diff --git a/backend/src/main/java/org/raddatz/familienarchiv/importing/MassImportService.java b/backend/src/main/java/org/raddatz/familienarchiv/importing/MassImportService.java
index e09aaa76..955b883e 100644
--- a/backend/src/main/java/org/raddatz/familienarchiv/importing/MassImportService.java
+++ b/backend/src/main/java/org/raddatz/familienarchiv/importing/MassImportService.java
@@ -320,6 +320,17 @@ public class MassImportService {
         return new ProcessResult(processed, skippedFiles);
     }
 
+    private boolean isValidImportFilename(String filename) {
+        if (filename == null || filename.isBlank()) return false;
+        if (filename.contains("/")) return false;
+        if (filename.contains("\\")) return false;
+        if (filename.contains("..")) return false;
+        if (filename.equals(".")) return false;
+        if (filename.contains("\0")) return false;
+        if (Paths.get(filename).isAbsolute()) return false;
+        return true;
+    }
+
     // package-private: Mockito spy in tests can override to inject IOException
     InputStream openFileStream(File file) throws IOException {
         return new FileInputStream(file);
diff --git a/backend/src/test/java/org/raddatz/familienarchiv/importing/MassImportServiceTest.java b/backend/src/test/java/org/raddatz/familienarchiv/importing/MassImportServiceTest.java
index 126a6d74..4804ffcb 100644
--- a/backend/src/test/java/org/raddatz/familienarchiv/importing/MassImportServiceTest.java
+++ b/backend/src/test/java/org/raddatz/familienarchiv/importing/MassImportServiceTest.java
@@ -438,6 +438,62 @@ class MassImportServiceTest {
         verify(documentService).findByOriginalFilename("doc002.pdf");
     }
 
+    // ─── isValidImportFilename — security regression — do not remove ─────────
+
+    @Test
+    void isValidImportFilename_returnsFalse_whenFilenameIsNull() {
+        boolean result = ReflectionTestUtils.invokeMethod(service, "isValidImportFilename", (String) null);
+        assertThat(result).isFalse();
+    }
+
+    @Test
+    void isValidImportFilename_returnsFalse_whenFilenameIsBlank() {
+        boolean result = ReflectionTestUtils.invokeMethod(service, "isValidImportFilename", "   ");
+        assertThat(result).isFalse();
+    }
+
+    @Test
+    void isValidImportFilename_returnsFalse_whenFilenameContainsForwardSlash() {
+        boolean result = ReflectionTestUtils.invokeMethod(service, "isValidImportFilename", "etc/passwd");
+        assertThat(result).isFalse();
+    }
+
+    @Test
+    void isValidImportFilename_returnsFalse_whenFilenameContainsBackslash() {
+        boolean result = ReflectionTestUtils.invokeMethod(service, "isValidImportFilename", "..\\etc\\passwd");
+        assertThat(result).isFalse();
+    }
+
+    @Test
+    void isValidImportFilename_returnsFalse_whenFilenameContainsDotDot() {
+        boolean result = ReflectionTestUtils.invokeMethod(service, "isValidImportFilename", "doc..evil.pdf");
+        assertThat(result).isFalse();
+    }
+
+    @Test
+    void isValidImportFilename_returnsFalse_whenFilenameIsDotDot() {
+        boolean result = ReflectionTestUtils.invokeMethod(service, "isValidImportFilename", "..");
+        assertThat(result).isFalse();
+    }
+
+    @Test
+    void isValidImportFilename_returnsFalse_whenFilenameIsAbsolutePath() {
+        boolean result = ReflectionTestUtils.invokeMethod(service, "isValidImportFilename", "/etc/passwd");
+        assertThat(result).isFalse();
+    }
+
+    @Test
+    void isValidImportFilename_returnsFalse_whenFilenameContainsNullByte() {
+        boolean result = ReflectionTestUtils.invokeMethod(service, "isValidImportFilename", "file\0.pdf");
+        assertThat(result).isFalse();
+    }
+
+    @Test
+    void isValidImportFilename_returnsTrue_whenFilenameIsPlainBasename() {
+        boolean result = ReflectionTestUtils.invokeMethod(service, "isValidImportFilename", "document.pdf");
+        assertThat(result).isTrue();
+    }
+
     // ─── importSingleDocument — non-blank optional fields ────────────────────
 
     @Test