Mass Assignment in Buffalo with Hmac Signatures

Mass Assignment is a common web security risk where an attacker can set fields that were not intended to be user-controllable. In the Buffalo web framework for Go, this typically occurs during form or JSON binding when input is mapped directly to a model without explicit field allowlisting. When Hmac Signatures are used to protect a request—often to verify authenticity or integrity—developers may assume that the signed payload itself prevents tampering and therefore skip proper input validation. This assumption creates a dangerous gap: the signature confirms the request came from a trusted source, but it does not restrict which fields an authenticated user may set.

Consider a user profile update endpoint that accepts a signed JSON payload including an isAdmin field. If the handler binds the request body into a struct that embeds the user model without using exclude tags or explicit field selection, an attacker who obtains a valid Hmac-signed request (e.g., by replaying or manipulating a legitimate client request) can flip isAdmin to true. The Hmac Signature verifies the payload originated from a trusted client, but it does not prevent privilege escalation because the server trusted the client’s selection of fields. This pattern is especially problematic when the same signing key is used across multiple endpoints or when the signature covers only a subset of the request (e.g., only query parameters), leaving bound body parameters unchecked.

In Buffalo, because models are often bound directly in App.Resource or via pop.Binder, omitting whitelisting mechanisms means any incoming parameter that matches a model column can be mass-assigned. Hmac Signatures do not mitigate this because they operate at the transport or session level rather than at the field-level authorization layer. Real-world attack patterns such as IDOR combined with mass assignment can exploit this to modify other users’ records or escalate privileges. For example, an attacker who can intercept or forge a valid Hmac-signed request may change sensitive fields like subscription_status or billing_role. The OWASP API Top 10 category 1:2023 – Broken Object Level Authorization captures this risk, and the presence of Hmac Signatures does not reduce severity if input validation is missing.

Additional exposure arises when endpoints accept nested structures or embedded relations and bind them without scoping. A developer might sign a payload to prevent tampering but fail to validate that the bound struct does not include unintended associations, such as setting a foreign key that changes ownership. MiddleBrick’s LLM/AI Security checks highlight this by detecting patterns where public endpoints with weak signing are probed for excessive agency or data exfiltration via bound models. The key takeaway is that Hmac Signatures provide integrity and origin verification, but they are not a substitute for explicit field authorization and input validation at the model-binding layer.

To remediate mass assignment when using Hmac Signatures in Buffalo, you must enforce strict field allowlisting at the binding layer, independent of the signature verification. Treat the Hmac signature as a gate that confirms the request’s provenance, and treat model binding as a separate gate that enforces what fields may be set.

Example 1: Using exclude tags to block sensitive fields

Define a dedicated update struct that omits sensitive fields and use it for binding. Keep the model used for verification separate from the model used for updates.

// models/user.go
package models

type User struct {
	ID           uuid.UUID `json:"id"`
	Email        string    `json:"email"`
	IsAdmin      bool      `json:"is_admin"`
	Subscription string    `json:"subscription"`
}

// handlers/users.go — safe update struct
 type UserUpdateParams struct {
	Email        string `json:"email" validate:"required,email"`
	Subscription string `json:"subscription" validate:"required"`
	ExcludeFields []string `param:"exclude" json:"-"`
}

func UpdateUser(c buffalo.Context) error {
	var params UserUpdateParams
	if err := c.Bind(¶ms); err != nil {
		return c.Error(400, err)
	}
	// Verify Hmac signature separately (pseudo-code)
	if !verifyHmac(c.Request()) {
		return c.Error(401, errors.New("invalid signature"))
	}

	user := &models.User{}
	if err := c.DB().Find(user, c.Params().Get("user_id")); err != nil {
		return c.Error(404, err)
	}

	// Apply only allowed fields
	user.Email = params.Email
	user.Subscription = params.Subscription
	// Do not bind IsAdmin from params

	if err := c.DB().Save(user); err != nil {
		return c.Error(500, err)
	}
	return c.Render(200, r.HTML("users.edit.html"))
}

Example 2: Explicit field selection with pop.Binder and validation

Use a custom binder to whitelist fields and reject unknown keys, ensuring Hmac-signed payloads cannot introduce unexpected parameters.

// handlers/users.go — custom binder
func BindUserUpdate(obj *models.User, params map[string]interface{}) error {
	allowed := map[string]bool{
		"email":        true,
		"subscription": true,
	}
	for key := range params {
		if !allowed[key] {
			return errors.New("field not allowed: " + key)
		}
	}
	// Safe manual mapping
	if email, ok := params["email"].(string); ok {
		obj.Email = email
	}
	if sub, ok := params["subscription"].(string); ok {
		obj.Subscription = sub
	}
	return nil
}

func UpdateUser(c buffalo.Context) error {
	var raw map[string]interface{}
	if err := c.Bind(&raw); err != nil {
		return c.Error(400, err)
	}
	if !verifyHmac(c.Request()) {
		return c.Error(401, errors.New("invalid signature"))
	}

	user := &models.User{}
	if err := c.DB().Find(user, c.Params().Get("user_id")); err != nil {
		return c.Error(404, err)
	}

	if err := BindUserUpdate(user, raw); err != nil {
		return c.Error(400, err)
	}

	if err := c.DB().Save(user); err != nil {
		return c.Error(500, err)
	}
	return c.Render(200, r.HTML("users.edit.html"))
}

Example 3: Middleware to enforce signed requests before binding

Add a handler-level middleware that validates the Hmac signature and injects a cleaned parameter map into the context, preventing unchecked binding downstream.

func HmacAuth(next buffalo.Handler) buffalo.Handler {
	return func(c buffalo.Context) error {
		if !verifyHmac(c.Request()) {
			return c.Error(401, errors.New("invalid hmac signature"))
		}
		// Optionally parse and filter body here, then store safe params
		c.Set("safeParams", filteredParams(c.Request()))
		return next(c)
	}
}

// In routes:
app.POST("/users/:user_id", HmacAuth(UpdateUser))

These examples demonstrate that Hmac Signatures verify integrity and origin but do not limit which fields a trusted client can set. By combining signature verification with explicit field allowlisting, struct exclusion tags, and custom binders, you mitigate mass assignment while preserving the integrity checks provided by Hmac Signatures.