How an intruder attacks SSH

One particular Linux service needs no introduction: Secure Shell (SSH) is synonymous with logging into remote Linux devices of all varieties. You can use SSH to log into a Raspberry Pi, a mail server, a web server, or even embedded Linux devices such as those running Internet of Things (IoT) applications.

SSH emerged in the 1990s, when it became clear that the unencrypted Telnet was not suitable for communication on the open Internet. SSH version 1 was popular for years, but experts eventually began to warn that it had its own security problems. SSH version 2 was a major rewrite, due to the numerous issues that plagued version 1, including vulnerability to man-in-the-middle attacks. In the Linux world, the SSH software of choice for both client and server is called OpenSSH [1].

This article looks at some of the approaches attackers and ethical hackers use to compromise SSH servers. I will also look at how to prevent a common type of attack against SSH servers. It should go without saying: Only use the tools discussed in this article on servers that you own or explicitly have permission to test against. A number of these approaches could cause downtime or ultimately lock you out of the target SSH instance.

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