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: calc 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” 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.

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 env = pb.environment;

pb.directory(new File("TrustedDir"));

Process p = pb.start;

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

System.Diagnostics.Process process = new System.Diagnostics.Process;

System.Diagnostics.ProcessStartInfo startInfo = new System.Diagnostics.ProcessStartInfo;

startInfo.FileName = "validatedCommand";

startInfo.Arguments = "validatedArg1 validatedArg2 validatedArg3";

process.StartInfo = startInfo;

process.Start;

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

Other Cheatsheets

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