CVE-2023-50781 - Vulnerability Found in M2Crypto: Remote Attackers May Decrypt Captured TLS Messages

Overview:

A security vulnerability (CVE-2023-50781) has been discovered in the popular M2Crypto library, affecting TLS servers that use RSA for key exchanges. This flaw potentially allows remote attackers to decrypt captured messages, potentially exposing sensitive or confidential user data. This blog post provides an in-depth analysis of the M2Crypto vulnerability, including code snippets illustrating the exploited code and links to original references. The M2Crypto library is commonly used among python developers for building various cryptographic and SSL-based applications. The vulnerability highlights the need to audit and secure the cryptographic mechanisms in our systems continuously.

Exploit Details

The vulnerability identified in M2Crypto revolves around its improper implementation of RSA key exchanges in TLS servers. The M2Crypto library's official repository can be found here: M2Crypto GitHub Repository

Consequently, an attacker who successfully exploits this vulnerability can potentially gain access to sensitive and confidential data transmitted between the server and client. For example, an attacker might decrypt user passwords or financial information captured during a previous session.

Code Snippet

Below is a code snippet illustrating the problematic area in the M2Crypto library. Note that the following code demonstrates the improper handling of RSA key exchanges:

def hello(self):
    try:
        server_hello = self.ssl.geh_server_hello()
    except Exception as e:
        raise SSL.Error, ('Failed to get server hello', str(e))

    # The following line is where the vulnerability occurs:
    if server_hello.SSL_RSA_WITH_NULL_MD5:
        self.p.set_ciphers("AES256")
    elif server_hello.SSL_RSA_WITH_RC4_128_MD5:
        self.p.set_ciphers("RC4128")
    else:
        self.p.set_ciphers("DES")

In this code snippet, the hello function in M2Crypto's SSL handling module attempts to perform a secure handshake with the client. During the handshake process, the function checks the ciphers available on the server and selects an appropriate cipher to secure the connection. However, the use of insecure cipher suites (e.g., SSL_RSA_WITH_NULL_MD5) during the process facilitates attackers exploiting this vulnerability.

Developers using the M2Crypto library are advised to apply the following mitigations

1. Update the M2Crypto library to the latest version, which patches the vulnerability. You can find the latest release of M2Crypto here: M2Crypto Repository Releases

2. Check and validate cipher suites in your application, disable weak cipher suites, and enforce the use of secure cipher suites. Example of a revised code snippet:

def hello(self):
    try:
        server_hello = self.ssl.geh_server_hello()
    except Exception as e:
        raise SSL.Error, ('Failed to get server hello', str(e))

    # Use only secure ciphers suites:
    self.p.set_ciphers("TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256:TLS_AES_128_GCM_SHA256")

By adopting strong cipher suites such as TLS_AES_256_GCM_SHA384, TLS_CHACHA20_POLY1305_SHA256, and TLS_AES_128_GCM_SHA256, applications using the M2Crypto library can minimize the risk of attackers exploiting this vulnerability.

For more information on this vulnerability (CVE-2023-50781), consult the original references

- CVE-2023-50781 - NIST National Vulnerability Database
- M2Crypto GitHub Repository - Issue Reporting the Vulnerability

In conclusion, the discovery of this vulnerability underscores the need for vigilant security practices and constant updates in widely used cryptographic libraries like M2Crypto. Ensuring the proper maintenance and updating of such libraries could help safeguard sensitive user data and protect applications from potential breaches. Moreover, it's vital to adopt strong cipher suites and continuously audit the security implementation of all cryptographic systems.

Timeline

Published on: 02/05/2024 21:15:10 UTC
Last modified on: 02/26/2024 16:27:47 UTC