diff --git a/src/moongreet/users.py b/src/moongreet/users.py
index 358c0c1..cf1f508 100644
--- a/src/moongreet/users.py
+++ b/src/moongreet/users.py
@@ -2,9 +2,12 @@
 # ABOUTME: Provides User dataclass and helper functions for the greeter UI.
 
 import configparser
+import re
 from dataclasses import dataclass
 from pathlib import Path
 
+VALID_THEME_NAME = re.compile(r"^[A-Za-z0-9_-]+$")
+
 NOLOGIN_SHELLS = {"/usr/sbin/nologin", "/sbin/nologin", "/bin/false", "/usr/bin/nologin"}
 MIN_UID = 1000
 MAX_UID = 65533
@@ -102,6 +105,9 @@ def get_user_gtk_theme(config_dir: Path | None = None) -> str | None:
         return None
 
     if config.has_option("Settings", "gtk-theme-name"):
-        return config.get("Settings", "gtk-theme-name")
+        theme = config.get("Settings", "gtk-theme-name")
+        # Validate against path traversal — only allow safe theme names
+        if theme and VALID_THEME_NAME.match(theme):
+            return theme
 
     return None
diff --git a/tests/test_integration.py b/tests/test_integration.py
index 7ebcf6f..75fec69 100644
--- a/tests/test_integration.py
+++ b/tests/test_integration.py
@@ -11,6 +11,7 @@ from pathlib import Path
 import pytest
 
 from moongreet.greeter import faillock_warning, FAILLOCK_MAX_ATTEMPTS
+from moongreet.i18n import load_strings
 from moongreet.ipc import create_session, post_auth_response, start_session, cancel_session
 
 
@@ -182,22 +183,26 @@ class TestFaillockWarning:
     """Tests for the faillock warning message logic."""
 
     def test_no_warning_on_first_attempt(self) -> None:
-        assert faillock_warning(1) is None
+        strings = load_strings("de")
+        assert faillock_warning(1, strings) is None
 
     def test_warning_on_second_attempt(self) -> None:
-        warning = faillock_warning(2)
+        strings = load_strings("de")
+        warning = faillock_warning(2, strings)
         assert warning is not None
         assert "1" in warning  # 1 Versuch übrig
 
     def test_warning_on_third_attempt(self) -> None:
-        warning = faillock_warning(3)
+        strings = load_strings("de")
+        warning = faillock_warning(3, strings)
         assert warning is not None
-        assert "gesperrt" in warning.lower()
+        assert warning == strings.faillock_locked
 
     def test_warning_beyond_max_attempts(self) -> None:
-        warning = faillock_warning(4)
+        strings = load_strings("de")
+        warning = faillock_warning(4, strings)
         assert warning is not None
-        assert "gesperrt" in warning.lower()
+        assert warning == strings.faillock_locked
 
     def test_max_attempts_constant_is_three(self) -> None:
         assert FAILLOCK_MAX_ATTEMPTS == 3
diff --git a/tests/test_users.py b/tests/test_users.py
index 0180ca8..f357f8c 100644
--- a/tests/test_users.py
+++ b/tests/test_users.py
@@ -187,7 +187,7 @@ class TestGetUserGtkTheme:
         assert result is None
 
     def test_handles_interpolation_characters(self, tmp_path: Path) -> None:
-        """Theme names with % characters should not trigger interpolation errors."""
+        """Theme names with % characters are rejected by validation."""
         gtk_dir = tmp_path / ".config" / "gtk-4.0"
         gtk_dir.mkdir(parents=True)
         settings = gtk_dir / "settings.ini"
@@ -195,7 +195,18 @@ class TestGetUserGtkTheme:
 
         result = get_user_gtk_theme(config_dir=gtk_dir)
 
-        assert result == "My%Theme"
+        assert result is None
+
+    def test_rejects_path_traversal_theme_name(self, tmp_path: Path) -> None:
+        """Theme names with path traversal characters should be rejected."""
+        gtk_dir = tmp_path / ".config" / "gtk-4.0"
+        gtk_dir.mkdir(parents=True)
+        settings = gtk_dir / "settings.ini"
+        settings.write_text("[Settings]\ngtk-theme-name=../../../../etc/evil\n")
+
+        result = get_user_gtk_theme(config_dir=gtk_dir)
+
+        assert result is None
 
     def test_ignores_symlinked_accountsservice_icon(self, tmp_path: Path) -> None:
         """AccountsService icon as symlink should be ignored to prevent traversal."""