Twisted is a popular event-driven networking engine for Python 3.6+. A recent vulnerability has been discovered in the Twisted SSH client and server, affecting versions prior to 22.2.. This vulnerability, labeled as CVE-2022-21716, allows an attacker to flood the client or server with an infinite amount of data, resulting in a buffer using up all available memory on the target system. In this post, we'll delve into the details of this vulnerability, demonstrate how it can be exploited, provide a code snippet of the vulnerable code, and discuss the patch that addresses this issue.

Vulnerability Details

The main issue with the affected versions of Twisted is that the client and server implementation of the SSH protocol will accept an unlimited amount of data for the peer's SSH version identifier. This results in a buffer that continually grows, consuming all available memory on the target system, leading to a memory exhaustion scenario.

The vulnerability can be exploited simply by connecting to the vulnerable SSH service and sending an infinite stream of data. An example of how to do this using the netcat command-line utility is as follows:

nc -rv localhost 22 < /dev/zero

This command will connect to a locally-running vulnerable SSH server on port 22 and send an endless stream of null bytes (\x00) to the target.

Vulnerable Code Snippet

While specific code involved in the vulnerability is not directly available, the flaw resides in the function that handles the SSH version identifier.

def some_handler_function(self):
    # Vulnerable code snippet
    while True:
        # Accept an unlimited amount of data for the peer's SSH version identifier
        data_received = self.receive_some_data()
        if data_received is None:
            break

        self.process_data(data_received)

Patch and Solution

The vulnerability has been patched in Twisted version 22.2., so the best course of action to protect your systems from this vulnerability is to upgrade Twisted as soon as possible. The patch ensures that the server and client implementations handle data in a controlled manner by limiting the amount of data that can be buffered.

def patched_handler_function(self):
    # Patched code snippet
    max_data_size = 1024  # Limit the allowed data buffer size

    while True:
        # Only accept a limited amount of data for the peer's SSH version identifier
        data_received = self.receive_some_data(max_data_size)
        if data_received is None:
            break

        self.process_data(data_received)

        # Adjust the remaining allowed data buffer size
        max_data_size -= len(data_received)
        if max_data_size <= :
            break

Additionally, the patch release includes fixes to other minor issues and improvements in the codebase. It is highly recommended to update Twisted to the latest version to take advantage of these enhancements.

References & Further Reading

1. Original advisory for CVE-2022-21716: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2022-21716
2. Twisted Project: https://twistedmatrix.com/trac/
3. Twisted GitHub Repository: https://github.com/twisted/twisted

Conclusion

CVE-2022-21716 is a critical memory exhaustion vulnerability in Twisted's SSH client and server implementations. It is essential to upgrade to Twisted version 22.2. or later to protect your systems from potential exploitation. Ensure that you regularly check for and apply patches to all your software, as vulnerabilities like this can have severe consequences if left unaddressed.

Timeline

Published on: 03/03/2022 21:15:00 UTC
Last modified on: 07/03/2022 03:15:00 UTC