GHSA-32pv-mpqg-h292

### 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 outside the sync directory.

**Finding 2: Arbitrary directory read — `GET /sync/upload_finished` (sync.js line 288)**

The `dir_name` query parameter is used directly in `path.join()` without sanitization:

```javascript const syncDir = path.join( rootFolder.location, "mobile_app", "sync", dir_name ); let entries = await fs.readdir(syncDir); ```

Also unauthenticated. An attacker can list directory contents and read files named `translated-ids.json`, `unique-conflicts.json`, `data-conflicts.json`, or `error.json` from any directory.

**Contrast — fixed endpoint in the same file (line 342):**

The `clean_sync_dir` endpoint correctly uses `File.normalise_in_base()`:

```javascript const syncDir = File.normalise_in_base( path.join(rootFolder.location, "mobile_app", "sync"), dir_name ); if (syncDir) await fs.rm(syncDir, { recursive: true, force: true }); ```

### PoC

```bash # Write arbitrary file to /tmp/ curl -X POST http://TARGET:3000/sync/offline_changes \ -H "Content-Type: application/json" \ -d '{ "newSyncTimestamp": "../../../../tmp/saltcorn_poc", "oldSyncTimestamp": "0", "changes": {"proof": "path_traversal_write"} }' # Result: /tmp/saltcorn_poc_public/changes.json created with attacker content

# List /etc/ directory curl "http://TARGET:3000/sync/upload_finished?dir_name=../../../../etc" ```

### Impact

- **Unauthenticated arbitrary directory creation** anywhere on the filesystem - **Unauthenticated arbitrary JSON file write** (`changes.json`) to any writable directory - **Unauthenticated directory listing** of arbitrary directories - **Unauthenticated read** of specific JSON files from arbitrary directories - Potential for **remote code execution** via writing to sensitive paths (cron, systemd, Node.js module paths)

### Remediation

Apply `File.normalise_in_base()` to both endpoints, matching the existing pattern in `clean_sync_dir`:

```javascript // offline_changes fix const syncDirName = `${newSyncTimestamp}_${req.user?.email || "public"}`; const syncDir = File.normalise_in_base( path.join(rootFolder.location, "mobile_app", "sync"), syncDirName ); if (!syncDir) { return res.status(400).json({ error: "Invalid sync directory name" }); }

// upload_finished fix const syncDir = File.normalise_in_base( path.join(rootFolder.location, "mobile_app", "sync"), dir_name ); if (!syncDir) { return res.json({ finished: false }); } ```

Additionally, add `loggedIn` middleware to endpoints that modify server state.