Checking for a Security Compromise: Backdoors and Intruders

By | February 1, 2015

In our imperfect world it is all too possible for a cloud server to get hacked. We can, however, find the culprit and make sure our cloud server security prevents a future security breach. In this article we will learn some techniques and tools useful for investigating our cloud servers if we suspect they’ve been compromised.


Cloud servers can be compromised as a result of various factors.

If your cloud server has been compromised, don’t panic. Panic leads to poor decisions, and then the situation could become worse. Instead, try to understand what happened and make sure your cloud server does not get compromised again in the same manner. This article’s objective is simple: Learn from your mistakes and don’t make the same mistakes twice.

In this article we will cover the tools we can use before going into rescue mode (to be covered in the next articleChecking for a Security Compromise: Rescue Mode Investigation). The cloud server used for this article series was running Ubuntu 8.10; however, the steps demonstrated will be similar for other Linux distributions.

Important Warning

Before proceeding any further, you must make an important decision: Do you plan to involve law enforcement and prosecute the attacker? If you do, then leave the compromised system alone and make no changes to it. Any changes you make post-attack could complicate and taint the evidence. Because of that, a common policy is to power off a system once a compromise is detected and leave it off until law enforcement is ready to investigate.

Checking Network Connections

Let’s begin our investigation by checking our cloud server’s network connections.


netstat -an

This command helps you check for any backdoors which have been opened on your cloud server.

netstat -an
Active Internet connections (servers and established)
Proto Recv-Q Send-Q             Local Address                     Foreign Address            State
tcp        0               0                                    *                  LISTEN
tcp        0               0                                    *                  LISTEN
tcp        0               0                                    *                  LISTEN
tcp        0             284                                           ESTABLISHED

In this case we see port 6697 is open. This is a port commonly used by IRC servers. This is not a good sign, unless we’re running our own chat server. We can sniff any connections to that port using tcpdump.

tcpdump src port 6697

This will capture all the packets with destination port 6697.

Using lsof

lsof is a command line utility which stands for list open files. It is used in many Unix-like systems to report a list of all open files and the processes that opened them. By default Linux treats everything, including devices, as a file. This makes lsof a very powerful tool. For example, we can use lsof to see what user has a particular file open:

sudo lsof /etc/passwd

If we discover the username under the intruder’s control, lsof can be used to display all his running processes:

sudo lsof -u hisUserName

lsof also helps us check our network connections. Investigating various aspects of our cloud server with multiple tools is important because if we suspect the system is compromised, we can’t be sure which commands will provide reliable results. Also, lsof provides some options which netstat does not.

To list all the open IP sockets associated with your cloud server’s SSH server, run the following command:

sudo lsof -i:22


We have now learned some techniques that can be used to discover backdoors and track intruders on our slice. This will help us avoid a repeat of whatever situation or mistake led up to the compromise, so we’re less likely to get hacked again in the same way. In the next article, Checking for a Security Compromise: Rescue Mode Investigation, we will learn how to investigate our cloud server in rescue mode.


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