Sandbox Escape in Echo Go with Hmac Signatures

A sandbox escape in the context of Echo Go with Hmac Signatures typically arises when signature verification is implemented inconsistently between internal routing and external entry points, allowing an attacker to bypass intended isolation. Consider an Echo Go service that exposes both a public HTTP API and an internal administrative endpoint, each using Hmac Signatures for request authentication but with different key handling or verification strictness.

For example, if the public route applies signature validation only for selected methods or paths, while an internal route trusts additional context (such as internal headers or cookies), an attacker may craft a request that reaches the public endpoint with a valid Hmac for a benign operation, then leverage server-side redirects or forwarded calls to invoke the privileged internal route without satisfying its stricter checks. This can occur when middleware logic fails to enforce verification uniformly, effectively breaking the sandbox boundary between untrusted and trusted execution scopes.

Echo Go’s routing groups and middleware chains can inadvertently create such disparities if developers apply signature checks at the group level but omit them on specific handlers, or if they condition verification on route patterns that an attacker can circumvent. A common real-world pattern involves using a shared Hmac key for signing webhook payloads and API calls; if the service reuses that key across contexts and does not scope the signature to the intended operation or tenant, an attacker who obtains or guesses the key might forge requests that traverse from a public-facing handler into backend administrative logic, achieving a sandbox escape.

In practice, this vulnerability maps to the BOLA/IDOR and Authentication checks in middleBrick’s scan, which can detect inconsistent verification across endpoints. An unauthenticated scan may reveal routes that accept Hmac-signed requests but lack strict origin validation, allowing unauthorized access to functionality that should be isolated. The presence of Hmac Signatures does not inherently prevent sandbox escape; it depends on how the verification logic is integrated into Echo Go’s request lifecycle and whether all execution paths enforce the same integrity checks.

For instance, if an Echo Go server defines a handler like /admin/reset that is invoked internally after a public webhook handler verifies an Hmac signature, but the internal call skips re-validation, the boundary between the webhook sandbox and the admin sandbox collapses. middleBrick’s checks for Property Authorization and Unsafe Consumption can surface such inconsistencies by comparing required versus applied security controls across the API surface, highlighting routes where signature-based isolation is incomplete.

Remediation focuses on applying Hmac Signatures consistently, scoping them to the intended operation, and avoiding key reuse across trust boundaries. Below are concrete, working examples for Echo Go that demonstrate secure signature verification and routing hygiene.

Example 1: Consistent Hmac verification middleware

Define a middleware that validates the Hmac for all incoming requests before routing, ensuring uniform protection across public and internal routes.

package main

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

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

const sharedSecret = "your-secure-shared-secret"

func hmacMiddleware(next echo.HandlerFunc) echo.HandlerFunc {
	return func(c echo.Context) error {
		signature := c.Request().Header.Get("X-Signature")
		if signature == "" {
			return c.String(http.StatusUnauthorized, "missing signature")
		}
		body, err := c.Get("rawBody").([]byte)
		if err != nil || len(body) == 0 {
			return c.String(http.StatusBadRequest, "unable to read body")
		}
		h := hmac.New(sha256.New, []byte(sharedSecret))
		h.Write(body)
		expected := hex.EncodeToString(h.Sum(nil))
		if !hmac.Equal([]byte(expected), []byte(signature)) {
			return c.String(http.StatusForbidden, "invalid signature")
		}
		return next(c)
	}
}

func publicHandler(c echo.Context) error {
	return c.String(http.StatusOK, "public, authenticated")
}

func adminHandler(c echo.Context) error {
	return c.String(http.StatusOK, "admin, authenticated")
}

func main() {
	e := echo.New()
	// parse body for Hmac
	e.Use(func(next echo.HandlerFunc) echo.HandlerFunc {
		return func(c echo.Context) error {
			buf := new(strings.Builder)
			tee := io.TeeReader(c.Request().Body, buf)
			defer c.Request().Body.Close()
			body, _ := io.ReadAll(tee)
			c.Set("rawBody", body)
			c.Request().Body = io.NopCloser(strings.NewReader(buf.String()))
			return next(c)
		}
	})
	e.Use(hmacMiddleware)
	e.GET("/public", publicHandler)
	e.POST("/admin/reset", adminHandler)
	e.Start(":8080")
}

Example 2: Scoped Hmac per operation to prevent cross-context use

Include an operation identifier within the signed payload so that a single key cannot be reused across different actions or tenants.

package main

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

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

type SignedRequest struct {
	Operation string          `json:"operation"`
	Payload   json.RawMessage `json:"payload"`
	Signature string          `json:"signature"`
}

func verifyScopedHmac(c echo.Context, operation string) (bool, error) {
	var req SignedRequest
	if err := json.NewDecoder(c.Request().Body).Decode(&req); err != nil {
		return false, err
	}
	// restore body for further use if needed
	c.Request().Body = io.NopClounter(bytes.NewBuffer(req.Payload))

	expected, err := hex.DecodeString(req.Signature)
	if err != nil {
		return false, err
	}
	h := hmac.New(sha256.New, []byte(sharedSecret))
	h.Write([]byte(operation))
	h.Write(req.Payload)
	return hmac.Equal(expected, h.Sum(nil)), nil
}

func scopedHandler(c echo.Context) error {
	ok, err := verifyScopedHmac(c, "reset-password")
	if err != nil || !ok {
		return c.String(http.StatusForbidden, "invalid scoped signature")
	}
	return c.JSON(http.StatusOK, map[string]string{"status": "ok"})
}

func main() {
	e := echo.New()
	e.POST("/scoped/reset", scopedHandler)
	e.Start(":8080")
}

Key remediation practices

• Apply Hmac verification uniformly via middleware rather than per-handler to reduce inconsistency.
• Scope signatures to an operation or tenant context to prevent reuse across different trust zones.
• Avoid relying on internal routing or forwarded calls to re-verify; validate at the entry point and propagate identity explicitly.
• Do not share Hmac keys between public and internal services; rotate keys and audit access regularly.

These changes reduce the risk of sandbox escape by ensuring that Hmac Signatures are enforced consistently and that no route implicitly trusts calls that bypass signature validation.