In early 2024, security researchers found a serious issue in Python’s ssl module. Catalogued as CVE-2024-0397, this vulnerability is about a memory race condition within certain SSLContext methods. It sounds technical, but the gist is simple: If handled wrong, your Python app could crash or expose sensitive memory contents during a TLS handshake.

Here’s everything you need to know, *exclusively explained in practical terms*.

What Is CVE-2024-0397?

In Python, the ssl module handles encrypted network communication using TLS. When your app uses SSLContext to manage certificates, two methods can cause trouble:

get_ca_certs()

If these are called *while* new certificates are being loaded—like during a TLS handshake—Python can get confused and mess up memory. That’s the race condition. Hackers might exploit this situation to make your program crash (denial of service) or, worse, read bits of memory they shouldn't be able to see.

Thread B: At the same time, Python loads new CA certificates as part of a handshake.

The SSLContext code wasn't locking memory properly. So, if both operations happen together, the internal data structure tracking certificates could get scrambled, leading to memory corruption.

The bug is hard to reproduce, but in the high-traffic world of servers and microservices, it can definitely happen.

In vulnerable versions, the following overlap is possible

import ssl
import threading

def load_certs(context):
    # Simulate certs loading during handshake
    context.load_verify_locations(capath="/etc/ssl/certs")

def get_stats(context):
    # Stats method that can trigger race
    print(context.cert_store_stats())
    print(context.get_ca_certs())

context = ssl.create_default_context()

t1 = threading.Thread(target=load_certs, args=(context,))
t2 = threading.Thread(target=get_stats, args=(context,))

t1.start()
t2.start()
t1.join()
t2.join()

*This race condition could lead to a crash, or possibly expose memory contents if an attacker can trigger it carefully.*

Exploit Details (Simple Version)

While no publicly available exploit is circulating (as of June 2024), here's a theoretical attack:

- An attacker triggers many handshake connections, causing the server to reload certificates from a directory.
- Simultaneously, they force your server-side code (maybe a status page or a logging process) to call get_ca_certs() or cert_store_stats().
- If the stars align, memory structures overlap. Python can crash, or information from another process/thread may leak into a TLS connection (imagine session cookies, keys, etc).

If you’re running Python in a threaded web server or application, this is a real threat—especially in environments where certificate directories are used and rotated often (think: auto-renewing LetsEncrypt setups).

Any *older* maintained lines

> The bug is fixed in CPython 3.10.14, 3.11.9, 3.12.3, and 3.13.a5

How Was It Fixed?

Python core developers added proper thread locking around certificate store access. Internal data structures won’t overlap anymore during these method calls.

Reference to the actual GitHub patch on CPython.

Upgrade to 3.10.14, 3.11.9, 3.12.3, or higher.

- If you can't upgrade, avoid calling SSLContext.get_ca_certs() and SSLContext.cert_store_stats() in code that might run in parallel with handshake operations—*but that’s hard to guarantee in production*.

References

- Python CVE-2024-0397 Security Advisory
- Official CPython ChangeLog Entry
- Python ssl module docs

Final Words

CVE-2024-0397 is a timely reminder: even in “safe” languages, fast-moving, multi-threaded code can cause weird, hard-to-detect bugs. Whether you run web servers or just care about code quality, keeping your dependencies and python runtimes up to date is your best weapon.

Timeline

Published on: 06/17/2024 16:15:10 UTC
Last modified on: 07/03/2024 01:44:41 UTC