting

pip install black isort
isort . && black .
npx prtfm

License

Apache-2.0