In this lesson,
we're going to discuss backdoors,
focusing specifically on rootkits, trojans, and web shells.
These backdoors are crucial tools in the attacker's arsenal,
allowing them to maintain access to compromised systems
long after the initial breach.
First, let's define what a backdoor is.
A backdoor is a method used
by attackers the bypass normal authentication mechanisms
and gain access to a system, network, or application.
This access can be used to execute commands, retrieve data,
or deploy further malicious activities.
The key characteristic of backdoors is
that they allow the attacker
to re-enter a system without the need
to exploit the vulnerability again.
Think of it as having a secret key to a locked door;
even if the main entrance is secured,
the attacker can still come and go as they please
through the hidden backdoor.
Now, let's dive into rootkits.
A root kit is a type of software that is designed
to hide the existence of certain processes
or programs from normal methods of detection
and enable continued privileged access to a computer.
Now, rootkits can operate at the kernel level
of the operating system, which gives them the ability
to intercept and alter system calls.
This makes them extremely difficult to detect and remove.
For instance, the ZeroAccess rootkit was notorious
for creating a hidden file system on Windows machines,
allowing attackers to conceal their activities
and maintain control over the infected system.
Rootkits like ZeroAccess can disable security software,
hide network connections,
and even prevent the system from being patched,
ensuring the attacker maintains access
for as long as possible.
To better understand rootkits,
let's look at how they integrate into a system.
When a rootkit infects a computer,
it typically gains the same level of privileges
as the operating system's kernel.
This high-level access allows the rootkit
to modify core system files
and processes, making it invisible to traditional antivirus
and anti-malware programs.
For example, the Stuxnet worm,
which targeted Iran's nuclear program,
used rootkit techniques to hide its presence
and avoid detection while it carried out its mission
of sabotaging centrifuges.
The rootkit component of the worm allowed it
to remain undetected for a long time,
demonstrating the stealth
and persistence these types of backdoors can achieve.
Next, we have trojans. A trojan, or a trojan horse,
it's a type of malware that disguises it itself
as legitimate software.
Unlike viruses and worms,
trojans do not replicate themselves,
but rely on users to execute them.
Once installed, a trojan can open a backdoor
for an attacker, giving them control
over the infected system.
Trojans are often spread through phishing emails,
malicious websites, or bundled with legitimate software.
A well-known example is the Zeus trojan, which was used
to steal banking information by logging keystrokes
and capturing screenshots.
Zeus was particularly effective
because it could inject itself into web browsers,
allowing it to capture sensitive information like usernames,
passwords, and credit card details as users typed them in.
Trojans can also be highly specialized.
For instance, some trojans are designed
to disable security measures, such as antivirus software
and firewalls, creating a safe environment
for other malware to operate.
Others might focus on creating botnets,
where infected machines are controlled remotely
by the attacker to perform coordinated attacks,
like Distributed Denial-of-Service attacks,
or DDoS attacks, sending spam emails,
or also mining cryptocurrency.
An example of a trojan designed
to create a botnet is the Mirai trojan,
which infected IoT devices
and used them to launch massive DDoS attacks, taking down
large swathes of the Internet.
Lastly, let's talk about web shells.
A web shell is a script that can be uploaded to a web server
to enable remote administration of the machine.
Web shells are often written in web development languages,
such as PHP, ASP, or JSP,
and allow an attacker to execute commands on the server
through a web browser interface.
For example, the China Chopper web shell is a tiny script
that provides a simple
but powerful interface for attackers to run commands,
upload files, and manage compromised servers.
Once a web shell is in place, an attacker can use it
to pivot further into the network, retrieve sensitive data,
or launch additional attacks.
Web shells are dangerous
because they are often overlooked
during standard security checks.
They can be easily hidden
within legitimate web application files,
making them difficult to detect.
Moreover, web shells provide attackers with a versatile tool
for maintaining long-term access to the compromised server.
For instance, the Equifax breach in 2017,
which exposed the personal information
of over 147 million people, involved attackers
exploiting a vulnerability in a web application
to upload a web shell.
This allowed them to maintain access to Equifax's systems
for several months, during which they
exfiltrated vast amounts of sensitive data.
To understand the full scope of web shells,
let's consider how they operate.
When an attacker uploads a web shell to a server,
they can use it to execute arbitrary commands,
manipulate the files, and also create additional backdoors.
Another example to consider is
where an attacker uses a web shell to upload
and execute a payload that gives them persistent access
to the server even if the original vulnerability is patched.
Now, this can include installing other types of malware,
such as key loggers, ransomware,
or additional backdoors,
ensuring the attacker retains control over the system.
So remember, backdoors. like rootkits, trojans,
and web shells, are powerful tools used by attackers
to maintain access to compromised systems.
Rootkits hide their presence and other malware, operating
at the kernel level to evade detection.
Trojans disguise themselves
as legitimate software, relying on user interaction
to install and then open backdoors for attackers.
And then, web shells provide a web-based interface
for remote control over a server, allowing attackers
to execute commands and manipulate files.
Understanding these methods is important for detecting
and defending against persistent threats in your network.