Fix Mass Assignment in Sanic
Mass Assignment in Sanic occurs when untrusted user input is directly bound to internal data models or objects without filtering. By injecting unexpected keys into the JSON payload, an attacker can escalate privileges, bypass business logic, or overwrite sensitive fields like 'is_admin', 'role', or 'account_balance'. In Sanic, this typically happens when developers pass 'request.json' directly into a database model's constructor or update method.
The Vulnerable Pattern
from sanic import Sanic, response from models import Userapp = Sanic(“VulnerableApp”)
@app.patch(“/profile/<user_id>”) async def update_profile(request, user_id): user = await User.get(id=user_id) # VULNERABLE: Directly updating the object with raw request data # An attacker can send {“is_admin”: true} to escalate privileges for key, value in request.json.items(): setattr(user, key, value) await user.save() return response.json({“status”: “success”})
The Secure Implementation
The fix involves moving from a 'blacklist' or 'blind-trust' approach to an explicit 'whitelist' approach. By using a validation library like Marshmallow or Pydantic, you define exactly which fields are mutable by the user. The 'EXCLUDE' policy ensures that any extra keys sent by an attacker (like 'is_admin') are silently discarded before they ever touch your data model. This enforces the Principle of Least Privilege at the input layer.
from sanic import Sanic, response
from marshmallow import Schema, fields, EXCLUDE
from models import User
app = Sanic(“SecureApp”)
Define a strict whitelist schema
class ProfileUpdateSchema(Schema):
bio = fields.Str()
display_name = fields.Str()
class Meta:
unknown = EXCLUDE # Drop any keys not explicitly defined
@app.patch(“/profile/<user_id>”)
async def update_profile(request, user_id):
user = await User.get(id=user_id)
# SECURE: Validate and filter input through the schema
schema = ProfileUpdateSchema()
data = schema.load(request.json)
for key, value in data.items():
setattr(user, key, value)
await user.save()
return response.json({"status": "success"})</code></pre>
Your Sanic API
might be exposed to Mass Assignment
74% of Sanic apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.
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