Mass Assignment in Fiber with Jwt Tokens

Mass assignment occurs when an API binds incoming request data directly to a data model or struct without filtering which fields can be set. In the Go Fiber framework, this commonly happens when developers use c.BodyParser(&myStruct) or c.ParamsParser(&myStruct) to map JSON or form values into a struct that contains sensitive fields such as Role, IsAdmin, or Permissions. When JWT tokens are used for identity verification, the risk is compounded because the token payload is often decoded into a claims struct that may itself contain authorization attributes. If those claims are merged or copied into a data model via mass assignment, an attacker can supply extra form fields or JSON keys that overwrite critical authorization properties, effectively escalating privileges.

Consider a typical authentication flow where a protected endpoint validates a JWT token and then updates a user profile. The token payload may include a sub claim for the user ID, and the developer might decode the token into a Claims struct. Later, they might bind the incoming request body to a UserUpdate struct that also contains an Role field. Because Fiber’s parameter binding is convenience-oriented, it will populate any field present in the request that matches a struct key, including Role, unless explicitly restricted. If the developer inadvertently uses the same struct for both token claims and user updates, or copies values from claims into the update struct via assignment, they may allow a user to modify their own role by including "role": "admin" in the request body.

This becomes an authorization bypass when the JWT token is trusted but the request data is not. For example, an authenticated user sends a PATCH request to /users/me with a JSON body that includes both profile fields and {"Role": "super_admin"}. If the server deserializes this into a struct that is later used to update the database, the mass assignment vulnerability enables privilege escalation. The presence of the JWT token gives the request legitimacy in the eyes of the server, but the token’s contents should not be conflated with safe update payloads. Attack patterns like this align with OWASP API Top 10 A01:2023 — Broken Object Level Authorization, where attackers manipulate object references to access or modify unauthorized data.

Additionally, when using JWT tokens, developers sometimes place claims into context values and later reuse those values in business logic without validating which fields are intended for authorization versus which are user-controlled input. If a handler copies claims into a domain object via a loop or manual assignment and also performs mass assignment from the request, the boundary between trusted and untrusted data blurs. This increases the likelihood that an attacker can inject unexpected keys through nested structures or JSON pointers, especially when the API supports PATCH or dynamic updates. Real-world examples include CVE patterns where role or permission fields are inadvertently writable due to broad struct binding rules.

To detect this during a scan, middleBrick runs security checks that examine binding behavior and compare it against declared struct fields, looking for signs that untrusted input can overwrite sensitive authorization attributes. The tool also inspects JWT handling code paths to identify places where decoded claims might be merged with request payloads. Findings are mapped to compliance frameworks such as OWASP API Top 10 and SOC2, providing prioritized remediation guidance rather than attempting to automatically patch the code.

Remediation centers on strict separation of concerns: keep JWT claims distinct from data model binding, and explicitly define which fields are allowed for updates. Instead of binding directly to a broad struct, use selective parsing or a whitelist approach. Below are concrete, working Fiber code examples that demonstrate secure patterns.

Example 1: Separate token validation and explicit field mapping

Validate the JWT token and extract claims into a dedicated struct, then bind the request to a different struct that excludes sensitive fields.

package main

import (
	"github.com/gofiber/fiber/v2"
	"github.com/gofiber/fiber/v2/middleware/jwt"
)

type Claims struct {
	UserID string `json:"sub"`
	Role   string `json:"role"`
}

type UserUpdateSafe struct {
	Email string `json:"email"`
	Name  string `json:"name"`
}

func UpdateProfile(c *fiber.Ctx) error {
	// Validate JWT and retrieve claims
	claims := jwt.FromCtx(c).GetClaims()
	claimsStruct := claims.(Claims)

	// Explicitly bind only safe fields
	var update UserUpdateSafe
	if err := c.BodyParser(&update); err != nil {
		return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": err.Error()})
	}

	// Use claimsStruct.UserID to ensure users can only update their own account
	// Do not copy Role or other authorization fields from claims into update struct
	// Proceed with update using update.Email, update.Name, and claimsStruct.UserID
	return c.JSON(fiber.Map{"status": "ok"})
}

Example 2: Using a copy function to avoid accidental field overwrite

Do not assign claims directly to a model that is also used for mass assignment. Use explicit copying only for allowed fields.

func ApplyProfileUpdate(userModel *User, update UserUpdateSafe) {
	// Explicitly map only intended fields
	if update.Email != "" {
		userModel.Email = update.Email
	}
	if update.Name != "" {
		userModel.Name = update.Name
	}
	// Do NOT copy Role, IsAdmin, or other sensitive fields from update or from claims
}

Example 3: Strict binding with only tags to ignore sensitive fields

Fiber supports binding options that can help reduce risk when used carefully. Combine with manual validation.

import "github.com/valyala/fasthttp"

type UserProfile struct {
	Email string `json:"email" form:"email"`
	Name  string `json:"name" form:"name"`
	Role  string `json:"-"` // ignored by binding
}

func SafeProfile(c *fiber.Ctx) error {
	var profile UserProfile
	if err := c.BodyParser(&profile); err != nil {
		return c.SendStatus(fiber.StatusBadRequest)
	}
	// Role is not populated from request
	return c.JSON(fiber.Map{"email": profile.Email, "name": profile.Name})
}

In all cases, ensure that JWT validation occurs before any binding, and that authorization checks (e.g., verifying that the user ID in the token matches the resource being modified) are performed separately. Avoid using a single struct for both authentication claims and update payloads, and prefer explicit field assignment over broad assignment patterns.

middleBrick scans can identify places where JWT claims and request binding overlap in risky ways, flagging potential mass assignment vectors. The tool does not fix the code but provides remediation guidance, such as separating claim handling from model binding and using strict field whitelists.