Mass Assignment Exploit in Buffalo

Mass assignment is a vulnerability that occurs when an application automatically binds user-supplied input to internal object properties without proper filtering. In Buffalo, this commonly happens when using the built-in c.Bind method to deserialize request data directly into a model struct.

Buffalo's typical pattern in a handler is:

func UsersCreate(c buffalo.Context) error {
  user := &models.User{}
  if err := c.Bind(user); err != nil {
    return c.Error(500, err)
  }
  // ... save user to database
}

The c.Bind method populates the user struct from the request body (JSON, form data, etc.). If the User model includes fields that should not be set by the client (such as IsAdmin, Role, ID, or CreatedAt), an attacker can include these fields in the request and have them blindly accepted.

Example attack: Suppose the User model is defined as:

type User struct {
  ID        uuid.UUID `json:"id" db:"id"`
  Email     string    `json:"email" db:"email"`
  Password  string    `json:"password" db:"password"`
  IsAdmin   bool      `json:"is_admin" db:"is_admin"`
  CreatedAt time.Time `json:"created_at" db:"created_at"`
}

An attacker could send a JSON payload:

{
  "email": "[email protected]",
  "password": "secret",
  "is_admin": true
}

If the handler uses c.Bind(user) without restrictions, the IsAdmin field will be set to true, granting the attacker admin privileges upon user creation. Similarly, during updates (PUT/PATCH), an attacker could modify the IsAdmin flag of existing users.

This vulnerability is not limited to boolean flags; it can also include fields like role, permissions, balance, or any property that influences authorization or business logic. In Buffalo, because c.Bind by default binds all fields present in the request that match struct fields (based on JSON tags or form names), mass assignment becomes a significant risk if developers do not explicitly limit the bound fields.

Impact: Successful mass assignment can lead to privilege escalation (e.g., becoming an admin), unauthorized data modification, bypassing business rules (e.g., setting is_verified=true), or even remote code execution in some cases if dangerous fields like script or command are bound.

Detecting mass assignment vulnerabilities in Buffalo APIs can be done through code review and dynamic testing. Code review involves checking for uses of c.Bind (or similar binding methods) that directly bind to a model struct containing sensitive fields without a whitelist. Look for patterns:

if err := c.Bind(&model); err != nil { ... }

especially when model has fields that should not be client-controlled.

Dynamic testing is more reliable: you can manually send requests with extra parameters and observe the response. However, this is time-consuming and error-prone. This is where automated security scanners like middleBrick come in.

middleBrick is an API security scanner that performs black-box testing of your endpoints. It includes a specific check for mass assignment (under its Property Authorization category). The scanner sends crafted requests to each endpoint, adding fields that are not typically present in normal requests (e.g., is_admin, role, id, user_id). It then analyzes the responses to see if these fields are accepted, stored, or reflected. For example, after creating a user with an extra is_admin field, middleBrick may issue a follow-up request to retrieve the user and check if the flag persisted. It also tests update endpoints by trying to modify sensitive properties.

The scan takes only 5–15 seconds and requires no configuration or credentials. You simply submit your API URL. The resulting report includes a risk score (A–F) and a breakdown per category. If mass assignment is found, it will be listed under Property Authorization with a severity rating (typically High) and specific remediation guidance tailored to Buffalo.

By integrating middleBrick into your development workflow—via the web dashboard, CLI tool, or GitHub Action—you can continuously monitor for such vulnerabilities. For instance, you can add the middleBrick GitHub Action to your CI pipeline to fail builds if a scan detects a mass assignment issue.

Fixing mass assignment in Buffalo requires restricting which fields are bound from the request. Buffalo provides several mechanisms:

1. Whitelist binding with c.Bind: Pass the names of allowed fields as additional arguments to c.Bind. Only those fields will be populated.
   
   Vulnerable:
   

   user := &models.User{}
   if err := c.Bind(user); err != nil {
     return c.Error(500, err)
   }

   Fixed:
   

   user := &models.User{}
   if err := c.Bind(user, "email", "password"); err != nil {
     return c.Error(500, err)
   }

   This ensures that even if the request contains is_admin, it will be ignored.
2. Use separate Data Transfer Objects (DTOs): Define a struct that contains only the fields that are safe to be set by the client. Bind to that struct, then copy the allowed values to the model.
   

   type UserCreateDTO struct {
     Email    string `json:"email"`
     Password string `json:"password"`
   }
   
   func UsersCreate(c buffalo.Context) error {
     dto := &UserCreateDTO{}
     if err := c.Bind(dto); err != nil {
       return c.Error(500, err)
     }
     user := &models.User{
       Email:    dto.Email,
       Password: dto.Password,
     }
     // ... save user
   }

   This approach is more explicit and scales well for complex models.
3. For updates, use partial DTOs or whitelists: When handling PATCH or PUT, be cautious not to allow updates to sensitive fields. Use a DTO that includes only updatable fields, or use the whitelist method with c.Bind and list only the fields that can be changed.
4. Avoid binding directly to models with sensitive fields: If a model contains fields like IsAdmin, Role, ID, etc., it's safer to never bind directly to it. Always use a DTO or whitelist.
5. Additional server-side checks: Even with binding restrictions, it's good practice to validate that the user is authorized to modify the specific fields. Buffalo's validation package can be used to enforce business rules. For example, after binding, you can check if user.IsAdmin is being changed and reject it unless the requester has appropriate privileges.
6. Struct tags for exclusion? Buffalo's c.Bind does not natively support a binding:"-" tag to exclude fields. Therefore, relying on whitelists or DTOs is the recommended approach.

After applying the fix, rescan your API with middleBrick to confirm that the mass assignment vulnerability is no longer detected. The scanner will verify that extra fields are ignored and not persisted.