What is XML external entity (XXE) injection? - ICSS

Introduction

XML External Entity Injection (also known as XXE) is a web security flaw that allows an attacker to interfere with an application's XML data processing. It frequently enables an attacker to examine files on the application server's disc and interact with any back-end or external systems that the programme can access.

An attacker can escalate a XXE assault to compromise the underlying server or other back-end infrastructure in some cases by exploiting the XXE vulnerability to launch server-side request forgery (SSRF) attacks.

What are the different forms of XXE attacks?

There are several forms of XXE attacks: Using XXE to obtain files, in which an external entity is defined that contains the contents of a file and is returned in the application's response.

Using XXE to launch SSRF attacks, where an external entity is defined by a URL to a back-end system.
Exfiltrate data out-of-band using blind XXE, where sensitive data is sent from the application server to a system controlled by the attacker.

Using blind XXE to retrieve data via error messages, the attacker can cause a parsing error message containing sensitive data.

Using XXE to obtain files

To launch a XXE injection attack that obtains an arbitrary file from the server's filesystem, you must change the provided XML in two ways:

Introduce (or modify) a DOCTYPE element that establishes an external entity providing the file's path.
To use the declared external entity, edit a data value in the XML that is returned in the application's response.

Assume a shopping application checks a product's stock level by delivering the following XML to the server:

<?xml version="1.0" encoding="UTF-8"?><stockChck><prdctId>556</prdctId></stockChck>

Because the application has no XXE defences, you can use the XXE vulnerability to retrieve the /etc/passwd file by providing the following XXE payload:

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE foo [ <!ENTITY xxe SYSTEM "file:///etc/passwd"> ]>

<stockChck><prdctId>&xxe;</prdctId></stockChck>

This XXE payload defines an external entity &xxe; whose value is the contents of the /etc/passwd file, and it makes use of the entity within the prdctId value. The contents of the file are included in the application's response as a result of this:

Invalid product ID: root:x:0:0:root:/root:/bin/bash

daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin

bin:x:2:2:bin:/bin:/usr/sbin/nologin

...

Blind XXE vulnerabilities

Many XXE vulnerabilities remain unnoticed. This indicates that the application does not return the values of any defined external entities in its answers, making direct access to server-side files impossible.Blind XXE vulnerabilities can still be discovered and exploited, but more sophisticated techniques are required.

Out-of-band techniques can occasionally be used to uncover vulnerabilities and attack them in order to exfiltrate data. Furthermore, you can occasionally cause XML parsing mistakes, which result in the revealing of sensitive data within error messages.

XXE attacks via file upload

How to Identify and Test XXE Vulnerabilities

Burp Suite's web vulnerability scanner can find the vast majority of XXE vulnerabilities fast and reliably.

Manually testing for XXE vulnerabilities often entails the following steps:

Testing for file retrieval by constructing an external object based on a well-known operating system file and incorporating that entity into data returned by the application's response.
Testing for blind XXE vulnerabilities involves creating an external entity based on a URL to a system that you control and watching for interactions with that system.
The Burp Collaborator client is ideal for this.
Using an XInclude attack to try to obtain a well-known operating system file, we tested for susceptible inclusion of user-supplied non-XML data within a server-side XML document.

How to prevent XXE vulnerabilities

Almost all XXE vulnerabilities originate as a result of the application's XML parsing library supporting potentially harmful XML features that the programme does not require or intends to utilise. Disabling those functionalities is the simplest and most efficient technique to avoid XXE assaults.

In general, disabling external entity resolution and XInclude support is sufficient. This is normally accomplished using configuration settings or by altering default behaviour programmatically. For further information on how to disable superfluous capabilities, consult the documentation for your XML parsing library or API.

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