This site is the archived OWASP Foundation Wiki and is no longer accepting Account Requests.
To view the new OWASP Foundation website, please visit https://owasp.org

Difference between revisions of "OS Command Injection Defense Cheat Sheet"

From OWASP
Jump to: navigation, search
(rename)
 
Line 3: Line 3:
  
 
{| style="padding: 0;margin:0;margin-top:10px;text-align:left;" |-
 
{| style="padding: 0;margin:0;margin-top:10px;text-align:left;" |-
| valign="top" style="border-right: 1px dotted gray;padding-right:25px;" |
+
| valign="top" style="border-right: 1px dotted gray;padding-right:25px;" |
 
Last revision (08/09/16): '''{{REVISIONMONTH}}/{{REVISIONDAY}}/{{REVISIONYEAR}}'''
 
Last revision (08/09/16): '''{{REVISIONMONTH}}/{{REVISIONDAY}}/{{REVISIONYEAR}}'''
  
Line 83: Line 83:
 
<pre>
 
<pre>
 
ProcessBuilder pb = new ProcessBuilder("TrustedCmd", "TrustedArg1", "TrustedArg2");
 
ProcessBuilder pb = new ProcessBuilder("TrustedCmd", "TrustedArg1", "TrustedArg2");
 +
 
Map<String, String> env = pb.environment();
 
Map<String, String> env = pb.environment();
  
Line 88: Line 89:
  
 
Process p = pb.start();
 
Process p = pb.start();
 +
</pre>
 +
 +
=== .Net ===
 +
In .Net use System.Diagnostics.Process.Start to call underlying OS functions.
 +
 +
<pre>
 +
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();
 +
 
</pre>
 
</pre>
  

Revision as of 21:03, 16 November 2017

Cheatsheets-header.jpg

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

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

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();

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

Other Cheatsheets