The process of detecting basic OS Command Injection vulnerabilities is the same process for exploiting such vulnerabilities. We attempt to append our command through various injection methods. If the command output changes from the intended usual result, we have successfully exploited the vulnerability. This may not be true for more advanced command injection vulnerabilities because we may utilize various fuzzing methods or code reviews to identify potential command injection vulnerabilities. We may then gradually build our payload until we achieve command injection. This module will focus on basic command injections, where we control user input that is being directly used in a system command execution a function without any sanitization.
Command Injection Methods
To inject an additional command to the intended one, we may use any of the following operators:
| Injection Operator | Injection Character | URL-Encoded Character | Executed Command |
|---|---|---|---|
| Semicolon | ; | %3b | Both |
| New Line | \n | %0a | Both |
| Background | & | %26 | Both (second output generally shown first) |
| Pipe | | | %7c | Both (only second output is shown) |
| AND | && | %26%26 | Both (only if first succeeds) |
| OR | | | %7c%7c | Second (only if first fails) |
| Sub-Shell | `` | %60%60 | Both (Linux-only) |
| Sub-Shell | $() | %24%28%29 | Both (Linux-only) |
We can use any of these operators to inject another command so both or either of the commands get executed. We would write our expected input (e.g., an IP), then use any of the above operators, and then write our new command. |
In addition to the above, there are a few unix-only operators, that would work on Linux and macOS, but would not work on Windows, such as wrapping our injected command with double backticks (``) or with a sub-shell operator ($()).
In general, for basic command injection, all of these operators can be used for command injections regardless of the web application language, framework, or back-end server. So, if we are injecting in a PHP web application running on a Linux server, or a .Net web application running on a Windows back-end server, or a NodeJS web application running on a macOS back-end server, our injections should work regardless.
The only exception may be the semi-colon ;, which will not work if the command was being executed with Windows Command Line (CMD), but would still work if it was being executed with Windows PowerShell.