Discover
PER INIETTARE ALTRI COMANDI INSERIRE PRIMA QUELLO CHE IL SITO SI ASPETTA E POI UTILIZZARE && E TUTTI I COMANDI CHE VUOI. This vulnerability exists because applications often use functions in programmi
PER INIETTARE ALTRI COMANDI INSERIRE PRIMA QUELLO CHE IL SITO SI ASPETTA E POI UTILIZZARE && E TUTTI I COMANDI CHE VUOI.
This vulnerability exists because applications often use functions in programming languages such as PHP, Python and NodeJS to pass data to and to make system calls on the machine’s operating system. For example, taking input from a field and searching for an entry into a file. Take this code snippet below as an example:
In this code snippet, the application takes data that a user enters in an input field named $title to search a directory for a song title. Let’s break this down into a few simple steps.
1. The application stores MP3 files in a directory contained on the operating system.
2. The user inputs the song title they wish to search for. The application stores this input into the $title variable.
3. The data within this $title variable is passed to the command grep to search a text file named songtitle.txt for the entry of whatever the user wishes to search for.
4. The output of this search of songtitle.txt will determine whether the application informs the user that the song exists or not.
Now, this sort of information would typically be stored in a database; however, this is just an example of where an application takes input from a user to interact with the application’s operating system.
An attacker could abuse this application by injecting their own commands for the application to execute. Rather than using grep to search for an entry in songtitle.txt, they could ask the application to read data from a more sensitive file.
Command Injection can be detected in mostly one of two ways:
- Blind command injection
- Verbose command injection
BIND
I have defined these two methods in the table below, where the two sections underneath will explain these in greater detail.
Blind is a type of injection is where there is no direct output from the application when testing payloads. You will have to investigate the behaviours of the application to determine whether or not your payload was successful.
For this type of command injection, we will need to use payloads that will cause some time delay. For example, the ping and sleep commands are significant payloads to test with. Using ping as an example, the application will hang for x seconds in relation to how many pings you have specified.
Another method of detecting blind command injection is by forcing some output. This can be done by using redirection operators such as >. If you are unfamiliar with this, I recommend checking out the Linux fundamentals module. For example, we can tell the web application to execute commands such as whoami and redirect that to a file. We can then use a command such as cat to read this newly created file’s contents.
The curl command is a great way to test for command injection. This is because you are able to use curl to deliver data to and from an application in your payload. Take this code snippet below as an example, a simple curl payload to an application is possible for command injection.
curl http://vulnerable.app/process.php%3Fsearch%3DThe%20Beatles%3B%20whoami
VERBOSE
Verbose is a type of injection is where there is direct feedback from the application once you have tested a payload. For example, running the whoami command to see what user the application is running under. The web application will output the username on the page directly.
Detecting command injection this way is arguably the easiest method of the two. Verbose command injection is when the application gives you feedback or output as to what is happening or being executed.
For example, the output of commands such as ping or whoami is directly displayed on the web application.
Useful payloads
I have compiled some valuable payloads for both Linux & Windows into the tables below.
Linux
- whoami = See what user the application is running under.
- ls = List the contents of the current directory. You may be able to find files such as configuration files, environment files (tokens and application keys), and many more valuable things.
- ping = This command will invoke the application to hang. This will be useful in testing an application for blind command injection.
- sleep = This is another useful payload in testing an application for blind command injection, where the machine does not have ping installed.
- nc = Netcat can be used to spawn a reverse shell onto the vulnerable application. You can use this foothold to navigate around the target machine for other services, files, or potential means of escalating privileges.
- whoami = See what user the application is running under.
- dir = List the contents of the current directory. You may be able to find files such as configuration files, environment files (tokens and application keys), and many more valuable things.
- ping = This command will invoke the application to hang. This will be useful in testing an application for blind command injection.
- timeout = This command will also invoke the application to hang. It is also useful for testing an application for blind command injection if the ping command is not installed.