In the world of cybersecurity, vulnerabilities are constantly being discovered and fixed. A recent vulnerability, identified as CVE-2023-23931, has been found in the cryptography package used by Python developers. This long read post will cover the details of the vulnerability, provide code examples, and link to the original references. We will also provide a high-level explanation of the exploit and how to address it.

Cryptography Package

The cryptography package is designed to make it easy for Python developers to include cryptographic primitives and recipes. It is widely used for secure communication, encryption, and hashing. However, a vulnerability was discovered in the Cipher.update_into function, which affects certain versions of the package.

Details of the Vulnerability (CVE-2023-23931)

This vulnerability arises when Cipher.update_into accepts Python objects that implement the buffer protocol but provide only immutable buffers. As a result, immutable objects such as bytes can be mutated, violating fundamental rules of Python and leading to corrupted output. This problem has been present since update_into was first introduced in cryptography 1.8.

Here's a code snippet that demonstrates the issue with the Cipher.update_into function

from cryptography.hazmat.backends import default_backend
from cryptography.hazmat.primitives import hashes, hmac

data_to_encrypt = b'sensitive_data'
h = hmac.HMAC(b'secret_key', hashes.SHA256(), backend=default_backend())
h
.update(data_to_encrypt)

# Incorrect use of update_into with an immutable object (bytes)
buffer = bytearray(50)
immutable_data = b'immutable'
result = h.update_into(immutable_data, buffer)  # Mutable buffer is provided

In this example, an immutable object (immutable_data) is passed to the update_into function along with a mutable buffer (buffer). According to the Python rules, since the passed object is immutable, the output should remain unchanged.

However, due to the vulnerability, the immutable object will be mutated, and the output will be corrupted.

Exploit Details

An attacker could potentially exploit this vulnerability to cause unexpected behavior in encryption or data manipulation, leading to incorrect output, data corruption, or information leakage.

Resolution and Fix

The developers of the cryptography package have addressed this issue by making sure that an exception is raised when an attempt is made to mutate an immutable object. This will prevent the violation of Python's rules and ensure the correct output.

It is strongly recommended that you upgrade your cryptography package to the latest version to protect your Python applications from possible exploitation.

Original References

1. CVE Details: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2023-23931
2. Cryptography Documentation: https://cryptography.io/en/latest/
3. Cryptography Package Changelog: https://cryptography.io/en/latest/changelog/
4. GitHub Issue: https://github.com/pyca/cryptography/issues/2323

Conclusion

The discovery and resolution of the CVE-2023-23931 vulnerability demonstrate the importance of constantly monitoring and updating your code to protect against potential security threats. By staying informed of the latest vulnerabilities and their fixes, developers can ensure that their Python applications remain secure and reliable.

Timeline

Published on: 02/07/2023 21:15:00 UTC
Last modified on: 02/16/2023 16:57:00 UTC