Mass Assignment in Chi with Mutual Tls

Mass assignment in Chi occurs when request data (JSON, form values, query parameters) is mapped directly into a struct or a database model without explicit field filtering. When you enable mutual TLS (mTLS) in Chi, the server verifies client certificates during the TLS handshake, which authenticates the client. However, mTLS does not reduce application-layer risks such as mass assignment. An authenticated client with a valid certificate can still send requests that over-permission structs or models, and Chi will happily bind those fields if the code does not explicitly restrict them.

For example, a Chi endpoint that decodes JSON into a user update struct may inadvertently allow a client to set isAdmin or role if those fields are not omitted from the binding. Even with mTLS ensuring the request comes from a trusted client, the server must still enforce authorization at the field level. The combination of mTLS and permissive binding creates a scenario where trust in the client certificate is mistakenly treated as sufficient authorization for sensitive fields, increasing the likelihood of privilege escalation via mass assignment.

Consider a handler that decodes into a struct with sensitive fields and then updates a record without filtering. mTLS ensures the request originates from an authorized client, but it does not ensure the client is allowed to modify every mapped field. The risk is highest when developers assume mTLS replaces input validation and field-level authorization. Real-world parallels include scenarios where an API key or certificate is leaked or when a trusted client is compromised; mass assignment then becomes a direct path to unintended data modification.

To remediate mass assignment in Chi while using mutual TLS, apply explicit field filtering and binding discipline. Do not rely on mTLS to enforce field-level permissions. Use a two-step approach: first validate the client certificate via mTLS (handled by Chi’s underlying HTTP server), then ensure only intended fields are bound and applied to your domain models.

Below is a concrete Chi route example showing safe handling with mTLS context available via request attributes. The example decodes a subset of fields, ignores unknown fields, and merges them into a domain struct without exposing sensitive attributes.

// File: handlers/user_update.go
package handlers

import (
	"context"
	"net/http"

	"github.com/go-chi/chi/v5"
)

// Trusted payload: only fields a client may set.
type UserUpdateInput struct {
	DisplayName string `json:"displayName"`
	Email       string `json:"email"`
}

// Domain model may contain additional fields not part of the input.
type User struct {
	ID           string
	DisplayName  string
	Email        string
	IsAdmin      bool
	Role         string
}

// SafeUpdate builds a User update from a request, applying only allowed fields.
func SafeUpdate(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		// mTLS already verified by the server; client identity is in TLS state.
		// You can inspect request.TLS if needed, but field binding remains separate.
		var input UserUpdateInput
		if err := r.ParseForm(); err != nil {
			http.Error(w, "invalid form", http.StatusBadRequest)
			return
		}
		if err := r.ParseJSON(&input); err != nil {
			http.Error(w, "invalid json", http.StatusBadRequest)
			return
		}

		// Explicitly map only safe fields; do not rely on automatic binding to full User.
		update := User{
			DisplayName: input.DisplayName,
			Email:       input.Email,
		}

		// Here you would apply update to storage, ensuring no mass assignment occurs.
		_ = update // placeholder for service/repository call

		w.WriteHeader(http.StatusOK)
	})
}

func Routes() http.Handler {
	router := chi.NewRouter()
	router.Use(SafeUpdate)
	router.Put("/users/{id}", func(w http.ResponseWriter, r *http.Request) {
		userID := chi.URLParam(r, "id")
		// Perform per-request authorization here, e.g., ensure the authenticated client
		// (via mTLS) has rights to update this user.
		_, _ = userID, w
	})
	return router
}

For JSON payloads, prefer binding to a DTO (data transfer object) that contains only permitted fields, then apply changes to your domain entity. This pattern ensures mass assignment protection irrespective of mTLS presence. In Chi, you can also leverage middleware to attach authorization decisions to the request context, but field filtering must still happen at the binding layer.