CVE-2022-48566: An In-Depth Analysis of the Python HMAC Compare_Digest Issue and Exploit Details

Recently, a vulnerability was discovered in Python's standard library module HMAC, specifically in the compare_digest function. This security issue was assigned the identifier CVE-2022-48566. In this post, we will look at the details of this vulnerability, along with the related code snippets, links to the original references, and exploit information.

Background

The vulnerability is present in the compare_digest function of Python's hmac module (Lib/hmac.py) up to version 3.9.1. The compare_digest function, as the name implies, compares two hashes for equality in a time that is dependent solely on the length of the operands, rather than the contents, thus providing a constant time comparison that helps protect against timing attacks. The vulnerability at hand allows for constant-time-defeating optimizations in the accumulator variable, leading to the possibility of timing attacks.

Code Snippet for the Vulnerable Function

def compare_digest(a, b):
    if sys.version_info < (2, 7, 7):
        return _slow_compare_digest(a, b) #pre 2.7.7 could allow timing attacks
    else:
        return a == b

Exploit Details

An attacker capable of sending a large number of requests to the server that processes HMAC digests could potentially exploit the timing information leak. By carefully measuring the time taken for each request and comparing the hashes, the attacker may deduce information about the contents of the digests being compared. This could lead to the recovery of secret keys in some scenarios.

Original Security Advisory

https://bugs.python.org/issue45276

National Vulnerability Database (NVD) Entry

https://nvd.nist.gov/vuln/detail/CVE-2022-48566

Python HMAC Documentation

https://docs.python.org/3/library/hmac.html

Timing Attacks

https://en.wikipedia.org/wiki/Timing_attack

Mitigation

As of Python 3.9.2, this issue has been addressed. To protect against this vulnerability, users should update to Python 3.9.2 or later. To update your Python version, follow the instructions on the official Python website:
https://www.python.org/downloads/

Conclusion

CVE-2022-48566 is a vulnerability present in hmac.compare_digest in Python's standard library up to version 3.9.1. This security issue allows for the possibility of timing attacks due to constant-time-defeating optimizations in the accumulator variable. To mitigate this vulnerability, users should update to Python 3.9.2 or later.

Timeline

Published on: 08/22/2023 19:16:00 UTC
Last modified on: 10/13/2023 17:04:00 UTC