Mass Assignment in Buffalo with Bearer Tokens

Mass Assignment in the Buffalo web framework occurs when a developer binds incoming request parameters directly to a model without filtering which fields are permitted. This becomes especially risky when Bearer Tokens are used for authentication, because an authenticated request can include unexpected form fields or JSON keys that overwrite sensitive attributes. In Buffalo, a typical handler might call params.Decode(&user) to map request data into a struct. If the struct contains fields such as IsAdmin, ConfirmationToken, or RoleID, and the handler does not whitelist which fields are allowed, an attacker who obtains or guesses a valid Bearer Token can submit crafted JSON or form data to escalate privileges or alter other users' data.

When Bearer Tokens are used, the token itself is often passed in an Authorization: Bearer <token> header, and the token is validated before the request body is processed. If the token is valid, Buffalo may trust the request body implicitly. This trust interaction means that even though authentication (via Bearer Token) is working correctly, the authorization boundary enforced by mass assignment can be bypassed. For example, an authenticated user with a standard token might send a POST or PATCH request that includes { "is_admin": true }. Without explicit parameter filtering, Buffalo can assign that value to the model, unintentionally granting elevated permissions.

The vulnerability is a specific instance of the broader BOLA/IDOR and BFLA/Privilege Escalation checks that middleBrick scans for. An attacker who controls the request body while holding a valid Bearer Token can attempt to modify attributes that should be immutable after creation, such as confirmed_at, activated, or tenant identifiers in multi-tenant schemas. Because Buffalo does not implicitly protect against over-posting, developers must explicitly define which fields are permitted for each context. The risk is compounded when APIs are also exposed via OpenAPI specs that do not accurately reflect read-only or server-controlled fields, as this mismatch allows runtime inputs to diverge from documented expectations.

Real-world attack patterns mirror this behavior: an authenticated request with a Bearer Token sends extra fields that map to sensitive model attributes. If input validation and property authorization are not enforced at the handler level, the server applies the values directly. This can lead to privilege escalation (BFLA), unauthorized data modification, or exposure of sensitive properties, which are among the OWASP API Top 10 and PCI-DSS relevant findings that middleBrick evaluates across its 12 parallel checks.

To detect this during a scan, middleBrick compares the runtime request shapes observed during the unauthenticated-style probe (using the Bearer Token when present) against the expected schema defined in the OpenAPI specification. Discrepancies where writable fields are not explicitly guarded or where per-context allowlists are missing are surfaced with severity and remediation guidance. This helps teams understand that authentication does not imply safe assignment, and that each endpoint must enforce its own property authorization logic.

Remediation focuses on explicit parameter filtering and using Buffalo's built-in mechanisms to control which fields can be mass-assigned. Instead of decoding all parameters into a model, developers should use selective binding or manual assignment for sensitive attributes. Below are concrete, syntactically correct examples for Buffalo handlers using Bearer Tokens.

Example 1: Safe decoding with allowed fields

Define an allowlist of fields that the endpoint is permitted to set, and decode only those fields. This prevents over-posting even when a valid Bearer Token is present.

import "github.com/gobuffalo/buffalo"
import "github.com/gobuffalo/packr/v2"
import "github.com/gobuffalo/validate/v3"

// User model fields: ID, Email, Name, IsAdmin, ConfirmationToken
func updateUser(c buffalo.Context) error {
    // Only these fields are safe to assign from user input
    allowed := []string{"email", "name"}

    var input struct {
        Email           string `json:"email"`
        Name            string `json:"name"`
        IsAdmin         bool   `json:"is_admin"`         // not allowed
        ConfirmationToken string `json:"confirmation_token"` // not allowed
    }

    if err := c.Bind(&input); err != nil {
        return c.Render(400, r.JSON(map[string]string{"error": err.Error()}))
    }

    // Manually assign only allowed fields
    user := &User{}
    if err := c.Session().Find(user, c.Params().Get("user_id")); err != nil {
        return c.Render(404, r.JSON(map[string]string{"error": "not found"}))
    }

    if contains(allowed, "email") {
        user.Email = input.Email
    }
    if contains(allowed, "name") {
        user.Name = input.Name
    }

    if err := user.Validate(); err != nil {
        return c.Render(422, r.JSON(validatorErrorMap(err)))n    }

    if err := c.Session().Update(user); err != nil {
        return c.Render(500, r.JSON(map[string]string{"error": "update failed"}))
    }

    return c.Render(200, r.JSON(user))
}

func contains(slice []string, item string) bool {
    for _, s := range slice {
        if s == item {
            return true
        }
    }
    return false
}

Example 2: Using Buffalo Params with explicit selection

Bind to a DTO (data transfer object) that only includes safe fields, then map to the model. This keeps handler logic clean and avoids accidental assignment of sensitive fields when a Bearer Token is validated earlier in the middleware stack.

import "github.com/gibuffalo/buffalo"

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

func updateUserSafe(c buffalo.Context) error {
    var dto UserUpdateDTO
    if err := c.Bind(&dto); err != nil {
        return c.Render(400, r.JSON(map[string]string{"error": "invalid payload"}))
    }

    user := &User{}
    if err := c.Session().Find(user, c.Params().Get("user_id")); err != nil {
        return c.Render(404, r.JSON(map[string]string{"error": "not found"}))
    }

    user.Email = dto.Email
    user.Name = dto.Name
    // Do not assign IsAdmin or ConfirmationToken from the DTO

    if err := c.Session().Update(user); err != nil {
        return c.Render(500, r.JSON(map[string]string{"error": "update failed"}))
    }

    return c.Render(200, r.JSON(user))
}

Example 3: Middleware and token-aware validation

When Bearer Tokens are validated in middleware, ensure that subsequent handlers do not assume all claims are safe to assign. Combine token validation with per-endpoint property authorization to mitigate risk.

import "github.com/gobuffalo/buffalo"

func RequireBearerToken(next buffalo.Handler) buffalo.Handler {
    return func(c buffalo.Context) error {
        auth := c.Request().Header.Get("Authorization")
        if auth == "" || !isValidBearerToken(auth) {
            return c.Render(401, r.JSON(map[string]string{"error": "unauthorized"}))
        }
        // Optionally attach token claims to context for later use
        return next(c)
    }
}

func isValidBearerToken(auth string) bool {
    // Simplified stub: real implementation validates signature/expiry
    return len(auth) > 7 && auth[:7] == "Bearer "
}

These examples emphasize that authentication via Bearer Tokens does not equate to safe mass assignment. By explicitly defining allowed fields and avoiding automatic decoding into models with sensitive attributes, developers can prevent privilege escalation and data exposure even when a valid token is present. These practices align with the checks performed by middleBrick, which flags missing property authorization and overly permissive parameter binding.