OS Command Injection Defense Cheat Sheet

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Last revision (08/09/16): 11/24/2017

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.

Comand-Injection.png

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 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

  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:
    • 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:
 ^[a-z0-9]{3,10}$  

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

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:

String[] specialChars = new String[]{"&", "&&", "|", "||"};
String payload = "cmd /c whoami";
String cmdTemplate = "java -version %s " + payload;
String cmd;
Process p;
int returnCode;
for (String specialChar : specialChars) {
	cmd = String.format(cmdTemplate, specialChar);
	System.out.printf("#### TEST CMD: %s\n", cmd);
	p = Runtime.getRuntime().exec(cmd);
	returnCode = p.waitFor();
	System.out.printf("RC    : %s\n", returnCode);
	System.out.printf("OUT   :\n%s\n", IOUtils.toString(p.getInputStream(), "utf-8"));
	System.out.printf("ERROR :\n%s\n", IOUtils.toString(p.getErrorStream(), "utf-8"));
}
System.out.printf("#### TEST PAYLOAD ONLY: %s\n", payload);
p = Runtime.getRuntime().exec(payload);
returnCode = p.waitFor();
System.out.printf("RC    : %s\n", returnCode);
System.out.printf("OUT   :\n%s\n", IOUtils.toString(p.getInputStream(), "utf-8"));
System.out.printf("ERROR :\n%s\n", IOUtils.toString(p.getErrorStream(), "utf-8"));

Result of the test:

#### TEST CMD: java -version & cmd /c whoami
RC    : 0
OUT   :

ERROR :
java version "1.8.0_31"

#### TEST CMD: java -version && cmd /c whoami
RC    : 0
OUT   :

ERROR :
java version "1.8.0_31"

#### TEST CMD: java -version | cmd /c whoami
RC    : 0
OUT   :

ERROR :
java version "1.8.0_31"

#### TEST CMD: java -version || cmd /c whoami
RC    : 0
OUT   :

ERROR :
java version "1.8.0_31"

#### TEST PAYLOAD ONLY: cmd /c whoami
RC    : 0
OUT   :
mydomain\simpleuser

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.

ProcessBuilder pb = new ProcessBuilder("TrustedCmd", "TrustedArg1", "TrustedArg2");

Map<String, String> 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

How to Avoid Vulnerabilities

How to Review Code

How to Test

External References

Authors and Primary Editors

Katy Anton - katy.anton@owasp.org

Jim Manico - jim.manico@owasp.org


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