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#REDIRECT [[Command_Injection_Defense_Cheat_Sheet]]
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== Description ==
Command injection (or OS Command Injection)  is a type of injection where the software, that constructs  a system command using externally-influenced input, does not correctly neutralizes the input from special  elements that can modify the initially intended command.
For example, if the supplied value is:
when typed in a Windows command prompt, the application “Calculator” is displayed.
However, if the supplied value  has been tempered with, and now it is:
calc & echo “test”
this changes the meaning of the initial intended value: now, both the “Calculator” application and the value “test”  are displayed.
The problem is exacerbated if the compromised process does not follow the principle of least privilege principle and attacker-controlled commands end up running with special system privileges that increases the amount of damage.
== Primary Defenses  ==
=== Defense Option 1: Avoid calling OS commands directly ===
The primary defense is to avoid calling OS commands directly. Built-in library functions are a very good alternative to OS Commands, and they cannot be manipulated to perform tasks other than those it is intended to do. For example use “mkdir()” instead of system(“mkdir /dir_name”).
If there are availble libraries or APIs for the language you used, this is the preferred method.
=== Defense option 2: Parametrization  in conjunction with Input Validation ===
If it is considered unavoidable the call to a system command incorporated with user-supplied, the following two layers of defense should be used within software in order to prevent attacks
* '''Parametrization''' - If available, use structured mechanisms that automatically enforce the separation between data and command. These mechanisms can help to provide the relevant quoting, encoding.
* '''Input validation''' - the values for commands and the relevant  arguments should be both validated. There are different degrees of validation for the actual command and its arguments: 
** When it comes to the commands used, these must be validated against a whitelist of allowed commands.
** In regards to the arguments used for these commands, they should be validated using the following options:
*** Positive or “whitelist” input validation - where are the arguments allowed explicitly defined
*** White list Regular Expression - where is explicitly defined a whitelist of good characters allowed and the maximum length of the string.  Ensure that metacharacters like  & |  ; $ > < ` \ ! and white-spaces are not part of the Regular Expression. For example, the following  regular expression only allows lowercase letters and numbers, and does not contain metacharacters. The length is also being limited to 3-10 characters:
== Additional Defenses ==
=== Least privilege ===
On top of primary defences, parameterizations and input validation, we also recommend adopting all of these additional defenses in order to provide defense in depth. These additional defenses are:
*Applications should run using the lowest privileges that are required to accomplish the necessary tasks .
*If possible, create isolated accounts with limited privileges that are only used for a single task.
== Code examples ==
=== C/C++ ===
Use function from exec()  family (  execl(), execve(), etc) , instead of system()
The exec family of functions does not use a full shell interpreter, so it is not vulnerable to command-injection attacks.
=== Java ===
In Java,  use ProcessBuilder and the command must be separated from its arguments.
==== Incorrect Usage  ====
ProcessBuilder b = new ProcessBuilder("C:\DoStuff.exe -arg1 -arg2");
In this example, the command together with the arguments are passed as a one string, making easy to manipulate that expression and inject malicious strings.
==== Correct Usage ====
Here is an example that starts a process with a modified working directory. The command, and each of the arguments are passed separately, This make it easy to validated each term and reduces the risk to insert malicious strings. 
ProcessBuilder pb = new ProcessBuilder("TrustedCmd", "TrustedArg1", "TrustedArg2");
Map<String, String> env = pb.environment(); File("TrustedDir"));
Process p = pb.start();
=== PHP ===
In PHP  use escapeshellarg() or escapeshellcmd() rather than exec() / system() / passthru()
== Related articles ==
'''Description of Command Injection Vulnerability'''
* [[Command Injection|OWASP Command Injection]]
'''How to Avoid Vulnerabilities'''
C Coding: [ Do not call system()]
'''How to Review Code'''
* OWASP - [[Reviewing Code for OS Injection]]
'''How to Test'''
* [[OWASP Testing Guide]] article on [[Testing for Command Injection]]
'''External References'''
[ CWE Entry 77 on Command Injection]

Latest revision as of 18:15, 13 November 2017