Http Request Smuggling in Gin with Hmac Signatures

HTTP request smuggling occurs when an attacker can craft requests that are interpreted differently by a frontend proxy/load balancer and the backend server. In Gin, using HMAC signatures for request authentication can inadvertently interact with parsing behavior when body handling, header ordering, or content-length/transfer-encoding mismatches exist, creating conditions where a smuggled request bypasses signature verification or reaches a different route than intended.

Gin uses httprouter under the hood and relies on standard Go HTTP parsing. If the application reads the request body for HMAC verification before ensuring the request has not been split or repeated across two request messages, an attacker can exploit differences in how the proxy and Gin parse Content-Length or Transfer-Encoding headers. For example, sending a request with both Content-Length: 13 and a second request immediately after in the same TCP stream can cause Gin to read the body as the first request while the proxy treats the remaining bytes as a new request. If the HMAC is computed only on selected headers and a predictable body, the second request may lack a valid signature yet still reach a handler because Gin processes it after the first request’s verification completes.

Specific patterns that amplify risk include accepting both Transfer-Encoding: chunked and Content-Length in the same service, inconsistent header normalization (e.g., case-sensitive comparison of X-Signature), and computing the HMAC over a body that has already been partially consumed by middleware. Middleware that calls c.GetRawData() or c.ShouldBindJSON() before signature validation can change the request body reader’s position, causing the signature check to operate on incomplete data and allowing a tampered or second request to pass if the attacker knows the algorithm but not the secret. The vulnerability is not in HMAC itself but in how and when the signature is derived and verified relative to request parsing in Gin, especially when the unauthenticated attack surface includes endpoints that accept varied content-encoding combinations.

To mitigate request smuggling when using HMAC signatures in Gin, ensure the signature is computed over a canonical representation of the entire request (method, path, sorted headers, and body) and that body parsing does not interfere with verification. Always read the raw body before any middleware transformations and validate the HMAC before binding. Use strict header normalization and reject requests with ambiguous transfer encodings.

Remediation steps and code example

• Read the raw body once using c.GetRawData() and reuse the bytes for both HMAC verification and downstream binding.
• Normalize headers by canonicalizing header names (e.g., sort and lower-case) before signing/verifying to avoid case-sensitive mismatches.
• Explicitly set Content-Length on responses and reject requests that mix Transfer-Encoding and Content-Length in a way that could confuse frontends.
• Fail closed: if the signature is missing or invalid, abort before any route-specific logic.

// Example: Secure HMAC verification in Gin
package main

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

	"github.com/gin-gonic/gin"
)

const signatureHeader = "X-Signature"
const sharedSecret = "your-256-bit-secret"

func computeSignature(method, path string, headers http.Header, body []byte) string {
	// Canonical header list: include relevant headers, sorted lower-case
	var keys []string
	for k := range headers {
		keys = append(keys, strings.ToLower(k))
	}
	sort.Strings(keys)

	var b strings.Builder
	b.WriteString(method + "\n")
	b.WriteString(path + "\n")
	for _, k := range keys {
		b.WriteString(k + ":" + headers.Get(k) + "\n")
	}
	b.WriteString(hex.EncodeToString(body))
	mac := hmac.New(sha256.New, []byte(sharedSecret))
	mac.Write([]byte(b.String()))
	return hex.EncodeToString(mac.Sum(nil))
}

func authMiddleware() gin.HandlerFunc {
	return func(c *gin.Context) {
		// Read raw body once and preserve it for binding later
		raw, err := c.GetRawData()
		if err != nil {
			c.AbortWithStatusJSON(http.StatusBadRequest, gin.H{"error": "failed to read body"})
			return
		}

		expected := computeSignature(c.Request.Method, c.Request.RequestURI, c.Request.Header, raw)
		provided := c.Request.Header.Get(signatureHeader)
		if !hmac.Equal([]byte(expected), []byte(provided)) {
			c.AbortWithStatusJSON(http.StatusUnauthorized, gin.H{"error": "invalid signature"})
			return
		}

		// Restore body for downstream binding (e.g., JSON)
		c.Request.Body = http.NoBody
		c.Request.ContentLength = int64(len(raw))
		_ = c.BindJSON(&struct{ Name string }{}) // example binding
		c.Next()
	}
}

func main() {
	r := gin.Default()
	r.Use(authMiddleware())
	r.POST("/webhook", func(c *gin.Context) {
		var payload struct{ Event string `json:"event"` }
		if c.ShouldBindJSON(&payload) == nil {
			c.JSON(http.StatusOK, gin.H{"received": payload.Event})
		}
	})
	http.ListenAndServe(":8080", r)
}

Additional practices: disable chunked transfer encoding at the proxy when possible, enforce Content-Length consistently, and run continuous scans with the middleBrick CLI to detect regressions in the unauthenticated attack surface. The Pro plan supports automated scans on a schedule and can integrate into CI/CD via the GitHub Action to fail builds if risk scores degrade.