fix: ensure user inputs are normalized (#724)

Co-authored-by: Elias Schneider <login@eliasschneider.com>

backend/internal/dto/dto_normalize.go

@@ -0,0 +1,94 @@
+package dto
+
+import (
+	"net/http"
+	"reflect"
+
+	"github.com/gin-gonic/gin"
+	"github.com/gin-gonic/gin/binding"
+	"golang.org/x/text/unicode/norm"
+)
+
+// Normalize iterates through an object and performs Unicode normalization on all string fields with the `unorm` tag.
+func Normalize(obj any) {
+	v := reflect.ValueOf(obj)
+	if v.Kind() != reflect.Ptr || v.IsNil() {
+		return
+	}
+	v = v.Elem()
+
+	// Handle case where obj is a slice of models
+	if v.Kind() == reflect.Slice {
+		for i := 0; i < v.Len(); i++ {
+			elem := v.Index(i)
+			if elem.Kind() == reflect.Ptr && !elem.IsNil() && elem.Elem().Kind() == reflect.Struct {
+				Normalize(elem.Interface())
+			} else if elem.Kind() == reflect.Struct && elem.CanAddr() {
+				Normalize(elem.Addr().Interface())
+			}
+		}
+		return
+	}
+
+	if v.Kind() != reflect.Struct {
+		return
+	}
+
+	// Iterate through all fields looking for those with the "unorm" tag
+	t := v.Type()
+loop:
+	for i := range t.NumField() {
+		field := t.Field(i)
+
+		unormTag := field.Tag.Get("unorm")
+		if unormTag == "" {
+			continue
+		}
+
+		fv := v.Field(i)
+		if !fv.CanSet() || fv.Kind() != reflect.String {
+			continue
+		}
+
+		var form norm.Form
+		switch unormTag {
+		case "nfc":
+			form = norm.NFC
+		case "nfkc":
+			form = norm.NFKC
+		case "nfd":
+			form = norm.NFD
+		case "nfkd":
+			form = norm.NFKD
+		default:
+			continue loop
+		}
+
+		val := fv.String()
+		val = form.String(val)
+		fv.SetString(val)
+	}
+}
+
+func ShouldBindWithNormalizedJSON(ctx *gin.Context, obj any) error {
+	return ctx.ShouldBindWith(obj, binding.JSON)
+}
+
+type NormalizerJSONBinding struct{}
+
+func (NormalizerJSONBinding) Name() string {
+	return "json"
+}
+
+func (NormalizerJSONBinding) Bind(req *http.Request, obj any) error {
+	// Use the default JSON binder
+	err := binding.JSON.Bind(req, obj)
+	if err != nil {
+		return err
+	}
+
+	// Perform normalization
+	Normalize(obj)
+
+	return nil
+}