XML is widely used in software systems for persistent data, exchanging data between a web service and client, and in configuration files. A misconfigured XML parser can leave a critical flaw in an application. Processing of untrusted XML streams can result in a range of exploits, including remote code execution and sensitive data being read. This tutorial will explain to information security specialists and programmers the fundamentals of XML and XML external entity (XXE) injection and it will go through the major XXE issues found on Google and Facebook servers. Moreover, this tutorial provides a hands-on lab for identifying and exploiting XXE vulnerabilities, along with practical guidance on how to secure a code-supporting XML input parsing.
What is XML?
XML (eXtensible Markup Language) is a markup language that defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. It is a markup language used for storing and transporting data.
Why we use XML?
1. XML is platform-independent and programming language independent, thus it can be used on any system and supports the technology change when that happens.
2. The data stored and transported using XML can be changed at any point in time without affecting the data presentation.
3. XML allows validation using DTD and Schema. This validation ensures that the XML document is free from any syntax error.
4. XML simplifies data sharing between various systems because of its platform-independent nature. XML data doesn’t require any conversion when transferred between different systems. Full Tutorial About XML what is xml and why we use
Syntax
Every XML document mostly starts with what is known as XML Prolog.
<?xml version="1.0" encoding="UTF-8"?>
Above the line is called XML prolog and it specifies the XML version and the encoding used in the XML document. This line is not compulsory to use but it is considered a `good practice` to put that line in all your XML documents.
Every XML document must contain a `ROOT` element. For example:
<?xml version="1.0" encoding="UTF-8"?>
<mail>
<to>Kumar Atul Jaiswal</to>
<from>HackerBoY</from>
<subject>About XML</subject>
<text>Teach about XML</text>
</mail>
In the above example the <mail> is the ROOT element of that document and <to>, <from>, <subject>, <text> are the children elements. If the XML document doesn't have any root element then it would be consideredwrong or invalid XML doc.
Another thing to remember is that XML is a case sensitive language. If a tag starts like <to> then it has to end by </to> and not by something like </To>(notice the capitalization of T)
Like HTML we can use attributes in XML too. The syntax for having attributes is also very similar to HTML. For example:
<text category = "message">You need to learn about XXE</text>
In the above example category is the attribute name and message is the attribute value.
So now let's understand how that DTD validates the XML. Here's what all those terms used in note.dtd mean
!DOCTYPE note - Defines a root element of the document named note
!ELEMENT note - Defines that the note element must contain the elements: "to, from, heading, body"
!ELEMENT to - Defines the to element to be of type "#PCDATA"
!ELEMENT from - Defines the from element to be of type "#PCDATA"
!ELEMENT heading - Defines the heading element to be of type "#PCDATA"
!ELEMENT body - Defines the body element to be of type "#PCDATA"
NOTE: #PCDATA means parseable character data.
Video Tutorial :-
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- Hacking Truth by Kumar Atul Jaiswal
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