Broken Authentication in Buffalo with Hmac Signatures

Buffalo is a web framework for Go that encourages rapid development and provides built-in mechanisms for handling authentication. When HMAC-based signatures are used to authenticate requests, incorrect implementation can lead to Broken Authentication, a category covered by the OWASP API Top 10. A common pattern is to sign requests using a shared secret combined with selected headers and a timestamp to prevent replay attacks. If the server-side verification does not properly validate the signature, or if it uses a weak comparison, an attacker can forge valid requests.

Consider a scenario where a client computes an HMAC over the HTTP method, the request path, selected headers, and a timestamp, then sends the signature and timestamp in headers. If the server reconstructs the signing string inconsistently—perhaps by including the query string in one place and omitting it in another, or by normalizing whitespace differently—it may accept a forged signature. For example, an attacker could manipulate the timestamp window, replay a valid request outside the allowed tolerance, or exploit case-sensitivity in header names to bypass intended validation logic. The framework does not enforce a strict canonicalization of the signed components, so differences in how the client and server serialize the data can lead to acceptance of an invalid signature.

Another risk arises from how the shared secret is stored and accessed. If the secret is embedded in configuration files that are checked into version control, or if it is transmitted over insecure channels during deployment, an attacker who gains access to the secret can generate valid HMACs for any request. Additionally, insufficient validation of the timestamp can allow replay attacks within the permitted window. If the server does not track used nonces or enforce a tight skew window (for example, five minutes), an intercepted request can be reused. These issues are particularly dangerous in APIs where authentication is performed via Hmac Signatures without additional protections such as TLS termination inspection or strict header canonicalization.

In a Buffalo application, route handlers often rely on middleware to perform authentication. If the middleware parses headers inconsistently—such as reading X-API-Timestamp with different key casing or failing to enforce required headers—the verification step may be bypassed. For instance, omitting a mandatory header from the signature computation, or failing to reject requests with missing timestamps, weakens the scheme. Moreover, if the server does not explicitly reject requests with timestamps too far in the past or future, an attacker can manipulate time-based skew to reuse valid signatures. The combination of flexible parsing, missing mandatory checks, and weak comparison logic creates conditions where authentication can be broken without requiring knowledge of the secret.

To illustrate, an attacker could intercept a valid request, modify an innocuous header that is not included in the signature, and forward it to the server. If the server does not enforce a strict set of signed headers, the modified request may be accepted as valid. This highlights the importance of defining a canonical set of headers, method, path, and timestamp, and ensuring both client and server implement identical serialization logic. Without such rigor, Hmac Signatures in Buffalo can expose authentication mechanisms to manipulation, leading to unauthorized access and data compromise.

Remediation focuses on strict canonicalization, constant-time comparison, and proper secret management. Define a precise set of headers to include in the signature, such as the HTTP method, the request path without query parameters, selected canonical headers, and a timestamp. Ensure both client and server serialize these components in the same order and format. Use a consistent timestamp format, such as Unix epoch seconds, and enforce a tight skew window, for example 300 seconds.

Below is a complete, working example of Hmac Signature generation and verification in a Buffalo application. The client computes the signature over a canonical string and sends it in headers. The server reconstructs the string identically and validates using a constant-time comparison.

// client side
package main

import (
	"crypto/hmac"
	"crypto/sha256"
	"fmt"
	"net/http"
	"sort"
	"strconv"
	"strings"
	"time"
)

func signRequest(method, url string, headers map[string]string, secret string) (string, int64, error) {
	timestamp := strconv.FormatInt(time.Now().Unix(), 10)
	// canonicalize: include method, path (no query), selected headers, timestamp
	path := "" // in practice, extract path from url without query
	// For simplicity, assume path is derived separately
	keys := make([]string, 0, len(headers))
	for k := range headers {
		keys = append(keys, k)
	}
	sort.Strings(keys)
	var b strings.Builder
	b.WriteString(method)
	b.WriteString("\n")
	b.WriteString(path)
	b.WriteString("\n")
	for _, k := range keys {
		b.WriteString(k)
		b.WriteString(":")
		b.WriteString(headers[k])
		b.WriteString("\n")
	}
	b.WriteString(timestamp)
	message := b.String()
	h := hmac.New(sha256.New, []byte(secret))
	h.Write([]byte(message))
	signature := fmt.Sprintf("%x", h.Sum(nil))
	return signature, timestamp, nil
}

