Cross Site Request Forgery in Buffalo with Hmac Signatures

Cross Site Request Forgery (CSRF) in Buffalo becomes a risk when anti-CSRF protections rely solely on Hmac Signatures without proper per-request randomness and strict validation. Buffalo is an HTTP web framework for Go; if you use Hmac Signatures to sign requests but do not include a per-request nonce or timestamp, an attacker can replay a valid signed request. The vulnerability occurs when the server verifies the Hmac signature but does not ensure the request context is unique per user action, enabling an attacker to trick a logged-in user into executing unwanted state-changing operations via a malicious site.

Consider a scenario where an endpoint uses an Hmac signature derived from a static or predictable string (e.g., a shared secret plus a user identifier but no nonce). An attacker can construct a request with a valid signature by reusing a captured signed request from the victim. Because Buffalo does not inherently enforce per-request uniqueness in the signature verification logic, the server may accept the malicious request as legitimate. For example, an endpoint that transfers funds or changes an email might rely only on the Hmac signature for integrity without additional CSRF tokens or same-site cookie attributes, making it susceptible to CSRF despite the use of Hmac Signatures.

Additionally, if the Hmac signature is computed over a subset of request parameters and an attacker can control or guess other parameters, they may forge requests that still produce a valid signature. This is especially relevant when the signature excludes critical anti-CSRF fields or when the signature is computed before certain user-supplied values are bound to the session. The risk is compounded if the server does not validate the request origin or enforce strict referrer checks, allowing cross-origin requests with valid Hmac signatures to succeed.

To mitigate CSRF when using Hmac Signatures in Buffalo, ensure each signed request includes a per-request nonce and a timestamp, and validate these on the server. Use a cryptographically secure random nonce combined with the current timestamp, include them in the signature base string, and verify them on each request. Below are concrete code examples for generating and verifying Hmac signatures with nonce and timestamp in Buffalo.

First, generate the signature on the client or server side by including a nonce and timestamp:

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/hex"
    "fmt"
    "strconv"
    "time"
)

func generateHmacSignature(secret, method, path, body, nonce string, ts int64) string {
    message := fmt.Sprintf("%s|%s|%s|%s|%d|%s", method, path, body, nonce, ts, secret)
    key := []byte(secret)
    h := hmac.New(sha256.New, key)
    h.Write([]byte(message))
    return hex.EncodeToString(h.Sum(nil))
}

// Example usage:
// secret := "your-256-bit-secret"
// nonce := "7d9f2a1c-4e8b-4f3a-9c6d-2e5f8a1b3c7d" // generate per request
// ts := time.Now().Unix()
// signature := generateHmacSignature(secret, "POST", "/api/transfer", `{\"to\":\"123\"}`, nonce, ts)

On the server side in Buffalo, verify the nonce (must be unique and previously unused) and timestamp (must be within an acceptable window) before validating the Hmac signature:

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

func verifyHmacSignature(secret, method, path, body, nonce string, ts int64, receivedSignature string) bool {
    // Reject if timestamp is too old (e.g., >5 minutes)
    if time.Now().Unix()-ts > 300 {
        return false
    }
    // Reject if nonce has been used before (store used nonces in a cache/db)
    if isReplay(nonce) {
        return false
    }
    expected := generateHmacSignature(secret, method, path, body, nonce, ts)
    return hmac.Equal([]byte(expected), []byte(receivedSignature))
}

func hmacMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        // Extract headers/params: X-Signature, X-Nonce, X-Timestamp
        signature := r.Header.Get("X-Signature")
        nonce := r.Header.Get("X-Nonce")
        tsStr := r.Header.Get("X-Timestamp")
        ts, _ := strconv.ParseInt(tsStr, 10, 64)
        body := `{}` // read request body as needed
        if !verifyHmacSignature("your-256-bit-secret", r.Method, r.URL.Path, body, nonce, ts, signature) {
            http.Error(w, "invalid signature", http.StatusForbidden)
            return
        }
        next.ServeHTTP(w, r)
    })
}

Additionally, pair Hmac Signatures with standard CSRF defenses: set SameSite cookies, use anti-CSRF tokens for forms, and validate the Origin/Referer headers where appropriate. This layered approach ensures that even if a signature is somehow reused, the per-request nonce and timestamp prevent replay attacks that lead to CSRF.