Spring4shell in Echo Go with Hmac Signatures

The Spring4shell vulnerability (CVE-2022-22965) affects applications using Spring MVC and Spring WebFlux when data binding is performed on object properties, potentially allowing attackers to execute arbitrary code via crafted payloads. In an Echo Go service that uses Hmac Signatures for request integrity, the presence of Spring4shell does not directly exploit Hmac verification, but the combination can expose issues if signature validation is inconsistently applied or if the application processes untrusted input before signature checks.

When Echo Go routes rely on middleware that parses JSON or form data into Go structs (e.g., using c.Bind), and that data is later used to influence behavior or forwarded to downstream services, an attacker may attempt to inject malicious payloads that could bypass business logic or reach components that do not validate signatures. If the Hmac signature is computed only on certain headers or a subset of the request, an attacker might manipulate unsigned parts to trigger unexpected behavior in Spring-like integrations or in service-to-service communication that expects trusted data.

In practice, the risk arises when signature verification is applied inconsistently—for example, verifying only on selected routes while other endpoints trust parsed binding results. An unauthenticated scan with middleBrick would highlight missing signature coverage on data-binding entry points, insecure deserialization patterns, and missing integrity checks on parameters that could be leveraged in SSRF or injection chains. middleBrick tests for Input Validation and Unsafe Consumption, flagging cases where untrusted data reaches handlers without prior integrity validation, even if Hmac Signatures are used elsewhere.

Because Echo Go does not have a built-in mechanism to enforce Hmac coverage across all routes automatically, developers must ensure every request path validates the signature before processing bound data. middleBrick’s OpenAPI/Swagger analysis (with full $ref resolution) cross-references spec definitions with runtime findings to highlight endpoints where security checks are absent, helping teams avoid gaps between documented behavior and actual implementation.

To mitigate risks related to inconsistent Hmac signature verification in Echo Go, enforce signature validation on all routes that accept external input, and ensure binding occurs only after integrity checks pass. Below are concrete, working examples demonstrating how to implement Hmac Signatures correctly.

Example 1: Validate Hmac before binding in Echo Go

This example shows how to verify an Hmac header before deserializing request body, preventing tampered data from being processed.

package main

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

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

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

func verifyHmac(next echo.HandlerFunc) echo.HandlerFunc {
	return func(c echo.Context) error {
		expectedMAC := c.Request().Header.Get("X-API-Signature")
		if expectedMAC == "" {
			return echo.NewHTTPError(http.StatusBadRequest, "missing signature")
		}

		body, err := io.ReadAll(c.Request().Body)
		if err != nil {
			return echo.NewHTTPError(http.StatusBadRequest, "failed to read body")
		}
		// Restore body for subsequent handlers
		c.Request().Body = io.NopCloser(io.LimitReader(io.Reader(io.MultiReader(io.NewSectionReader(c.Request().Body, 0, int64(len(body))), body), int64(len(body)))))

		mac := hmac.New(sha256.New, []byte(sharedSecret))
		mac.Write(body)
		actualMAC := hex.EncodeToString(mac.Sum(nil))

		if !hmac.Equal([]byte(actualMAC), []byte(expectedMAC)) {
			return echo.NewHTTPError(http.StatusUnauthorized, "invalid signature")
		}

		return next(c)
	}
}

func handler(c echo.Context) error {
	var req struct {
		Action string `json:"action"`
		Value  int    `json:"value"`
	}
	if err := c.Bind(&req); err != nil {
		return echo.NewHTTPError(http.StatusBadRequest, "invalid request")
	}
	return c.JSON(http.StatusOK, map[string]string{"status": "ok"})
}

func main() {
	e := echo.New()
	e.POST("/api/action", verifyHmac(handler))
	e.Start(":8080")
}

Example 2: Hmac verification with middleware that covers all routes

This example registers the Hmac verification as a global middleware, ensuring every request is validated before reaching route handlers.

package main

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
	"io"

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

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

func HmacMiddleware() echo.MiddlewareFunc {
	return func(next echo.HandlerFunc) echo.HandlerFunc {
		return func(c echo.Context) error {
			expectedMAC := c.Request().Header.Get("X-API-Signature")
			if expectedMAC == "" {
				return echo.NewHTTPError(http.StatusBadRequest, "missing signature")
			}

			body, err := io.ReadAll(c.Request().Body)
			if err != nil {
				return echo.NewHTTPError(http.StatusBadRequest, "failed to read body")
			}
			c.Request().Body = io.NopCloser(io.LimitReader(io.Reader(io.MultiReader(io.NewSectionReader(c.Request().Body, 0, int64(len(body))), body), int64(len(body)))))

			mac := hmac.New(sha256.New, []byte(sharedSecret))
			mac.Write(body)
			actualMAC := hex.EncodeToString(mac.Sum(nil))

			if !hmac.Equal([]byte(actualMAC), []byte(expectedMAC)) {
				return echo.NewHTTPError(http.StatusUnauthorized, "invalid signature")
			}

			return next(c)
		}
	}
}

func safeHandler(c echo.Context) error {
	// Process only after signature verification
	return c.String(http.StatusOK, "verified")
}

func main() {
	e := echo.New()
	e.Use(HmacMiddleware())
	e.GET("/webhook", safeHandler)
	e.POST("/api/data", safeHandler)
	e.Start(":8080")
}

Key remediation guidance

• Always compute the Hmac over the exact request body bytes before any binding or deserialization.
• Use hmac.Equal to compare signatures to prevent timing attacks.
• Apply the verification middleware globally or to all entry points that accept external data to avoid inconsistent coverage.
• Combine with input validation and rate limiting to reduce exposure from malformed or replayed requests.

middleBrick can validate this approach by scanning your endpoints and confirming that Hmac verification is present on all relevant routes, highlighting any endpoints missing integrity checks.