GHSA-q9m2-fhv9-3jcf

## Summary `validate_path_security` uses string-prefix containment (`startswith`) for boundary checks. This allows paths that are **outside** the intended project directory but share its prefix string (e.g., `/tmp/potato_proj_demo_evil/...` vs `/tmp/potato_proj_demo`) to be accepted.

## Details ### Affected source location (root cause) **File:** `potato/server_utils/config_module.py`

**Snippet (lines 370–373):** ```python real_path = os.path.realpath(normalized_path) real_base = os.path.realpath(base_dir) if not real_path.startswith(real_base): raise ConfigSecurityError(...) ```

**Snippet (lines 384–389):** ```python real_path = os.path.realpath(normalized_path) check_dir = project_dir if project_dir else base_dir real_check_dir = os.path.realpath(check_dir) if not real_path.startswith(real_check_dir): raise ConfigSecurityError(...) ```

`startswith()` is string-based, so `/tmp/potato_proj_demo_evil/...` passes when checked against `/tmp/potato_proj_demo`.

### Confirmed affected call sites **File:** `potato/server_utils/config_module.py`

1) `validate_file_paths` task_dir branch (line 2113) ```python validated_task_dir = validate_path_security(task_dir, project_dir) ```

2) `validate_file_paths` data_files branch (line 2151) ```python validated_path = validate_path_security(file_path, base_dir, project_dir) ```

3) `validate_training_config` training.data_file branch (line 2286) ```python validated_path = validate_path_security(data_file, base_dir, project_dir) ```

## PoC ```python from potato.server_utils.config_module import validate_path_security

base = '/tmp/potato_proj_demo' vuln = '/tmp/potato_proj_demo_evil/file.txt'

try: print('inside=', validate_path_security('/tmp/potato_proj_demo/file.txt', base, base)) except Exception as e: print('inside_error=', type(e).__name__, e)

try: validate_path_security('/tmp/other_demo/file.txt', base, base) except Exception as e: print('baseline=', type(e).__name__, e)

print('trigger=', validate_path_security(vuln, base, base)) ```

## Impact - Can allow unauthorized sibling-prefix file access outside intended project boundary. - Can affect read paths (`data_files`, `training.data_file`, `base_css`, `header_logo`) and output/path placement depending on configuration.