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Automated Security Protocol

How to fix Mass Assignment
in Dart Frog

Executive Summary

Mass Assignment in Dart Frog occurs when an application takes user-provided JSON and maps it directly to a database model or internal state without filtering. In Dart Frog's functional routing style, this usually happens when developers use a generic 'fromJson' constructor on an incoming request body. An attacker can inject unexpected fields—like 'is_admin', 'permissions', or 'balance'—to escalate privileges or manipulate data that should be read-only.

The Vulnerable Pattern

VULNERABLE CODE
// routes/profile/update.dart
Future onRequest(RequestContext context) async {
  final body = await context.request.json() as Map;

// VULNERABLE: Blindly updating the user object with whatever the client sent.
// If the client sends {“is_premium”: true}, this code will process it.
final user = User.fromJson(body);
await database.users.update(user);


return Response.json(body: user.toJson());
}

The Secure Implementation

The fix involves decoupling the external API contract from the internal data model. By manually extracting fields or using a dedicated 'UpdateDTO' class, you create a whitelist of allowed attributes. Even if an attacker sends extra keys in the JSON payload, the Dart Frog handler ignores them because they are not explicitly mapped to the 'copyWith' method or the update logic. This ensures that sensitive fields like 'id', 'role', or 'created_at' remain immutable from the perspective of the client.

SECURE CODE
// routes/profile/update.dart
Future onRequest(RequestContext context) async {
  final body = await context.request.json() as Map;
  final currentUser = context.read();

// SECURE: Explicitly extract only the fields the user is allowed to change.
// Use a DTO (Data Transfer Object) or manual mapping to enforce a whitelist.
final allowedUpdate = {
‘displayName’: body[‘displayName’] as String?,
‘bio’: body[‘bio’] as String?,
};


final updatedUser = currentUser.copyWith(
displayName: allowedUpdate[‘displayName’] ?? currentUser.displayName,
bio: allowedUpdate[‘bio’] ?? currentUser.bio,
);


await database.users.save(updatedUser);
return Response.json(body: updatedUser.toJson());
}

System Alert • ID: 4511
Target: Dart Frog API
Potential Vulnerability

Your Dart Frog API might be exposed to Mass Assignment

74% of Dart Frog apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.

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