diff --git a/internal/webfinger/usecase.go b/internal/webfinger/usecase.go
new file mode 100644
index 0000000..67949b8
--- /dev/null
+++ b/internal/webfinger/usecase.go
@@ -0,0 +1,11 @@
+package webfinger
+
+import (
+	"context"
+
+	"source.toby3d.me/toby3d/home/internal/domain"
+)
+
+type UseCase interface {
+	Do(ctx context.Context, acct string) (*domain.Site, error)
+}
diff --git a/internal/webfinger/usecase/webfinger_ucase.go b/internal/webfinger/usecase/webfinger_ucase.go
new file mode 100644
index 0000000..6c76910
--- /dev/null
+++ b/internal/webfinger/usecase/webfinger_ucase.go
@@ -0,0 +1,37 @@
+package usecase
+
+import (
+	"context"
+	"fmt"
+	"strings"
+
+	"source.toby3d.me/toby3d/home/internal/domain"
+	"source.toby3d.me/toby3d/home/internal/site"
+	"source.toby3d.me/toby3d/home/internal/webfinger"
+)
+
+type webFingerUseCase struct {
+	sites site.Repository
+}
+
+func NewWebFingerUseCase(sites site.Repository) webfinger.UseCase {
+	return &webFingerUseCase{sites: sites}
+}
+
+func (ucase *webFingerUseCase) Do(ctx context.Context, acct string) (*domain.Site, error) {
+	parts := strings.Split(acct, "@")
+	if len(parts) != 2 {
+		return nil, fmt.Errorf("invalid 'acct' value format, got %s, want [user, host]", parts)
+	}
+
+	site, err := ucase.sites.Get(ctx, domain.LanguageUnd)
+	if err != nil {
+		return nil, fmt.Errorf("cannot read global site config: %w", err)
+	}
+
+	if !strings.EqualFold(parts[1], site.BaseURL.Hostname()) {
+		return nil, fmt.Errorf("requested %s user outside %s resource", parts[1], site.BaseURL.Hostname())
+	}
+
+	return site, nil
+}