OS Command Injection Defense Cheat Sheet



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Introduction
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: when execute, it 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.

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 available libraries or APIs for the language you used, this is the preferred method.

Defense option 2: Escape values added to OS commands specific to each OS
TODO
 * For examples, see escapeshellarg or escapeshellcmd in PHP.

Defense option 3: 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 ^[a-z0-9]{3,10}$
 * 1) 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.
 * 2) 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:
 * 3) * When it comes to the commands used, these must be validated against a whitelist of allowed commands.
 * 4) * In regards to the arguments used for these commands, they should be validated using the following options:
 * 5) ** Positive or “whitelist” input validation - where are the arguments allowed explicitly defined
 * 6) ** 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:

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.

Java
In Java, use ProcessBuilder and the command must be separated from its arguments.

Note about the Java's Runtime.exec method behavior:

There are many sites that will tell you that Java's Runtime.exec is exactly the same as C's system function. This is not true. Both allow you to invoke a new program/process. However, C's system function passes its arguments to the shell (/bin/sh) to be parsed, whereas Runtime.exec tries to split the string into an array of words, then executes the first word in the array with the rest of the words as parameters. Runtime.exec does NOT try to invoke the shell at any point and do not support shell metacharacters. The key difference is that much of the functionality provided by the shell that could be used for mischief (chaining commands using "&", "&&", "|", "||", etc, redirecting input and output) would simply end up as a parameter being passed to the first command, and likely causing a syntax error, or being thrown out as an invalid parameter.

Code to test the note above:

Result of the test:

RC   : 0 OUT  :
 * 1) TEST CMD: java -version & cmd /c whoami

ERROR : java version "1.8.0_31"

RC   : 0 OUT  :
 * 1) TEST CMD: java -version && cmd /c whoami

ERROR : java version "1.8.0_31"

RC   : 0 OUT  :
 * 1) TEST CMD: java -version | cmd /c whoami

ERROR : java version "1.8.0_31"

RC   : 0 OUT  :
 * 1) TEST CMD: java -version || cmd /c whoami

ERROR : java version "1.8.0_31"

RC   : 0 OUT  : mydomain\simpleuser
 * 1) TEST PAYLOAD ONLY: cmd /c whoami

ERROR :

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.

.Net
In .Net use System.Diagnostics.Process.Start to call underlying OS functions.

PHP
In PHP use escapeshellarg or escapeshellcmd rather than exec, system, passthru.

Related articles
Description of Command Injection Vulnerability
 * OWASP Command Injection

How to Avoid Vulnerabilities How to Review Code
 * C Coding: Do not call system
 * 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

Authors and Primary Editors
Katy Anton - katy.anton@owasp.org

Jim Manico - jim.manico@owasp.org

Other Cheatsheets

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