Saltcorn is an extensible, open source, no-code database application builder. Prior to 1.4.5, 1.5.5, and 1.6.0-beta.4, the POST /sync/offline_changes endpoint allows an unauthenticated attacker to create arbitrary directories and write a changes.json file with attacker-controlled JSON content anywhere on the server filesystem. The GET /sync/upload_finished endpoint allows an unauthenticated attacker to list arbitrary directory contents and read specific JSON files. This vulnerability is fixed in 1.4.5, 1.5.5, and 1.6.0-beta.4.

Affected: npm: @saltcorn/server

### Summary

Two unauthenticated path traversal vulnerabilities exist in Saltcorn's mobile sync endpoints. The `POST /sync/offline_changes` endpoint allows an unauthenticated attacker to create arbitrary directories and write a `changes.json` file with attacker-controlled JSON content anywhere on the server filesystem. The `GET /sync/upload_finished` endpoint allows an unauthenticated attacker to list arbitrary directory contents and read specific JSON files.

The safe path validation function `File.normalise_in_base()` exists in the codebase and is correctly used by the `clean_sync_dir` endpoint in the **same file** (fix for GHSA-43f3-h63w-p6f6), but was not applied to these two endpoints.

### Details

**Finding 1: Arbitrary file write — `POST /sync/offline_changes` (sync.js line 226)**

The `newSyncTimestamp` parameter from the request body is used directly in `path.join()` without sanitization:

```javascript const syncDirName = `${newSyncTimestamp}_${req.user?.email || "public"}`; const syncDir = path.join( rootFolder.location, "mobile_app", "sync", syncDirName ); await fs.mkdir(syncDir, { recursive: true }); // creates arbitrary dir await fs.writeFile( path.join(syncDir, "changes.json"), JSON.stringify(changes) // writes attacker content ); ```

No authentication middleware is applied to this route. Since `path.join()` normalizes `../` sequences, setting `newSyncTimestamp` to `../../../../tmp/evil` causes the path to resolve…