CVE-2023-0361 - Timing Side-Channel Vulnerability in GnuTLS RSA Key Exchange, and How to Exploit It

A critical vulnerability has been identified in GnuTLS, a popular open-source library that implements Transport Layer Security (TLS) protocols and provides secure communication over the internet. This vulnerability, dubbed CVE-2023-0361, exposes a timing side-channel in the handling of RSA ClientKeyExchange messages. An attacker can exploit this side-channel to recover the RSA key encrypted in the ciphertext, potentially performing a Bleichenbacher style attack over a network. This blog post will outline the details of the vulnerability, provide code snippets, and discuss ways to exploit it.

Vulnerability Explanation

The GnuTLS flaw lies in the RSA ClientKeyExchange message handling process when a GnuTLS server receives a message with RSA keys. Through accurate measurement of the server's response time to specially crafted messages, an attacker can deduce the RSA ciphertext. As a result, the attacker could potentially decrypt application data exchanged over the connection between the client and server.

This CVE-2023-0361 vulnerability is a timing side-channel attack (referred to as a Bleichenbacher attack). A side-channel attack is an exploit that does not target the cryptographic methods directly but manipulates the information leaked through other channels (e.g., timing). The Bleichenbacher attack exploits the timing information in a manner that enables the attacker to recover the encrypted RSA key.

To execute this vulnerability successfully, an attacker needs to send a large number of carefully crafted messages to the target server. As the server processes these messages, the attacker analyzes the server's response time and compares it against a known or generated value to identify discrepancies.

Exploit Details

The following code snippet demonstrates an example of how an attacker might generate a crafted ClientKeyExchange message to target the affected system:

`python
import gnutls

def crafted_RSA_key_exchange_msg(payload):
   # Function to generate a crafted RSA key exchange message
   msg = gnutls.ClientKeyExchange()
   msg.payload = payload

Timeline

Published on: 02/15/2023 18:15:00 UTC
Last modified on: 03/24/2023 16:15:00 UTC