fix: harden downloader against path traversal, URL escape, oversized bodies

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/data/remote/content_downloader.dart

@@ -70,12 +70,25 @@ class ContentDownloader {
   }
 
   Future<bool> _downloadPack(ManifestSeason season) async {
+    // seasonId becomes a filesystem path segment; only accept safe slugs so a
+    // hostile manifest can never write outside the cache dir.
+    if (!RegExp(r'^[a-zA-Z0-9_-]+$').hasMatch(season.seasonId)) return false;
+
+    if (season.packUrl.contains('..') ||
+        season.packUrl.startsWith('/') ||
+        season.packUrl.contains('://')) {
+      return false;
+    }
+
     try {
       final res = await _client
           .get(Uri.parse('$baseUrl/${season.packUrl}'))
           .timeout(const Duration(seconds: 30));
       if (res.statusCode != 200) return false;
 
+      // Packs are tens of KB; anything huge is a server error or an attack.
+      if (res.bodyBytes.length > 5 * 1024 * 1024) return false;
+
       // Verify in memory before any file is touched, so a bad pack can
       // never leave artifacts on disk.
       final digest = sha256.convert(res.bodyBytes).toString();

test/data/remote/content_downloader_test.dart

@@ -129,4 +129,51 @@ void main() {
     ).sync();
     expect(updated, isTrue);
   });
+
+  test('malicious seasonId never escapes the cache dir', () async {
+    final outside = Directory('${tmp.path}/../bs_escape_probe');
+    addTearDown(() async {
+      if (await outside.exists()) await outside.delete(recursive: true);
+    });
+    final client = MockClient((request) async {
+      if (request.url.path.endsWith('manifest.json')) {
+        return http.Response(
+          jsonEncode({
+            'schemaVersion': 1,
+            'minAppBuild': 1,
+            'current': 'x',
+            'seasons': [
+              {
+                'seasonId': '../bs_escape_probe',
+                'version': 1,
+                'packUrl': 'seasons/x/pack.json',
+                'sha256': shaOf(packBody),
+              },
+            ],
+          }),
+          200,
+        );
+      }
+      return http.Response(packBody, 200);
+    });
+    final downloader = ContentDownloader(
+      baseUrl: 'https://example.com/content',
+      cacheDir: tmp,
+      client: client,
+    );
+    expect(await downloader.sync(), isFalse);
+    expect(await outside.exists(), isFalse);
+  });
+
+  test('corrupt cache index is treated as empty and re-downloads', () async {
+    await File('${tmp.path}/cache_index.json').create(recursive: true);
+    await File('${tmp.path}/cache_index.json').writeAsString('{broken');
+    final downloader = ContentDownloader(
+      baseUrl: 'https://example.com/content',
+      cacheDir: tmp,
+      client: okClient(),
+    );
+    expect(await downloader.sync(), isTrue);
+    expect(downloader.cachedVersion('season_002'), 1);
+  });
 }