In this lesson, we're going
to discuss arbitrary code execution,
focusing on deserialization attacks and gaining web shells.
Arbitrary code execution vulnerabilities
are highly critical, as they allow attackers
to run malicious code on the target system,
leading to complete control
over the affected application or server.
Let's define what arbitrary code execution is.
Arbitrary code execution happens simply
when an attacker's able to run their own code
on a target system.
This type of vulnerability is particularly dangerous,
because it can allow attackers to perform a wide range
of malicious activities, including installing malware,
stealing data, and pivoting to other systems
within the network.
Next, let's discuss deserialization attacks.
Deserialization is a process of converting data
from a stored format, like JSON,
back into the object that the application can use.
Deserialization attacks occur when untrusted data is used
to take advantage of the flaws in how an application works
by injecting malicious objects.
If an application deserializes untrusted data
without proper validation,
an attacker can manipulate the serialized data
to include malicious payloads.
Imagine a web application that brings back user sessions
by converting the user input into objects.
If the application does not properly check this input,
an attacker can create a fake object
containing harmful code.
When the application processes this object,
it inadvertently runs the harmful code.
For instance, consider a scenario where an attacker
sends a specially crafted serialized object to a website.
This object looks like a normal session object,
but contains hidden malicious commands.
Look at this example of JSON serialized data.
The website, not validating the input,
deserializes this object
and unknowingly activates the malicious command.
This could include actions like listing files
from the root directory.
Next, let's discuss gaining a web shell
through arbitrary code execution.
A web shell is a script that allows an attacker
to execute commands on a web server through a web interface.
Web shells provide attackers
with remote control over the server,
enabling them to perform a variety of malicious activities,
such as exfiltrating data, pivoting to other systems,
and maintaining persistent access.
An attacker might gain a web shell
by exploiting a vulnerability
that allows for arbitrary code execution.
For instance, if an application
has a file upload functionality
that does not properly validate the uploaded files,
an attacker could upload a malicious script.
Once the script is uploaded, the attacker can access it
through a web browser and execute commands on the server.
Consider a PHP application that allows users
to upload profile pictures.
If the application does not validate the uploaded files,
an attacker could upload a PHP script
containing a web shell.
Accessing the script through the web browser
would give the attacker a command interface to the server.
To prevent attackers from gaining web shells,
ensure that all file uploads are thoroughly validated.
This includes checking file extensions,
MIME types, and content.
Additionally, restrict the locations
where files can be uploaded,
and ensure that executable files
cannot be uploaded to directories accessible via the web.
Using web application firewalls can also help detect
and block attempts to upload malicious files.
Arbitrary code execution can also result
from other vulnerabilities, such as command injection,
SQL injection, or cross-site scripting
when they're exploited in ways that allow attackers
to run commands or code on a server.
Each of these vulnerabilities leverages
the application's ability to execute code or commands,
manipulating it to include the attacker's payloads.
So, remember, arbitrary code execution
is a critical vulnerability that allows attacker
to run malicious code on a target system.
Deserialization attacks occur when untrusted data
is deserialized without proper validation,
potentially leading to arbitrary code execution.
Gaining a web shell is one of the ways attackers
can leverage arbitrary code execution
to gain remote control over a server.
To mitigate these risks,
avoid deserializing untrusted data,
validate all file uploads, restrict executable file uploads,
and use security mechanisms, like web application firewalls.
Regularly review and test your applications
for these vulnerabilities to maintain a secure environment.