Reviewing Code for OS Injection

OWASP Code Review Guide Table of Contents

Introduction
Injection flaws allow attackers to pass malicious code through a web application to another sub system. Depending on the subsystem different types of injection attack can be performed: RDBMS: SQL Injection WebBrowser/Appserver: SQL Injection OS-shell: Operating system commands Calling external applications from your application.

How to locate the potentially vulnerable code
Many developers believe text fields are the only areas for data validation. This is an incorrect assumption. Any external input must be data validated:

Text fields, List boxes, radio buttons, check boxes, cookies, HTTP header data, HTTP post data, hidden fields, parameter names and parameter values. … This is not an exhaustive list.

“Process to process” or “entity-to-entity” communication must be investigated also. Any code that communicates with an upstream or downstream process and accepts input from it must be reviewed.

All injection flaws are input validation errors. The presence if an injection flaw is an indication of incorrect data validation on the input received from an external source outside the boundary of trust, which gets more blurred every year.

Basically for this type of vulnerability we need to find all input streams into the application. This can be from a users browser, CLI or fat client but also from upstream processes that “feed” our application.

An example would be to search the code base for the use of API’s or packages that are normally used for communication purposes.

The java.io, java.sql, java.net, java.rmi, java.xml packages are all used for application communication. Searching for methods from those packages in the code base can yield results. A less “scientific” method is to search for common keywords such as “UserID”, “LoginID” or “Password”.

Vulnerable Patterns for OS injection
What we should be looking for are relationships between the application and the operating system. The application utilising functions of the underlying operating system.

In java using the Runtime object, java.lang.Runtime does this. In .NET calls such as System.Diagnostics.Process.Start are used to call underlying OS functions. In PHP we may look for calls such as exec or passthru.

Example:

We have a class that eventually gets input from the user via a HTTP request. This class is used to execute some native exe on the application server and return a result.

public class DoStuff { public string executeCommand(String userName) {	try { String myUid = userName; Runtime rt = Runtime.getRuntime; rt.exec("doStuff.exe " +”-“ +myUid); // Call exe with userID }catch(Exception e) 		{ e.printStackTrace; } 	} }

Ok, so the method executeCommand calls doStuff.exe via the java.lang.runtime static method getRuntime. The parameter passed is not validated in any way in this class. We are assuming that the data has not been data validated prior to calling this method. Transactional analysis should have encountered any data validation prior to this point. Inputting “Joe69” would result in the following MS DOS command: doStuff.exe –Joe69 Lets say we input Joe69 & netstat –a we would get the following response: The exe doStuff would execute passing in the User Id Joe69, but then the dos command netstat would be called. How this works is the passing of the parameter “&” into the application, which in turn is used as a command appender in MS DOS and hence the command after the & character is executed.

UNIX: An attacker might insert the string “; cat /etc/hosts” the contents of the UNIX hosts file might be exposed to the attacker.

.NET Example: namespace ExternalExecution { class CallExternal { static void Main(string[] args) { String arg1=args[0]; System.Diagnostics.Process.Start("doStuff.exe", arg1); } } }

Yet again there is no data validation to speak of here. Assuming no upstream validation occurring in another class.

Classic ASP Example: <%   option explicit dim wshell set wshell = CreateObject("WScript.Shell") wshell.run "c:\file.bat " & Request.Form("Args") set wshell = nothing %>

These attacks include calls to the operating system via system calls, the use of external programs via shell commands, as well as calls to backend databases via SQL (i.e., SQL injection). Complete scripts written in perl, python, shell, bat and other languages can be injected into poorly designed web applications and executed.

Good Patterns & procedures to prevent OS injection
See the Data Validation section.

Related Articles
Interpreter Injection

Command Injection