Mass Assignment in Buffalo with Dynamodb

Mass Assignment occurs when a web framework binds user-supplied input directly to a model or struct fields without filtering allowed fields. In Buffalo, developers often bind HTTP request parameters (e.g., JSON or form values) into structs that map to AWS DynamoDB item attributes. If the struct contains fields that should never be set by the client (such as Admin, Role, IsActive, or internal identifiers), and these fields are not explicitly excluded during binding, an attacker can provide them in requests to escalate privileges or alter behavior.

Buffalo does not provide automatic field filtering when binding to structs; it relies on the developer to whitelist fields (e.g., via pop.Binder options or manual assignment). When those structs are saved to DynamoDB using the AWS SDK for Go, any over-permissive binding writes unintended attributes into the item. This becomes a BOLA/IDOR-like issue when combined with DynamoDB’s key-based access model: a user might modify their own partition key but also inject attributes that change authorization checks performed server-side.

DynamoDB-specific exposure arises because the database itself does not enforce schema-level field restrictions; it stores a map of attribute-value pairs. If an attacker sets attributes like created_by or payment_status that your application logic trusts, the write succeeds and later reads may return poisoned data. For example, binding a JSON payload that includes "is_admin": true to a Buffalo handler struct and then calling dynamodb.PutItem can change an account’s authorization state without any server-side validation.

Consider a handler that creates a user profile:

// models/user.go
type User struct {
    UserID   string `json:"user_id"`
    Email    string `json:"email"`
    Username string `json:"username"`
    Role     string `json:"role"`     // sensitive, should not be client-set
    Active   bool   `json:"active"`   // sensitive
}

// handlers/users.go
func UsersCreate(c buffalo.Context) error {
    user := &User{}
    if err := c.Bind(user); err != nil {
        return errors.WithStack(err)
    }
    // Convert User to DynamoDB attribute value map
    av, err := dynamodbattribute.MarshalMap(user)
    if err != nil {
        return errors.WithStack(err)
    }
    _, err = db.Session().PutItem(&dynamodb.PutItemInput{
        TableName: aws.String("users"),
        Item:      av,
    })
    return errors.WithStack(err)
}

If the client sends {"email": "[email protected]", "username": "alice", "role": "admin", "active": true}, the Role and Active fields are populated from user input, leading to privilege escalation. Because the struct is bound directly and then marshaled to DynamoDB attribute values, there is no intermediate validation layer by default.

To detect this with middleBrick, a scan against your Buffalo + DynamoDB endpoint can uncover missing field filtering and improper binding practices, surfacing findings mapped to the OWASP API Top 10 Mass Assignment category and providing remediation guidance tailored to Go struct binding patterns.

Remediation focuses on whitelisting fields during binding and ensuring sensitive fields are set server-side, never derived from client input. Use Buffalo’s pop.Binder with exclude tags or manually assign only safe fields after binding. Additionally, validate and sanitize before constructing DynamoDB attribute maps.

Approach 1: Use pop.Binder with exclude tags

Define a separate request struct that omits sensitive fields and use it for binding. Then map to your domain model on the server side.

// models/user.go (domain model, keep sensitive fields)
type User struct {
    UserID   string `json:"user_id"`
    Email    string `json:"email"`
    Username string `json:"username"`
    Role     string `json:"role"`
    Active   bool   `json:"active"`
}

// handlers/users_request.go (safe binding model)
type UserCreateRequest struct {
    Email    string `json:"email" validate:"required,email"`
    Username string `json:"username" validate:"required,min=3,max=32"`
}

// handlers/users.go
func UsersCreate(c buffalo.Context) error {
    req := &UserCreateRequest{}
    if err := c.Bind(req); err != nil {
        return errors.WithStack(err)
    }
    // Validate using govalidator or similar
    verrs := validate.Run(req)
    if verrs != nil && verrs.HasAny() {
        return errorx.Erros(verrs)
    }

    // Server-side defaults and sensitive assignments
    user := &User{
        Email:    req.Email,
        Username: req.Username,
        Role:     "user",        // default, not from client
        Active:   true,          // server-controlled initial state
    }

    av, err := dynamodbattribute.MarshalMap(user)
    if err != nil {
        return errors.WithStack(err)
    }
    _, err = db.Session().PutItem(&dynamodb.PutItemInput{
        TableName: aws.String("users"),
        Item:      av,
    })
    return errors.WithStack(err)
}

Approach 2: Manual field filtering after Bind

If you must bind directly to the domain struct, clear sensitive fields after binding before marshaling.

func UsersCreate(c buffalo.Context) error {
    user := &User{}
    if err := c.Bind(user); err != nil {
        return errors.WithStack(err)
    }
    // Remove any client-supplied sensitive values
    user.Role = "user"
    user.Active = true

    av, err := dynamodbattribute.MarshalMap(user)
    if err != nil {
        return errors.WithStack(err)
    }
    _, err = db.Session().PutItem(&dynamodb.PutItemInput{
        TableName: aws.String("users"),
        Item:      av,
    })
    return errors.WithStack(err)
}

Additional DynamoDB hardening

• Use ConditionExpression with attribute existence checks to prevent clients from injecting unexpected attributes that could bypass conditional logic.
• Apply IAM policies that restrict which attributes can be written (e.g., deny writes to role or is_admin if possible at the table level via fine-grained access policies).
• Validate attribute types on the server (e.g., ensure active is boolean) before marshaling to avoid type confusion when DynamoDB returns items.

These patterns reduce the attack surface for Mass Assignment and align with secure coding practices for Go services interfacing with DynamoDB. middleBrick can validate your endpoints for missing field filtering and improper binding by running a scan; findings include references to CWE-915 (Improper Control of Modification of Object Prototype Attributes) and actionable remediation steps.

For teams using the Pro plan, continuous monitoring and GitHub Action integration can fail CI/CD pipelines if a scan detects mass assignment risks in your API definitions, helping catch regressions before deployment.