Prototype Pollution in Echo Go with Bearer Tokens

Prototype pollution in Echo Go combined with Bearer token handling can arise when user-supplied data modifies the prototype of objects used during token validation or parsing, and the API relies on bearer tokens for authorization. In JavaScript/Node, prototype pollution occurs when an attacker can inject properties like __proto__, constructor, or prototype into objects, potentially changing behavior for all objects. In an Echo Go context, this typically relates to server-side code that processes JSON payloads (e.g., configuration or dynamic routing rules) and uses bearer tokens for authentication.

Consider an Echo Go handler that merges incoming JSON into a routing or configuration object before validating a bearer token. If the merge is shallow or uses unchecked reflection, an attacker can submit fields intended to pollute the shared object prototype. When the application later performs token validation (e.g., checking scopes or roles attached to the token), the polluted prototype may alter logic, bypass checks, or cause unsafe behavior. For example, a polluted object might cause role checks to incorrectly evaluate, effectively allowing unauthorized access despite a valid bearer token being present.

Additionally, bearer tokens passed in request headers may be processed alongside user input in middleware. If the middleware copies or extends objects using user-controlled keys without sanitization, it can inadvertently modify shared structures. Even though Echo Go is a Go framework and does not have JavaScript-style prototypes, the term here refers to shared configuration or routing structures that behave like prototypes in the sense that changes affect many requests. When combined with bearer tokens, this can expose logic that should be immutable, such as scope validation or token binding checks.

Real-world patterns that increase risk include using mapinterface{} with unchecked unmarshalling, or using libraries that perform deep merges on configuration objects. If an API endpoint accepts JSON like { "__proto__": { "scope": "admin" } } and merges it into a routing config that later influences authorization decisions, the bearer token validation may be subverted. Although Go’s type system prevents classic JavaScript prototype pollution, the conceptual risk remains when shared state is mutated by untrusted input before authentication checks.

MiddleBrick’s 12 security checks run in parallel and include Input Validation and Authorization testing. It would flag unsafe merging of user data into authorization-related structures and highlight missing validation on fields that could influence token handling. By correlating OpenAPI/Swagger specs with runtime findings, it identifies places where bearer token usage intersects with dynamic data processing, helping you locate risky patterns before they are exploited.

Remediation centers on strict input validation, avoiding shared mutable state, and isolating bearer token processing from user-controlled data. Below are concrete examples showing an unsafe pattern and a secure implementation.

Unsafe Example (vulnerable)

package main

import (
    "github.com/labstack/echo/v4"
    "net/http"
)

type Config struct {
    Scope string `json:"scope"`
}

var sharedConfig = &Config{Scope: "user"}

func unsafeHandler(c echo.Context) error {
    var input map[string]interface{}
    if err := c.Bind(&input); err != nil {
        return err
    }
    // Dangerous: merging user input into shared structure
    for k, v := range input {
        sharedConfig[k] = v // type assertion omitted for brevity; unsafe
    }
    token := c.Request().Header.Get("Authorization")
    if !validateToken(token, sharedConfig.Scope) {
        return c.String(http.StatusUnauthorized, "invalid")
    }
    return c.String(http.StatusOK, "ok")
}

func validateToken(token, scope string) bool {
    // naive validation for example
    return token == "Bearer valid" && scope == "user"
}

Secure Example (remediated)

package main

import (
    "github.com/labstack/echo/v4"
    "net/http"
)

type Config struct {
    Scope string `json:"scope"`
}

func secureHandler(c echo.Context) error {
    token := c.Request().Header.Get("Authorization")
    if token == "" {
        return c.String(http.StatusUnauthorized, "missing token")
    }
    // Validate token format and extract claims before using any user data
    claims, valid := validateToken(token)
    if !valid {
        return c.String(http.StatusUnauthorized, "invalid token")
    }
    // Process user input separately, never merge into shared config
    var input struct {
        Param string `json:"param"`
    }
    if err := c.Bind(&input); err != nil {
        return err
    }
    // Use claims for authorization, not mutated shared state
    if claims.Scope != "admin" {
        return c.String(http.StatusForbidden, "insufficient scope")
    }
    // Safe: use input.Param for business logic, not auth decisions
    return c.JSON(http.StatusOK, map[string]string{"received": input.Param})
}

func validateToken(token string) (struct{ Scope string }, bool) {
    // In practice, parse JWT and validate signature, issuer, scopes
    // This is a stand-in for proper JWT validation
    if token == "Bearer valid" {
        return struct{ Scope string }{Scope: "user"}, true
    }
    return struct{ Scope string }{}, false
}

Key practices demonstrated:

• Do not merge or assign user-controlled keys into shared configuration or routing objects used for authorization.
• Validate and parse the bearer token early, and use the claims/Scopes for authorization rather than runtime state that can be influenced by user input.
• Keep token validation logic independent from request body processing; avoid passing user data into authentication checks.
• Use strict struct unmarshalling for expected payloads instead of map merging, and reject unexpected fields.

MiddleBrick’s CLI (middlebrick scan <url>) and Web Dashboard can help detect endpoints where bearer token handling intersects with dynamic data binding. The GitHub Action can enforce a minimum score before merging, and the MCP Server allows you to scan APIs directly from your IDE while applying these fixes.