Request Smuggling in Echo Go with Hmac Signatures

Request smuggling arises when an HTTP request is parsed differently by front-end infrastructure (such as a reverse proxy or load balancer) and the backend application. In Echo Go, when Hmac Signatures are used for request authentication but the application does not enforce strict message integrity checks before routing or parsing, smuggling can be introduced. This typically occurs when the proxy and the Echo server handle Transfer-Encoding and Content-Length headers inconsistently, allowing an attacker to smuggle a second request into the next pipeline.

Consider an API that uses Hmac Signatures to sign a canonical string composed of selected headers and the body. If Echo binds query parameters and headers early via echo.BindQuery or relies on lax header parsing before validating the signature, an attacker can inject an extra request by crafting a request like the following:

POST /api/v1/webhook HTTP/1.1
Content-Type: application/json
X-API-Key: example-key
X-Signature: sha256=abc123
Transfer-Encoding: chunked
Content-Length: 0

0

GET /admin/internal HTTP/1.1
Host: backend
Content-Length: 7

secret=1

An improperly configured proxy might interpret the first request as having no body (due to Content-Length: 0) and forward it to Echo Go, while the chunked body containing the second request is processed separately. If Echo Go binds and processes headers before verifying that the request body aligns with the signed canonical representation, the second request may be executed in the context of the first authenticated request. This violates the expectation that Hmac Signatures protect the entire request as a single, atomic unit.

The vulnerability is compounded when Echo routes requests based on early binding of path parameters or headers without re-validating the message integrity after routing. For example, using echo.Param("id") before confirming that the body and headers covered by the Hmac signature remain unchanged can allow path manipulation alongside the smuggling attack. In such cases, the attacker may bypass intended access controls or cause the backend to process an unintended action.

Remediation focuses on ensuring that the Hmac signature covers all parts of the request that influence routing and processing, and that Echo Go parses and validates the signature before any binding or routing occurs. This prevents attackers from smuggling requests by exploiting inconsistencies in header and body parsing.

First, construct a canonical string that includes the HTTP method, request path, selected headers, and the raw request body. Then compute the Hmac signature and compare it in constant time before any binding. The following example demonstrates a secure verification middleware in Echo Go:

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

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

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

        method := c.Request().Method
        path := c.Request().URL.Path
        headerSubset := c.Request().Header.Get("X-API-Key")
        body := c.Request().Body
        // consume body for verification; replace with io.ReadAll if needed
        payload, _ := io.ReadAll(body)
        c.Request().Body = io.NopCloser(bytes.NewBuffer(payload))

        canonical := method + "\n" + path + "\n" + headerSubset + "\n" + string(payload)
        mac := hmac.New(sha256.New, []byte(secret))
        mac.Write([]byte(canonical))
        expected := hex.EncodeToString(mac.Sum(nil))

        if !hmac.Equal([]byte(expected), []byte(receivedSig)) {
            return echo.NewHTTPError(http.StatusForbidden, "invalid signature")
        }
        return next(c)
    }
}

func handler(c echo.Context) error {
    // Safe to bind after signature verification
    var req MyRequest
    if err := c.Bind(&req); err != nil {
        return err
    }
    return c.JSON(http.StatusOK, req)
}

Second, configure Echo to reject ambiguous message parsing by disabling automatic handling of conflicting Transfer-Encoding and Content-Length. In production deployments, place a strict reverse proxy in front of Echo Go that normalizes these headers and rejects requests that contain both in a way that could enable smuggling. Ensure that the proxy does not change the request body before the signature is validated by the application.

Third, avoid using path parameters or headers for routing decisions before signature validation. Instead, bind and validate the signature using a dedicated middleware that runs on every request. If you use the CLI tool to generate clients, ensure that signing logic is consistent between client and server; you can integrate the scanner into your CI/CD pipeline using the GitHub Action to detect mismatches in API expectations:

middlebrick scan https://api.example.com

Finally, test your implementation with active probes that simulate request smuggling attempts. The MCP Server can be used from your AI coding assistant to validate that the middleware is invoked before any routing or binding, reinforcing a defense-in-depth posture against smuggling when Hmac Signatures are employed.