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Fix Mass Assignment in Gorilla

Mass Assignment in Gorilla/mux-based Go applications occurs when raw request bodies are decoded directly into internal domain models or database structs. If your struct includes sensitive fields like 'IsAdmin', 'Balance', or 'Role', an attacker can include these in their JSON payload to overwrite data they shouldn't touch. This is a classic 'over-posting' vulnerability that leads to privilege escalation.

The Vulnerable Pattern

type User struct {
    ID       int    `json:"id"` 
    Username string `json:"username"` 
    IsAdmin  bool   `json:"is_admin"` 
}

func UpdateUserHandler(w http.ResponseWriter, r *http.Request) {
var user User
// VULNERABLE: Direct binding to the DB model
// An attacker sends {“username”: “hacker”, “is_admin”: true}
if err := json.NewDecoder(r.Body).Decode(&user); err != nil {
http.Error(w, err.Error(), http.StatusBadRequest)
return
}
db.Model(&user).Updates(user)
}

The Secure Implementation

To kill Mass Assignment, you must decouple your API surface from your database schema. The secure implementation uses a 'DTO' or 'Input Struct' that only contains fields the user is explicitly allowed to change. By decoding the request into this restricted struct, any extra fields injected by an attacker (like 'is_admin') are ignored by the JSON decoder. This creates a hard-coded whitelist, ensuring that internal state remains protected regardless of what the client sends.

type UserUpdateInput struct {
    Username *string `json:"username"` 
}

func UpdateUserHandler(w http.ResponseWriter, r *http.Request) {
// SECURE: Use a DTO (Data Transfer Object) to whitelist allowed fields
var input UserUpdateInput
if err := json.NewDecoder(r.Body).Decode(&input); err != nil {
http.Error(w, err.Error(), http.StatusBadRequest)
return
}


var user User
db.First(&user, userId)

if input.Username != nil {
    user.Username = *input.Username
}

db.Save(&user)


}

System Alert • ID: 3542
Target: Gorilla API
Potential Vulnerability

Your Gorilla API might be exposed to Mass Assignment

74% of Gorilla apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.

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