Initial

backend/internal/csrf/errors.go

@@ -0,0 +1,7 @@
+package csrf
+
+import "errors"
+
+var (
+	ErrInternal = errors.New("internal error")
+)

backend/internal/csrf/handler.go

@@ -0,0 +1,21 @@
+package csrf
+
+import (
+	"net/http"
+)
+
+func UpdateCsrf(w http.ResponseWriter, cookieName string, secure bool) (string, error) {
+	cookieValue, err := generateToken(CsrfLength)
+	if err != nil {
+		return "", err
+	}
+	http.SetCookie(w, &http.Cookie{
+		Name:     cookieName,
+		Value:    cookieValue,
+		Path:     "/",
+		HttpOnly: false,
+		Secure:   secure,
+		SameSite: http.SameSiteLaxMode,
+	})
+	return cookieValue, nil
+}

backend/internal/csrf/middleware.go

@@ -0,0 +1,51 @@
+package csrf
+
+import (
+	"context"
+	"crypto/subtle"
+	"log/slog"
+	"net/http"
+)
+
+const CsrfKey = "csrf"
+const CsrfLength = 32
+
+func NewMiddleware(cookieName, formName string, secure bool) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			cookie, err := r.Cookie(cookieName)
+			var cookieValue string
+
+			if err != nil || cookie.Value == "" {
+				cookieValue, err = UpdateCsrf(w, cookieName, secure)
+				if err != nil {
+					slog.Error("update csrf", slog.String("op", "middleware.UpdateCsrf"), slog.Any("error", err))
+					http.Error(w, "Internal server error", http.StatusInternalServerError)
+					return
+				}
+			} else {
+				cookieValue = cookie.Value
+			}
+
+			ctx := context.WithValue(r.Context(), CsrfKey, cookieValue)
+			r = r.WithContext(ctx)
+
+			if r.Method == http.MethodGet || r.Method == http.MethodHead || r.Method == http.MethodOptions {
+				next.ServeHTTP(w, r)
+				return
+			}
+
+			formValue := r.FormValue(formName)
+			if formValue == "" {
+				formValue = r.Header.Get("X-CSRF-Token")
+			}
+
+			if subtle.ConstantTimeCompare([]byte(formValue), []byte(cookieValue)) != 1 {
+				http.Error(w, "CSRF Token Validation Failed", http.StatusForbidden)
+				return
+			}
+
+			next.ServeHTTP(w, r)
+		})
+	}
+}

backend/internal/csrf/service.go

@@ -0,0 +1,19 @@
+package csrf
+
+import (
+	"crypto/rand"
+	"encoding/base64"
+	"log/slog"
+)
+
+func generateToken(length int) (string, error) {
+	b := make([]byte, length)
+
+	_, err := rand.Read(b)
+	if err != nil {
+		slog.Error("generate csrf token", slog.String("op", "service.generateToken"), slog.Any("error", err))
+		return "", ErrInternal
+	}
+
+	return base64.RawURLEncoding.EncodeToString(b), nil
+}