Check for a security compromise: back doors and intruders

  • Last updated on: 2018-12-06
  • Authored by: Rose Contreras

Cloud servers can be compromised as a result of various factors: weak passwords, weak iptables, old software versions with known exploits, and so on.

If your cloud server has been compromised, don’t panic. Panic leads to poor decisions, which could make the situation worse. Instead, try to understand what happened and ensure that your cloud server is not compromised again in the same manner. This article’s objective is to learn from your mistakes and to not make the same mistakes twice.

This article describes some techniques and tools that you can use to investigate your servers if you suspect that they have been compromised. You should use these tools before going into rescue mode (which is covered in Checking for a security compromise: Rescue mode investigation). The cloud server used for this article was running Ubuntu® 8.10. However, the steps demonstrated are similar for other Linux® distributions.

Important warning

Before proceeding, 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 taint the evidence and complicate the investigation. Because of that, a common policy is to power off a system after a compromise is detected and leave it off until law enforcement is ready to investigate.

Check network connections

Begin your investigation by checking your cloud server’s network connections.

Use the netstat -an command, which produces output similar to the following example, to check for any back doors that 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 example, port 6697 is open. This port is commonly used by Internet Relay Chat (IRC) servers. This is not a good sign, unless you are running your own chat server. You can discover any connections to that port by using the following tcpdump command:

tcpdump src port 6697

Running this command captures all the packets with destination port 6697.

Use lsof

The list open files (lsof) command-line utility is used in many systems based on UNIX® 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.

Not all virtual machines (VMs) have lsof installed by default, so you might have to install it by using yum or apt-get if you see the following response:

-bash: lsof: command not found

For example, you can use lsof to see what user has a particular file open:

sudo lsof /etc/passwd

If you discover the username under the intruder’s control, you can use lsof to display all of the intruder’s running processes:

sudo lsof -u hisUserName

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

To list all the open Internet Protocol (IP) sockets associated with your cloud server’s Secure Shell (SSH) server, run the following command:

sudo lsof -i:22


In this article, you’ve learned some techniques to use to discover back doors and track intruders on your server. These techniques will help you avoid a repeat of whatever situation or mistake led to the compromise, so you are less likely to get hacked again in the same way. In the next article, Checking for a security compromise: Rescue mode investigation, you will learn how to investigate your cloud server in rescue mode.

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