Mass Assignment in Gorilla Mux with Hmac Signatures

Mass Assignment occurs when an API binds incoming JSON or form data directly to a data model, granting an attacker the ability to set fields they should not control. In Gorilla Mux, this risk is amplified when endpoints rely on Hmac Signatures for request authentication but do not enforce strict input binding. A common pattern is to validate the Hmac to confirm request integrity and origin, then decode the payload into a struct using a broad struct tag like json:",omitempty" without explicitly excluding sensitive fields.

Consider a user profile update endpoint that uses Hmac Signatures to verify the request comes from a trusted source. The handler decodes the JSON body into a UserUpdate struct that embeds the full User model. If the Hmac validation passes but the struct does not whitelist fields, an attacker can add extra keys such as is_admin or role in the payload. Those keys will be mapped into the struct and subsequently persisted, enabling privilege escalation even though the request was authenticated with a valid Hmac.

The combination of Gorilla Mux routing, Hmac Signatures for integrity, and unchecked decoding creates a bypass where authentication and integrity checks appear to pass, but authorization controls are missing. Because Hmac Signatures confirm the payload has not been altered in transit, developers may mistakenly assume the data is safe, failing to apply explicit field filtering. This maps to the BFLA/Privilege Escalation and Property Authorization checks in middleBrick’s scan set, where over-permissive bindings are flagged as high-severity findings.

In practice, this can lead to scenarios where an attacker crafts a request with a valid Hmac but with additional form fields that modify ownership or permissions. The scan will detect that the endpoint accepts unbounded input and that the Hmac is verified without corresponding property-level checks, highlighting a gap between integrity verification and authorization enforcement.

Remediation focuses on two layers: strict Hmac validation and precise struct binding. First, ensure the Hmac is computed over a canonical representation of the payload (e.g., the raw request body) and that the timestamp or nonce is included to prevent replay. Second, avoid binding the entire request body to a model that includes sensitive fields; instead, use explicit field-by-field assignment or a filtered struct.

Below is a concrete example of a secure handler using Gorilla Mux with Hmac Signatures. The handler reads the raw body, validates the Hmac, and then decodes only the allowed fields into a dedicated update struct that excludes sensitive attributes like role or permissions.

//go
package main

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"net/http"
	"strings"

	"github.com/gorilla/mux"
)

type ProfileUpdate struct {
	DisplayName string `json:"display_name"`
	Email       string `json:"email"`
}

func verifyHmac(body, receivedHmac, secret string) bool {
	h := hmac.New(sha256.New, []byte(secret))
	h.Write([]byte(body))
	expected := hex.EncodeToString(h.Sum(nil))
	return hmac.Equal([]byte(expected), []byte(receivedHmac))
}

func updateProfileHandler(secret string) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		vars := mux.Vars(r)
		userID := vars["id"]

		bodyBytes, err := io.ReadAll(r.Body)
		if err != nil {
			http.Error(w, `{"error": "invalid body"}`, http.StatusBadRequest)
			return
		}
		defer r.Body.Close()

		receivedHmac := r.Header.Get("X-Hmac-Signature")
		if !verifyHmac(string(bodyBytes), receivedHmac, secret) {
			http.Error(w, `{"error": "invalid signature"}`, http.StatusUnauthorized)
			return
		}

		var incoming ProfileUpdate
		if err := json.Unmarshal(bodyBytes, &incoming); err != nil {
			http.Error(w, `{"error": "invalid json"}`, http.StatusBadRequest)
			return
		}

		// Explicitly map only safe fields; do not embed the full User model
		// Assume a function applyProfileUpdate that uses userID and incoming.DisplayName/Email
		if err := applyProfileUpdate(userID, incoming.DisplayName, incoming.Email); err != nil {
			http.Error(w, `{"error": "update failed"}`, http.StatusInternalServerError)
			return
		}

		w.WriteHeader(http.StatusOK)
		w.Write([]byte(`{"status": "ok"}`))
	}
}

If you must work with a broader struct, use explicit field omission and avoid public fields that an attacker could set. For example, define a request struct with only exported fields you intend to update and omit sensitive fields entirely. Alternatively, decode into a map[string]interface{} and iterate over a whitelist of keys before applying changes.

In a production setup, combine this with middleware that standardizes Hmac validation and ensures replay protection via nonce or timestamp checks. middleBrick’s scan will highlight endpoints where the Hmac is verified but the struct binding remains permissive, guiding you toward the precise fix shown above.