Man In The Middle in Gin with Hmac Signatures

A Man In The Middle (MITM) attack against an API built with Gin becomes especially relevant when Hmac Signatures are used for request authentication but are implemented or verified incorrectly. In this context, the vulnerability occurs when an attacker can observe and manipulate in-flight HTTP traffic between a client and a Gin server and can either bypass signature verification or forge valid signatures due to weak shared secret handling or algorithm confusion.

Hmac Signatures rely on a shared secret known only to the client and server. The client creates a canonical representation of the request—often including selected headers, the request path, and the body—and computes an HMAC (for example, using SHA256). This signature is sent in a header, commonly X-API-Signature or similar. The Gin server independently computes the expected HMAC using the same shared secret and algorithm, then compares it to the value provided. If the comparison is not performed in constant time or if the server accepts multiple signature formats or algorithms, an attacker who can intercept and modify the request may exploit this to forge requests without knowing the secret.

Specific MITM risks with Gin and Hmac Signatures include:

• Downgrade or algorithm confusion: If the server supports multiple algorithms (e.g., SHA256 and SHA1) and selects one based on client input without strict validation, an attacker can force the use of a weaker algorithm and forge signatures.
• Non-canonical requests: If the client does not enforce a strict canonical format (headers included, case handling, sorted parameters, no extra whitespace), two clients with the same secret can produce different valid signatures for the same logical request. An attacker can modify non-essential headers or parameters, and if the server is permissive, the signature may still verify.
• Transmission in clear text: Without transport-layer encryption, an attacker can modify the request body or headers. If the server’s verification logic is lenient—for example, omitting the request body from the signature or including query parameters inconsistently—the attacker may alter the payload while producing a valid signature.
• Secret leakage via logs or error messages: If the shared secret is embedded in source code, configuration files checked into version control, or exposed through verbose error responses, an MITM who can read logs or responses can obtain the secret and sign arbitrary requests.

In a real-world scenario, an attacker on the same network observes a request to a Gin endpoint that uses Hmac Signatures. If the server does not enforce HTTPS strictly or if there is a misconfiguration that allows HTTP fallback, the attacker can intercept the request. If the signature is computed over an incomplete set of headers or does not include a nonce or timestamp, the attacker may replay the request or modify non-critical headers without invalidating the signature. Even with HTTPS, insufficient validation on the server side—such as not enforcing a strict comparison function—can allow the attacker to succeed by exploiting timing differences or algorithm ambiguity.

Because middleBrick scans unauthenticated attack surfaces and tests security controls, it can surface weaknesses in how Hmac Signatures are implemented on Gin endpoints. Findings may highlight missing integrity checks, inconsistent canonicalization, or insecure transport practices that facilitate MITM opportunities. These are reported with severity and remediation guidance rather than attempting to fix or block traffic.

To mitigate MITM risks when using Hmac Signatures in Gin, focus on canonicalization, strict algorithm selection, constant-time comparison, and secure transport. Below are concrete, realistic code examples that demonstrate a hardened approach.

1. Define a canonicalization function that deterministically selects headers, normalizes case, and sorts parameters. Include a timestamp or nonce to prevent replay, and ensure the body is included as-is (or excluded consistently across client and server).

// canonical.go
package main

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

// CanonicalRequest builds a string that both client and server use identically.
// It includes selected headers, the request path, and the body.
func CanonicalRequest(r *http.Request, body []byte, headersToInclude []string) string {
	var b bytes.Buffer
	// Include HTTP method
	b.WriteString(strings.ToUpper(r.Method) + "\n")
	// Include path
	b.WriteString(r.URL.Path + "\n")
	// Include sorted, selected headers
	headers := make([]string, 0, len(headersToInclude))
	for _, h := range headersToInclude {
		if v := r.Header.Get(h); v != "" {
			headers = append(headers, strings.ToLower(h)+":"+v)
		}
	}
	sort.Strings(headers)
	for _, h := range headers {
		b.WriteString(h + "\n")
	}
	// Include body if present
	if body != nil && len(body) > 0 {
		b.Write(body)
		b.WriteByte('\n')
	}
	// Optional: include a timestamp from a header to prevent replay
	// b.WriteString(r.Header.Get("X-Request-Timestamp") + "\n")
	return b.String()
}

// ComputeHmac returns hex-encoded HMAC-SHA256 using a shared secret.
func ComputeHmac(secret []byte, canonical string) string {
	h := hmac.New(sha256.New, secret)
	h.Write([]byte(canonical))
	return hex.EncodeToString(h.Sum(nil))
}

2. In the Gin handler, verify the signature using a constant-time comparison and reject ambiguous or missing algorithm specifications. Do not allow the client to dictate which hash function to use.

// handlers.go
package main

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

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

// Shared secret should be loaded securely (e.g., from environment, not hardcoded).
var sharedSecret = []byte(os.Getenv("API_HMAC_SECRET"))

func VerifyHmacSignature(headersToInclude []string) gin.HandlerFunc {
	return func(c *gin.Context) {
		// Read body exactly as it will be canonicalized
		rawBody, err := c.GetRawData()
		if err != nil && c.Request.Body != nil {
			c.AbortWithStatusJSON(http.StatusBadRequest, gin.H{"error": "unable to read body"})
			return
		}
		// Re-set body so downstream handlers can read it
		c.Request.Body = http.MaxBytesReader(c.Writer, bytes.NewBuffer(rawBody), >>20) // limit as needed

		// Require a specific signature header
		sig := c.GetHeader("X-API-Signature")
		if sig == "" {
			c.AbortWithStatusJSON(http.StatusUnauthorized, gin.H{"error": "missing signature"})
			return
		}

		// Build canonical string exactly as the client does
		canonical := CanonicalRequest(c.Request, rawBody, headersToInclude)
		expected := ComputeHmac(sharedSecret, canonical)

		// Constant-time comparison to avoid timing attacks
		if !hmac.Equal([]byte(expected), []byte(sig)) {
			c.AbortWithStatusJSON(http.StatusUnauthorized, gin.H2{"error": "invalid signature"})
			return
		}

		// Optional: enforce timestamp/nonce freshness to prevent replay
		// ts := c.GetHeader("X-Request-Timestamp")
		// if ts == "" || time.Since(parseTime(ts)) > allowedSkew {
		//  c.AbortWithStatusJSON(http.StatusUnauthorized, gin.H{"error": "stale request"})
		//  return
		// }

		c.Next()
	}
}

// Example usage in a Gin route
func main() {
	r := gin.Default()
	headersToInclude := []string{
		"Content-Type",
		"X-Request-Id",
		// Add other headers you require in the canonical string
	}
	r.Use(VerifyHmacSignature(headersToInclude))

	r.POST("/webhook", func(c *gin.Context) {
		var payload map[string]interface{}{}
		if err := c.BindJSON(&payload); err != nil {
			c.JSON(http.StatusBadRequest, gin.H{"error": "invalid JSON"})
			return
		}
		c.JSON(http.StatusOK, gin.H{"status": "ok"})
	})

	// r.Run(":8080")
}

Key remediation practices summarized:

These steps reduce the attack surface for MITM against Hmac Signatures in Gin by ensuring that signatures are deterministic, verifiable in constant time, and tied to the exact request content and transport security.

middleBrick can help by scanning your endpoints and reporting inconsistencies in signature handling, missing canonicalization safeguards, or use of weak algorithms, providing prioritized findings and remediation guidance rather than attempting automatic fixes.