Affected: masci banks

Banks generates meaningful LLM prompts using a simple template language. Prior to version 2.4.5, DirectoryPromptRegistry.set() in src/banks/registries/directory.py interpolates attacker-controlled Prompt.name and Prompt.version values into a Path without canonicalization or containment validation. Relative traversal such as ../victim/foo and an absolute Prompt.name can escape or discard the configured registry root, while overwrite=True permits replacement of existing target files. The poisoned name is persisted in index.json and reconstructed by _load(), allowing the out-of-root path to survive later registry loads. An application that forwards request data into these fields can therefore write Prompt.raw bytes to attacker-chosen paths writable by the application process. This issue is fixed in version 2.4.5.

Affected: PyPI: banks

## Summary

`DirectoryPromptRegistry.set()` interpolates the attacker-controllable `Prompt.name` into a `Path` expression with no canonicalization. An application that derives the prompt name from request data lets a caller write attacker-controlled bytes outside the configured registry directory.

## Details

`src/banks/registries/directory.py:44`

```python prompt_file = path / f"{prompt.name}.{prompt.version}.jinja" prompt_file.write_text(prompt.raw) ```

Two failure modes:

1. **Relative traversal.** `name="../victim/foo"` resolves to `<registry>/../victim/foo.0.jinja` — outside the configured root. 2. **Absolute-path bypass.** `pathlib` documents that `Path("/a") / Path("/b")` returns `Path("/b")`. So `name="/abs/path"` discards the registry root entirely; the registry is never consulted.

The poisoned `name` is then persisted to `index.json`, so the out-of-root path keeps reconstructing on later `_load()` calls (`directory.py:135-141`). With `overwrite=True`, existing files at the target path are replaced.

## Proof of Concept

```python import tempfile from pathlib import Path from banks import Prompt from banks.registries import DirectoryPromptRegistry

work = Path(tempfile.mkdtemp()) registry = work / "registry"; registry.mkdir() victim = work / "victim"; victim.mkdir()

reg = DirectoryPromptRegistry(str(registry))

# (1) Relative traversal reg.set(prompt=Prompt("pwn", name="../victim/pwned", version="0")) print((victim / "pwned.0.jinja").read_text())…