security(import): harden DocumentBuilderFactory against XXE in MassImportService

Extract XxeSafeXmlParser with all 6 OWASP-recommended features
(disallow-doctype-decl, external-general-entities, external-parameter-entities,
load-external-dtd, XInclude, expandEntityReferences). Make readOds()
package-private; add failing-then-passing regression test and valid-ODS guard test.

POI 5.5.0 does not mitigate this: the vulnerable parser is a custom
DocumentBuilderFactory call in readOds(), not inside POI's internal ODS reader.
The hardening is defence-in-depth, not redundant with POI defaults.

Closes #528

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/test/java/org/raddatz/familienarchiv/importing/MassImportServiceTest.java

@@ -21,9 +21,12 @@ import software.amazon.awssdk.services.s3.S3Client;
 import software.amazon.awssdk.services.s3.model.PutObjectRequest;
 
 import org.apache.poi.xssf.usermodel.XSSFWorkbook;
+import org.xml.sax.SAXParseException;
 
 import java.io.File;
 import java.io.OutputStream;
+import java.io.ByteArrayOutputStream;
+import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.time.LocalDate;
@@ -32,6 +35,8 @@ import java.util.ArrayList;
 import java.util.List;
 import java.util.Optional;
 import java.util.UUID;
+import java.util.zip.ZipEntry;
+import java.util.zip.ZipOutputStream;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatThrownBy;
@@ -520,6 +525,25 @@ class MassImportServiceTest {
         assertThat(result).isEqualTo("hello");
     }
 
+    // ─── readOds — XXE security regression ───────────────────────────────────
+
+    // Security regression — do not remove. Introduced by issue #528.
+    @Test
+    void readOds_rejects_xxe_doctype_payload(@TempDir Path tempDir) throws Exception {
+        File malicious = buildXxeOds(tempDir, "file:///etc/hostname");
+        assertThatThrownBy(() -> service.readOds(malicious))
+                .isInstanceOf(SAXParseException.class)
+                .hasMessageContaining("DOCTYPE is disallowed");
+    }
+
+    @Test
+    void readOds_parses_valid_ods_correctly(@TempDir Path tempDir) throws Exception {
+        File valid = buildValidOds(tempDir, "Mustermann");
+        List<List<String>> rows = service.readOds(valid);
+        assertThat(rows).isNotEmpty();
+        assertThat(rows.get(0)).contains("Mustermann");
+    }
+
     // ─── helpers ──────────────────────────────────────────────────────────────
 
     /**
@@ -553,4 +577,47 @@ class MassImportServiceTest {
                 ""         // 13: transcription
         );
     }
+
+    /** Creates a minimal ODS ZIP containing a content.xml with an XXE payload. */
+    private File buildXxeOds(Path dir, String entityTarget) throws Exception {
+        String xml = "<?xml version=\"1.0\"?>"
+                + "<!DOCTYPE foo [<!ENTITY xxe SYSTEM \"" + entityTarget + "\">]>"
+                + "<office:document-content"
+                + " xmlns:office=\"urn:oasis:names:tc:opendocument:xmlns:office:1.0\""
+                + " xmlns:table=\"urn:oasis:names:tc:opendocument:xmlns:table:1.0\""
+                + " xmlns:text=\"urn:oasis:names:tc:opendocument:xmlns:text:1.0\">"
+                + "<office:body><office:spreadsheet>"
+                + "<table:table><table:table-row><table:table-cell>"
+                + "<text:p>&xxe;</text:p>"
+                + "</table:table-cell></table:table-row></table:table>"
+                + "</office:spreadsheet></office:body>"
+                + "</office:document-content>";
+        return writeOdsZip(dir.resolve("malicious.ods"), xml);
+    }
+
+    /** Creates a minimal valid ODS ZIP containing a content.xml with the given cell value. */
+    private File buildValidOds(Path dir, String cellValue) throws Exception {
+        String xml = "<?xml version=\"1.0\"?>"
+                + "<office:document-content"
+                + " xmlns:office=\"urn:oasis:names:tc:opendocument:xmlns:office:1.0\""
+                + " xmlns:table=\"urn:oasis:names:tc:opendocument:xmlns:table:1.0\""
+                + " xmlns:text=\"urn:oasis:names:tc:opendocument:xmlns:text:1.0\">"
+                + "<office:body><office:spreadsheet>"
+                + "<table:table><table:table-row><table:table-cell>"
+                + "<text:p>" + cellValue + "</text:p>"
+                + "</table:table-cell></table:table-row></table:table>"
+                + "</office:spreadsheet></office:body>"
+                + "</office:document-content>";
+        return writeOdsZip(dir.resolve("valid.ods"), xml);
+    }
+
+    private File writeOdsZip(Path destination, String contentXml) throws Exception {
+        try (OutputStream fos = Files.newOutputStream(destination);
+             ZipOutputStream zip = new ZipOutputStream(fos)) {
+            zip.putNextEntry(new ZipEntry("content.xml"));
+            zip.write(contentXml.getBytes(StandardCharsets.UTF_8));
+            zip.closeEntry();
+        }
+        return destination.toFile();
+    }
 }