Broken Authentication in Fiber with Hmac Signatures

Broken authentication in the Fiber Go web framework when using Hmac Signatures often arises from implementation choices rather than the framework itself. When Hmac Signatures are used to secure API endpoints, the risk shifts to how the signature is generated, transmitted, and verified. If the signing key is weak, predictable, or exposed, an attacker can forge valid signatures and impersonate any user or service. Similarly, if the signature is transmitted in an easily intercepted location, such as a URL query parameter, it can be reused or leaked in logs.

A common pattern involves creating a signature over a canonical set of request properties, such as the HTTP method, path, timestamp, and a payload hash. If the server-side verification logic does not enforce a strict timestamp window, an attacker can capture a valid request and replay it later, a classic Broken Authentication issue known as a replay attack. The absence of nonce or one-time-use tracking enables this. Additionally, if the logic that determines which user identity is associated with a valid signature is flawed—such as mapping the signature directly to a user ID without additional context—it can lead to IDOR-like authorization issues where one user can act as another.

Another specific risk arises from inconsistent verification between client and server. For example, the client might include additional headers that influence the canonical string but are omitted during server verification, or vice versa. This discrepancy can allow an attacker to manipulate non-critical headers to bypass intended scope checks. Furthermore, if the signature generation uses insecure cryptographic primitives or an improper encoding scheme, it may be susceptible to collision attacks or string comparison vulnerabilities. In a black-box scan, these implementation-level inconsistencies are detectable as authentication bypass or integrity validation failures, which map to the broader Broken Authentication category in the OWASP API Top 10.

In the context of middleBrick’s security checks, an unauthenticated scan can identify these weaknesses by analyzing the request surface and testing the integrity of the Hmac flow without credentials. The scanner looks for endpoints where Hmac is used but where critical safeguards like replay protection, key management, and strict input validation appear to be missing. By correlating these runtime findings with OpenAPI/Swagger specifications, the tool can highlight mismatches between documented authentication expectations and actual behavior, providing a clearer picture of the authentication risk.

To remediate Broken Authentication risks when using Hmac Signatures in Fiber, focus on canonicalization, replay protection, and strict verification. Below are concrete code examples demonstrating a secure server-side verification handler and a client-side signing utility in Go.

First, the server should verify the signature with a strict time window and constant-time comparison to prevent timing attacks. The example assumes the client sends the timestamp and signature in headers, and the payload body is available for hashing.

package main

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

const (
	// In production, load this from a secure secret store
	sharedSecret = "your-256-bit-secret-that-is-long-and-random"
	// Allowable clock skew in seconds
	timeSkew = 30
)

func verifyHmac(next http.HandlerFunc) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		timestampStr := r.Header.Get("X-Request-Timestamp")
		signature := r.Header.Get("X-Request-Signature")

		if timestampStr == "" || signature == "" {
			http.Error(w, "missing authentication headers", http.StatusUnauthorized)
			return
		}

		timestamp, err := strconv.ParseInt(timestampStr, 10, 64)
		if err != nil {
			http.Error(w, "invalid timestamp", http.StatusBadRequest)
			return
		}

		// Reject requests with excessive clock skew
		now := time.Now().Unix()
		if timestamp < now-timeSkew || timestamp > now+timeSkew {
			http.Error(w, "request expired", http.StatusUnauthorized)
			return
		}

		// Build canonical string exactly as the client did
		// Example: method + path + timestamp + hex-encoded body hash
		bodyHash := sha256.Sum256([]byte(r.Body)) // r.Body must be read once and restored if needed
		canonical := strings.ToUpper(r.Method) + r.URL.Path + timestampStr + hex.EncodeToString(bodyHash[:])

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

		if !hmac.Equal([]byte(expected), []byte(signature)) {
			http.Error(w, "invalid signature", http.StatusUnauthorized)
			return
		}

		// Proceed to handler
		next.ServeHTTP(w, r)
	}
}

func secureHandler(w http.ResponseWriter, r *http.Request) {
	w.Write([]byte("Authenticated request"))
}

func main() {
	http.HandleFunc("/api/action", verifyHmac(secureHandler))
	http.ListenAndServe(":8080", nil)
}

On the client side, the signing utility must produce the exact canonical string used by the server. This includes the HTTP method in uppercase, the path without query parameters (unless explicitly included), the timestamp, and a consistent representation of the body.

package main

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"net/http"
	"strconv"
	"strings"
	"bytes"
)

func buildClientHeaders(method, url string, body []byte) (map[string]string, error) {
	sharedSecret := "your-256-bit-secret-that-is-long-and-random"
	timestamp := strconv.FormatInt(timeNow().Unix(), 10)

	bodyHash := sha256.Sum256(body)
	canonical := strings.ToUpper(method) + url + timestamp + hex.EncodeToString(bodyHash[:])

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

	headers := map[string]string{
		"X-Request-Timestamp": timestamp,
		"X-Request-Signature": signature,
	}
	return headers, nil
}

// timeNow is a replaceable function for testing
var timeNow = time.Now

func main() {
	client := &http.Client{}
	body := []byte(`{"action":"update"}`)
	headers, _ := buildClientHeaders("POST", "http://localhost:8080/api/action", body)

	req, _ := http.NewRequest("POST", "http://localhost:8080/api/action", bytes.NewBuffer(body))
	for k, v := range headers {
		req.Header.Set(k, v)
	}

	resp, err := client.Do(req)
	if err != nil {
		panic(err)
	}
	defer resp.Body.Close()
	fmt.Println(resp.StatusCode)
}

These examples emphasize deterministic canonicalization, time-bound validation, and constant-time comparison. For production, store the shared secret securely, rotate keys periodically, and ensure the body is read in a reproducible way (e.g., by buffering) before hashing. middleBrick’s scans can validate that these practices are reflected in runtime behavior and spec documentation.