func makeSignedRequest() (*http.Response, error) {
	secret := "my-shared-secret"
	h := http.Header{}
	h.Set("X-Custom-Header", "value")
	sig, ts, err := signRequest("GET", "https://api.example.com/resource", h, secret)
	if err != nil {
		return nil, err
	}
	req, _ := http.NewRequest("GET", "https://api.example.com/resource", nil)
	req.Header.Set("X-API-Timestamp", ts)
	req.Header.Set("X-API-Signature", sig)
	req.Header.Set("X-Custom-Header", "value")
	client := &http.Client{}
	return client.Do(req)
}

The server side verifies the signature using the same canonicalization and a constant-time comparison to avoid timing attacks:

// server side in a Buffalo middleware or handler
package main

import (
	"crypto/hmac"
	"crypto/sha256"
	"fmt"
	"io"
	"net/http"
	"sort"
	"strconv"
	"strings"
	"time"
)

func verifyHmac(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		const skewSeconds = 300
		sentSig := r.Header.Get("X-API-Signature")
		tsStr := r.Header.Get("X-API-Timestamp")
		if sentSig == "" || tsStr == "" {
			http.Error(w, "missing signature or timestamp", http.StatusUnauthorized)
			return
		}
		ts, err := strconv.ParseInt(tsStr, 10, 64)
		if err != nil {
			http.Error(w, "invalid timestamp", http.StatusBadRequest)
			return
		}
		now := time.Now().Unix()
		if now-ts > skewSeconds || ts-now > skewSeconds {
			http.Error(w, "timestamp skew too large", http.StatusUnauthorized)
			return
		}
		// reconstruct canonical string exactly as client did
		path := "" // derive path without query
		keys := make([]string, 0, len(r.Header))
		for k := range r.Header {
			// only include headers that were signed; use canonical set
			// For example, include only X-Custom-Header and ignore others
		}
		// Use a fixed canonical header set for verification
		canonicalHeaders := []string{"X-Custom-Header"}
		sort.Strings(canonicalHeaders)
		var b strings.Builder
		b.WriteString(r.Method)
		b.WriteString("\n")
		b.WriteString(path)
		b.WriteString("\n")
		for _, k := range canonicalHeaders {
			v := r.Header.Get(k)
			b.WriteString(k)
			b.WriteString(":")
			b.WriteString(v)
			b.WriteString("\n")
		}
		b.WriteString(tsStr)
		expected := b.String()
		h := hmac.New(sha256.New, []byte(secret))
		h.Write([]byte(expected))
		expectedSig := fmt.Sprintf("%x", h.Sum(nil))
		if !hmac.Equal([]byte(sentSig), []byte(expectedSig)) {
			http.Error(w, "invalid signature", http.StatusUnauthorized)
			return
		}
		next.ServeHTTP(w, r)
	})
}

Key remediation practices:

• Define a strict, immutable list of headers included in the signature to prevent header smuggling via case variations or omitted headers.
• Use hmac.Equal for signature comparison to mitigate timing attacks.
• Enforce a tight timestamp skew window and reject requests outside this window to prevent replay attacks.
• Store the shared secret securely, for example using environment variables or a secrets manager, and avoid hardcoding it in source code.
• Validate that required headers are present before computing or verifying the signature, and reject requests with missing mandatory components.

By standardizing the serialization and verification logic, and by applying these concrete code fixes, a Buffalo application can securely use Hmac Signatures for authentication, mitigating Broken Authentication risks.