OS command injection

Description

Command injection is an attack in which the goal is execution of arbitrary commands on the host operating system via a vulnerable application. Command injection attacks are possible when an application passes unsafe user supplied data (forms, cookies, HTTP headers etc.) to a system shell. In this attack, the attacker-supplied operating system commands are usually executed with the privileges of the vulnerable application. Command injection attacks are possible largely due to insufficient input validation.

This attack differs from Code Injection, in that code injection allows the attacker to add his own code that is then executed by the application. In Code Injection, the attacker extends the default functionality of the application without the necessity of executing system commands.

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 of 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.Startare used to call underlying OS functions. In PHP we may look for calls such as exec()or passthru().