Privilege Escalation in Echo Go with Hmac Signatures

A common pattern in Echo Go APIs is to use HMAC signatures to verify the integrity and origin of requests. While HMAC is a strong primitive, implementation decisions can invert its purpose and enable privilege escalation. The vulnerability arises when the server uses the same or a predictable key for multiple trust domains (e.g., public endpoints and admin endpoints) and does not enforce strict scope-binding between the signature and the intended authorization context.

Consider an Echo Go service that signs a JWT or a serialized payload with an HMAC key and uses that signature to authorize access to an endpoint. If the signature verification only checks validity but does not enforce claims like scope, role, or tenant, an authenticated low-privilege user can replay or mutate a signed request to gain higher privileges. For example, a user with read-only rights might discover that by altering a non-signature part of the request (such as a numeric resource ID or a role parameter) and re-signing with the same key (if the key is exposed or weak), the server accepts the tampered request as authorized. This is a BOLA/IDOR-like escalation mediated by HMAC misuse: the signature becomes a poor man’s access control token rather than a integrity check tied to a defined authorization model.

Another vector involves algorithm confusion or key reuse. If an Echo Go endpoint accepts multiple signing methods (e.g., HS256 and RS256) and does not explicitly require a specific algorithm, an attacker could supply an unsigned token or a token signed with a public key as an HMAC, and the server might fall back to a weaker verification path. In addition, logging or error messages that expose whether a signature is valid before checking authorization can leak decision timing or key validity, aiding further escalation. The root cause is treating the HMAC signature as the sole authorization check rather than one component of a least-privilege flow that includes explicit role/scope validation and strict input validation.

Real-world parallels include findings where API signatures were accepted without verifying the associated permissions, enabling horizontal or vertical escalation via crafted requests. This maps to common weaknesses such as CWE-285 (Improper Authorization) and CWE-345 (Insufficient Authentication). The risk is elevated when the API exposes endpoints that perform sensitive operations (e.g., modifying roles, billing, or configuration) and rely only on signature validity without contextual authorization checks.

Remediation focuses on binding the HMAC signature to the intended authorization context and hardening the verification pipeline. Never use a single key across disparate trust zones, and always validate claims before acting on a signature. Below are concrete, secure patterns for Echo Go.

Secure HMAC verification with scope/role binding

// secure_hmac.go
package main

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
	"net/http"
	"strings"

	"github.com/labstack/echo/v4"
)

// deriveKey binds the signing key to a specific purpose and scope.
func deriveKey(globalKey, scope string) []byte {
	h := hmac.New(sha256.New, []byte(globalKey))
	h.Write([]byte("echo-go:scope=" + scope + ":v1"))
	return h.Sum(nil)
}

// signPayload returns a hex-encoded HMAC for the payload + scope.
func signPayload(payload, scope, key string) string {
	k := deriveKey(key, scope)
	h := hmac.New(sha256.New, k)
	h.Write([]byte(payload))
	return hex.EncodeToString(h.Sum(nil))
}

// verifySignature ensures the signature matches the payload and required scope.
func verifySignature(payload, receivedSig, scope, key string) bool {
	expected := signPayload(payload, scope, key)
	return hmac.Equal([]byte(expected), []byte(receivedSig))
}

func adminHandler(c echo.Context) error {
	const adminScope = "admin:write"
	apiKey := c.Request().Header.Get("X-API-Key")
	signature := c.Request().Header.Get("X-Payload-Signature")
	body := c.Request().Body // simplified; in practice, read and rewind carefully

	// 1) Validate scope claim in the request (e.g., from JWT or a param)
	// For this example, scope is passed in a header; ensure it matches the endpoint.
	reqScope := c.Request().Header.Get("X-Scope")
	if reqScope != adminScope {
		return echo.NewHTTPError(http.StatusForbidden, "insufficient scope")
	}

	// 2) Verify HMAC with scope-bound key
	if !verifySignature(string(body), signature, reqScope, apiKey) {
		return echo.NewHTTPError(http.StatusUnauthorized, "invalid signature")
	}

	// 3) Enforce role/authorization checks independent of signature validity
	userRole := c.Get("user_role") // injected by earlier auth middleware
	if userRole != "admin" {
		return echo.NewHTTPError(http.StatusForbidden, "requires admin role")
	}

	return c.JSON(http.StatusOK, map[string]string{"status": "ok"})
}

Key hardening practices

• Key separation: Use distinct keys per scope or tenant. Derive keys with a scope binding as shown in deriveKey.
• Explicit scope/role checks: Validate claims (scope, role, tenant) before acting, independent of signature verification.
• Algorithm enforcement: If you use a higher-level framework that supports multiple algorithms, explicitly require HMAC-SHA256 and reject tokens with missing or mismatched alg headers.
• Constant-time comparison: Always use hmac.Equal to avoid timing attacks on signature verification.
• Input validation: Treat all request parameters and headers as untrusted; reject unexpected or mutable fields before signature verification.

By tying the HMAC to a concrete scope and layering explicit authorization checks, you prevent the signature from being abused to escalate privileges. This approach aligns with secure API design patterns and reduces the risk of BOLA/IDOR and privilege escalation in Echo Go services.