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CVE-2026-34730 — Copier `_external_data` allows path traversal and absolute-path local file read without unsafe mode

2026-10-02T23:41:09.825008+00:00

Affected: copier-org copier

Copier is a library and CLI app for rendering project templates. Prior to version 9.14.1, Copier's _external_data feature allows a template to load YAML files using template-controlled paths. If untrusted templates are in scope, a malicious template can read attacker-chosen YAML-parseable local files that are accessible to the user running Copier and expose their contents in rendered output. This issue has been patched in version 9.14.1.

GHSA-hgjq-p8cr-gg4h — Copier `_external_data` allows path traversal and absolute-path local file read without unsafe mode

2026-10-02T23:41:09.825087+00:00

Affected: PyPI: copier

### Summary

Copier's `_external_data` feature allows a template to load YAML files using template-controlled paths. The documentation describes these values as relative paths from the subproject destination, so relative paths themselves appear to be part of the intended feature model.

However, the current implementation also allows destination-external reads, including:

- Parent-directory paths such as `../secret.yml` - Absolute paths such as `/tmp/secret.yml`

and then exposes the parsed contents in rendered output.

This is possible without `--UNSAFE`, which makes the behavior potentially dangerous when Copier is run against untrusted templates. I am not certain this is unintended behavior, but it is security-sensitive and appears important to clarify.

### Details

The relevant flow is:

1. A template defines `_external_data` 2. Copier renders the configured path string 3. Copier calls `load_answersfile_data(dst_path, rendered_path, warn_on_missing=True)` 4. `load_answersfile_data()` opens `Path(dst_path, answers_file)` directly 5. Parsed YAML becomes available as `_external_data.` during rendering

Relevant code:

- -

The sink is:

```python with Path(dst_path, answers_file).open("rb") as fd: return yaml.safe_load(fd) ```

There is no…

PYSEC-2026-2135

2026-10-02T23:41:09.825157+00:00

Affected: PyPI: copier