Bola Idor in Echo Go with Hmac Signatures

Broken Object Level Authorization (BOLA) occurs when an API fails to enforce proper ownership or authorization checks between a subject and the resource they intend to access. In Echo Go, combining predictable resource identifiers with Hmac Signatures can inadvertently create BOLA when the signature validates the request format and integrity but does not bind the operation to the correct user context.

Consider an endpoint that processes a signature to verify that a request has not been tampered with, for example to confirm the request payload, timestamp, and action. If the signature is verified successfully but the resource ID (e.g., /users/{id}/profile) is taken directly from user-supplied input without confirming that the authenticated subject owns that ID, the API may allow one user to read, update, or delete another user’s profile. The Hmac Signature ensures the message has not been altered, but it does not by itself answer the question of whether the subject associated with the signature is allowed to act on the target resource.

In Echo Go, this can manifest when route parameters or query parameters are used to locate a datastore record without a tenant or ownership check. An attacker who knows or guesses another user’s resource ID can forge a valid Hmac-signed request using their own keyed credentials, and the server will validate the signature and then perform the operation on the attacker-supplied ID. Because the signature verifies integrity and not authorization, the operation proceeds, resulting in unauthorized access or modification. This pattern is common in scenarios where Hmac is used for replay protection or request signing but authorization is handled as a separate, implicit step rather than an explicit, per-request check tied to the subject.

For instance, an endpoint that updates a payment method might accept a JSON payload, a timestamp, and an Hmac signature that covers the payload and timestamp. If the user ID is included in the payload and the server uses it directly to locate the record without verifying that the user associated with the Hmac key matches that user ID, the endpoint is vulnerable. The signature prevents tampering with the payload content, but it does not prevent horizontal privilege escalation across user boundaries.

To summarize, BOLA in Echo Go with Hmac Signatures emerges when signature verification is treated as the sole security control, and authorization checks that align the subject with the resource are omitted or assumed. The presence of strong cryptographic integrity does not replace the need to confirm that the subject of the request is authorized to access or modify the targeted resource.

Remediation focuses on ensuring that after Hmac signature validation, the request is explicitly scoped to the subject performing the action. Signature verification should confirm integrity and authenticity, but an additional authorization step must assert that the subject owns or is permitted to act on the resource.

Below are concrete, working examples for Echo Go that demonstrate secure handling of Hmac Signatures combined with proper authorization checks.

Example 1: Validating Hmac and enforcing resource ownership

package main

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

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

// computeHmac returns hex-encoded Hmac-SHA256 of message using key.
func computeHmac(message, key string) string {
	h := hmac.New(sha256.New, []byte(key))
	h.Write([]byte(message))
	return hex.EncodeToString(h.Sum(nil))
}

// verifyHmacConstantTime performs a constant-time comparison.
func verifyHmacConstantTime(message, received, key string) bool {
	expected := computeHmac(message, key)
	return hmac.Equal([]byte(expected), []byte(received))
}

// getUserProfile handles GET /users/{id}/profile with Hmac validation and BOLA protection.
func getUserProfile(c echo.Context) error {
	// 1) Extract subject identity from a secure source (e.g., JWT claims, session).
	userID, err := getSubjectUserID(c)
	if err != nil {
		return echo.NewHTTPError(http.StatusUnauthorized, "invalid authentication")
	}

	// 2) Extract resource ID from route.
	requestedProfileID, err := strconv.Atoi(c.Param("id"))
	if err != nil {
		return echo.NewHTTPError(http.StatusBadRequest, "invalid profile id")
	}

	// 3) Ensure the subject can only access their own profile (BOLA enforcement).
	if requestedProfileID != userID {
		return echo.NewHTTPError(http.StatusForbidden, "you cannot access this profile")
	}

	// 4) Verify Hmac to ensure request integrity (example over query parameters).
	message := c.QueryParam("data")
	signature := c.QueryParam("sig")
	apiSecret := "your-secure-api-secret" // use env/config in production
	if !verifyHmacConstantTime(message, signature, apiSecret) {
		return echo.NewHTTPError(http.StatusBadRequest, "invalid signature")
	}

	// 5) At this point, signature is valid and subject is authorized.
	profile := fetchProfileFromStore(requestedProfileID)
	if profile == nil {
		return echo.NewHTTPError(http.StatusNotFound, "profile not found")
	}
	return c.JSON(http.StatusOK, profile)
}

func getSubjectUserID(c echo.Context) (int, error) {
	// Placeholder: extract user ID from authenticated context (e.g., JWT).
	// In real code, validate token and retrieve claims.
	return 42, nil
}

func fetchProfileFromStore(id int) map[string]interface{} {
	// Placeholder: simulate a database fetch.
	return map[string]interface{}{"id": id, "name": "Alice"}
}

Example 2: Hmac over JSON payload with user ID binding

package main

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
	"io/ioutil"
	"net/http"
	"strconv"

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

func computeHmac(data, key string) string {
	h := hmac.New(sha256.New, []byte(key))
	h.Write([]byte(data))
	return hex.EncodeToString(h.Sum(nil))
}

// verifyHmacJSON validates the signature and ensures the user ID in payload matches the subject.
func updatePaymentMethod(c echo.Context) error {
	// Subject identity from secure context.
	subjectID, err := getSubjectUserID(c)
	if err != nil {
		return echo.NewHTTPError(http.StatusUnauthorized, "unauthorized")
	}

	body, err := ioutil.ReadAll(c.Request().Body)
	if err != nil {
		return echo.NewHTTPError(http.StatusBadRequest, "invalid request body")
	}
	// Restore body for subsequent reads if needed (e.g., echo.BodyReuse).
	c.Request().Body = ioutil.NopCloser(ioutil.NopCloser(bytes.NewReader(body)))

	// Parse JSON to extract user_id and other fields.
	var payload map[string]interface{}
	if err := json.Unmarshal(body, &payload); err != nil {
		return echo.NewHTTPError(http.StatusBadRequest, "invalid json")
	}

	// Ensure the user ID in the payload matches the authenticated subject (BOLA protection).
	reqUserID, ok := payload["user_id"].(float64)
	if !ok || int(reqUserID) != subjectID {
		return echo.NewHTTPError(http.StatusForbidden, "user_id mismatch")
	}

	// Verify Hmac over the raw body.
	signature := c.Request().Header.Get("X-Request-Signature")
	if signature == "" {
		return echo.NewHTTPError(http.StatusBadRequest, "signature missing")
	}
	apiSecret := "your-secure-api-secret"
	if !hmac.Equal([]byte(computeHmac(string(body), apiSecret)), []byte(signature)) {
		return echo.NewHTTPError(http.StatusBadRequest, "invalid signature")
	}

	// Signature valid and subject matches resource; proceed with update.
	// updatePaymentMethodInStore(reqUserID, payload)
	return c.JSON(http.StatusOK, map[string]string{"status": "ok"})
}

func getSubjectUserID(c echo.Context) (int, error) {
	// Extract and validate subject, e.g., from JWT.
	return 101, nil
}

Key remediation practices:

• Always verify the Hmac signature over the exact bytes you intend to validate (full payload or selected canonical form).
• After signature validation, enforce explicit authorization that binds the subject to the resource (e.g., user ID in claims must match user ID in resource path or payload).
• Use constant-time comparison (hmac.Equal) to avoid timing attacks on the signature.
• Avoid relying on signature-only checks to determine authorization; treat signature as integrity/authenticity, not as an authorization token.