sftp: reject empty passwords in PasswordAuthenticator (#11938)

* sftp: reject empty passwords in PasswordAuthenticator

FileStore.ValidatePassword already refuses a zero-length stored or
supplied password, but the guard belongs at the authenticator too: an
empty password is never valid, and checking before the store call keeps
any user.Store implementation without its own empty-secret check from
matching on zero-length input.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* sftp: keep the brute-force delay on empty-password failures

The early rejection returned before the shared failure path, so an empty
password failed instantly while other failures slept 100-199 ms. Folding
the length check into the store call keeps the store untouched and every
failed attempt equally slow.

---------

Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

weed/sftpd/auth/password.go

@@ -37,8 +37,9 @@ func (a *PasswordAuthenticator) Authenticate(conn ssh.ConnMetadata, password []b
 		return nil, fmt.Errorf("password authentication disabled")
 	}
 
-	// Validate password against user store
-	if a.userStore.ValidatePassword(username, password) {
+	// An empty password is never valid; skip the store so a store without its
+	// own empty-secret check cannot match on zero-length input.
+	if len(password) != 0 && a.userStore.ValidatePassword(username, password) {
 		return &ssh.Permissions{
 			Extensions: map[string]string{
 				"username": username,

weed/sftpd/auth/password_test.go

@@ -0,0 +1,43 @@
+package auth
+
+import "testing"
+
+// acceptAllStore records whether the store was consulted and accepts any password.
+type acceptAllStore struct {
+	stubStore
+	called bool
+}
+
+func (s *acceptAllStore) ValidatePassword(string, []byte) bool {
+	s.called = true
+	return true
+}
+
+// An empty password must be rejected before the store is consulted, so a store
+// without its own empty-secret check cannot authenticate on zero-length input.
+func TestPasswordAuthenticatorRejectsEmptyPassword(t *testing.T) {
+	store := &acceptAllStore{}
+	a := NewPasswordAuthenticator(store, true)
+
+	for _, password := range [][]byte{nil, {}} {
+		if _, err := a.Authenticate(&fakeConnMetadata{user: "alice"}, password); err == nil {
+			t.Fatalf("expected empty password %v to be rejected", password)
+		}
+	}
+	if store.called {
+		t.Fatal("store must not be consulted for an empty password")
+	}
+}
+
+func TestPasswordAuthenticatorConsultsStore(t *testing.T) {
+	store := &acceptAllStore{}
+	a := NewPasswordAuthenticator(store, true)
+
+	perms, err := a.Authenticate(&fakeConnMetadata{user: "alice"}, []byte("s3cret"))
+	if err != nil {
+		t.Fatalf("expected non-empty password to reach the store: %v", err)
+	}
+	if perms.Extensions["username"] != "alice" {
+		t.Fatalf("expected username extension alice, got %q", perms.Extensions["username"])
+	}
+}