GHSA-xmj3-9gf7-h52wPublished 2026-10-23Low severity

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

A GitHub-reviewed security advisory, not necessarily under active exploitation the way our CISA KEV advisories are, but a real, disclosed vulnerability in an open-source dependency.

Affected packages

setuptoolspip

Vulnerable: >= 38.2.2, < 84.1.0

Patched in 84.1.0

Details

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.

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.

CWE-22

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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

### 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.