Windows path escape via namespace_packages.txt during legacy wheel installation Description

Summary

On Windows, attacker-controlled content in a wheel's
.dist-info/namespace_packages.txt can cause setuptools' legacy wheel
installation path to create an __init__.py outside the requested
destination_eggdir.

The attacker can influence the destination path, but not the contents of
the created file.

I confirmed this end-to-end using the real setuptools
Wheel.install_as_egg() path in a disposable local sandbox.

Details

Tested setuptools commit:

6ead555c5fb29bc57fe6105b1bffc163f56fd558

Relevant source:

setuptools/wheel.py

SHA-256:

359b7e0d5dd8c3135653abe08a8ed5dc0352eadfd7df22cb5e2b5303b7c3cafd

The relevant call path is:

Wheel(wheel_path).install_as_egg(destination)
→ _install_as_egg
→ _convert_metadata
→ _fix_namespace_packages

A wheel can provide attacker-controlled content in:

<distribution>.dist-info/namespace_packages.txt

The namespace value is eventually used approximately as:

mod_dir = os.path.join(destination_eggdir, *mod.split('.'))
mod_init = os.path.join(mod_dir, '__init__.py')

If mod is a Windows drive-qualified absolute path containing no dots,
mod.split('.') preserves it as a single component.
On Windows, os.path.join(destination_eggdir, absolute_component) discards
the original base directory and resolves to the attacker-supplied absolute
path.
The existing wheel filename and ZIP traversal checks do not validate this
metadata value before it is subsequently interpreted as a filesystem path.

```markdown
### PoC

The reproduction was performed entirely inside a disposable temporary
directory.

Layout:

```text
sandbox/
    destination/
    outside/
    input/
    canary.txt

A synthetic wheel was created containing:

demo_pkg-1.0.dist-info/namespace_packages.txt

whose content was the absolute path of:

sandbox\outside

The real acquired setuptools implementation was then invoked:

setuptools.wheel.Wheel(synthetic_wheel).install_as_egg(
    sandbox_destination
)

The operation completed without exception.

Expected metadata was created under:

sandbox\destination\EGG-INFO

but setuptools also created:

sandbox\outside\__init__.py

The file did not exist before execution.

Its contents were:

__import__('pkg_resources').declare_namespace(__name__)

The unrelated canary remained byte-identical.

The sandbox was deleted after recording the evidence.

### Impact

A crafted wheel can cause setuptools to create a fixed-content
`__init__.py` outside the intended installation directory at a
filesystem path influenced by the wheel, with the permissions of the
process performing the installation.

The demonstrated reproduction required the target parent directory to
already exist.

This report does **not** claim:

- arbitrary-content file write;
- overwrite of an existing `__init__.py`;
- remote code execution;
- persistence;
- privilege escalation.

I found CVE-2025-47273 / GHSA-5rjg-fvgr-3xxf, but that issue concerns
`PackageIndex._download_url` and URL-derived filenames rather than
`namespace_packages.txt` / `_fix_namespace_packages`.

I have not found a public advisory describing this specific path.