Business logic vulnerabilities in Hug-based APIs arise when developers trust client-provided state or bypass server-side verification of critical workflows. In Hug, the ease of parameter mapping often leads to IDOR or parameter tampering if the developer fails to implement a server-side source of truth. As a researcher, you must ensure that state transitions, pricing, and authorization checks are hard-coded to backend logic rather than relying on incoming request variables.

The Vulnerable Pattern

import hug
INSECURE: Trusting the client to define the price and quantity without server-side lookup
@hug.post(‘/purchase’)
def purchase_item(item_id: int, price: float, quantity: int):
total = price * quantity
# Logic to charge the user follows…
return {‘status’: ‘success’, ‘charged’: total, ‘item_id’: item_id}

The Secure Implementation

The vulnerable code allows 'Parameter Tampering'. An attacker can intercept the request and modify the 'price' field to 0.01, bypassing the intended business logic. The secure version removes the 'price' parameter from the user input entirely. Instead, it uses the 'item_id' to look up the authoritative price from a secure backend database. This ensures that the business invariant (the price of the item) cannot be manipulated by the end-user.

import hug
from my_database import get_item_by_id
SECURE: Fetching the source of truth from the database based on a validated ID
@hug.post(‘/purchase’)
def purchase_item(item_id: int, quantity: int):
item = get_item_by_id(item_id)
if not item:
return {‘error’: ‘Invalid item’}, 400
# Enforce server-side pricing
actual_price = item['price']
total = actual_price * quantity

# Logic to charge the user follows...
return {'status': 'success', 'charged': total}</code></pre>