@saltcorn/server arbitrary file and directory listing when accessing build mobile app results

Summary

### Summary

A user with admin permission can read arbitrary file and directory names on the filesystem by calling the `admin/build-mobile-app/result?build_dir_name=` endpoint. The `build_dir_name` parameter is not properly validated and it's then used to construct the `buildDir` that is read. The file/directory names under the `buildDir` will be returned.

### Details

- file: https://github.com/saltcorn/saltcorn/blob/v1.0.0-beta.13/packages/server/routes/admin.js#L2884-L2893

```js router.get( "/build-mobile-app/result", isAdmin, error_catcher(async (req, res) => { const { build_dir_name } = req.query; // [1] source const rootFolder = await File.rootFolder(); const buildDir = path.join( rootFolder.location, "mobile_app", build_dir_name // [2] ); const files = await Promise.all( fs .readdirSync(buildDir) // [3] sink .map(async (outFile) => await File.from_file_on_disk(outFile, buildDir)) ); [...] }) ); ```

### PoC

- log into the application as an admin user - visit the following url: `http://localhost:3000/admin/build-mobile-app/result?build_dir_name=/../../../../../../../../`

**NOTE**: it's possible to only see file and directory names but not to download their content.

### Impact

Information disclosure

### Recommended Mitigation

Resolve the `buildDir` and check if it starts with `${rootFolder.location}/mobile_app`.