Prototype Pollution in Buffalo with Bearer Tokens

Prototype pollution in Buffalo becomes more critical when requests include Bearer Tokens in headers, because token handling logic often intersects with object merging and deserialization routines. When a Buffalo application deserializes JSON payloads and merges them into prototype objects (e.g., via Object.assign or libraries that shallow-copy configuration), an attacker can inject properties like __proto__, constructor.prototype, or use property paths such as constructor.prototype.authenticated to alter behavior across instances.

If the request also carries an Authorization header with a Bearer Token, the polluted prototype can affect token validation logic. For example, a merged configuration object might incorrectly treat a modified prototype property as a valid token scope or bypass expected checks, effectively allowing privilege escalation or unauthorized access to token-protected endpoints. This combination exposes a scenario where an attacker can manipulate object inheritance chains while leveraging the token to reach authenticated routes that would otherwise be restricted.

Consider a Buffalo app that parses JSON and merges it into a session or configuration object before verifying the Bearer Token’s claims. An attacker sending:

{ "__proto__": { "skipAuth": true } }

can cause the server-side token validation to incorrectly consider the request as authenticated. Because Buffalo routes often rely on shared objects across request handling stages, the polluted prototype persists in memory and can affect subsequent requests in the same process, especially when token validation logic is not isolated per request.

The risk is compounded when the application uses middleware that merges headers, cookies, and body into a single context object. In such flows, a malicious payload like:

{ "constructor": { "prototype": { "userRole": "admin" } } }

can modify the prototype of objects used throughout the request lifecycle. If a Bearer Token is parsed and stored on the request context before prototype pollution is applied, the altered prototype may cause the token verification to accept tampered claims, leading to security bypasses that are hard to detect without explicit input validation and isolation.

To mitigate prototype pollution in Buffalo when Bearer Tokens are present, ensure token parsing and validation occur before any mutable merging of untrusted data, and avoid merging raw JSON into prototype-inherited objects. Use strict object creation with Object.create(null) and explicit copies that exclude prototype pollution vectors. Below are concrete code examples for a Buffalo handler that validates a Bearer Token safely.

Safe Token Parsing and Validation

Parse the Authorization header early, validate the token structure, and store claims in a plain object that does not inherit from Object.prototype. This prevents polluted properties from interfering with token logic.

// handlers/auth_middleware.go
package handlers

import (
    "context"
    "net/http"
    "strings"

    "github.com/gobuffalo/buffalo"
    "github.com/gobuffalo/buffalo/auth"
)

func AuthMiddleware(next buffalo.Handler) buffalo.Handler {
    return func(c buffalo.Context) error {
        authHeader := c.Request().Header.Get("Authorization")
        if authHeader == "" {
            return c.Error(http.StatusUnauthorized, "missing authorization header")
        }
        parts := strings.Split(authHeader, " ")
        if len(parts) != 2 || parts[0] != "Bearer" {
            return c.Error(http.StatusBadRequest, "invalid authorization format")
        }
        tokenString := parts[1]

        // Validate and parse token claims without using prototype-inherited objects
        claims := make(map[string]interface{})
        // In practice, use a JWT library to parse and validate token:
        // claims, err := auth.ParseToken(tokenString, secret)
        // For illustration, we simulate a safe parse:
        claims["sub"] = "user-123"
        claims["scope"] = "read write"

        // Store claims in a clean context value
        c.Set("claims", claims)
        return next(c)
    }
}

Safe JSON Merging with Isolated Prototypes

When merging JSON payloads (e.g., for request parameters or PATCH updates), use Object.create(null)-style patterns or explicit field-by-field assignment to avoid prototype pollution. In Buffalo, prefer struct binding with validation over raw map merging.

// services/update_profile.go
package services

import (
    "github.com/gobuffalo/buffalo"
    "golang.org/x/crypto/bcrypt"
)

type ProfileUpdate struct {
    DisplayName string `json:"displayName" validate:"required,max=64"`
    Email       string `json:"email" validate:"required,email"`
}

func ApplyProfileUpdate(c buffalo.Context, tx *pop.Connection) error {
    var updates ProfileUpdate
    if err := c.Bind(&updates); err != nil {
        return c.Error(http.StatusBadRequest, err)
    }
    if err := c.Validate(&updates); err != nil {
        return c.Error(http.StatusBadRequest, err)
    }

    // Fetch existing user (example)
    user := &models.User{}
    if err := tx.Find(user, c.Param("user_id")); err != nil {
        return c.Error(http.StatusInternalServerError, err)
    }

    // Explicit field assignment — no prototype pollution possible
    if updates.DisplayName != "" {
        user.DisplayName = updates.DisplayName
    }
    if updates.Email != "" {
        user.Email = updates.Email
    }

    // Re-hash password if needed, using bcrypt
    // bcrypt.GenerateFromPassword(...)

    return tx.Update(user)
}

Middleware to Reject Polluted Payloads

Add a validation step that rejects objects containing dangerous prototype pollution keys before they reach business logic. This works alongside Bearer Token checks to ensure only clean data is processed.

// middleware/security.go
package middleware

import (
    "net/http"

    "github.com/gobuffalo/buffalo"
)

func PreventPrototypePollution(next buffalo.Handler) buffalo.Handler {
    return func(c buffalo.Context) error {
        body := c.Request().Body
        // In practice, read and inspect the JSON for keys like __proto__, constructor, etc.
        // For simplicity, we assume a helper ValidateNoPollution exists.
        if containsPollutionKeys(body) {
            return c.Error(http.StatusBadRequest, "invalid payload: potential prototype pollution")
        }
        return next(c)
    }
}

// Example helper (simplified)
func containsPollutionKeys(body interface{}) bool {
    // Real implementation would recursively check maps/slices
    return false
}

By combining early Bearer Token parsing, strict claim storage, and explicit struct binding, Buffalo applications can avoid prototype pollution risks even when untrusted input is present.