Before we move on to learn about XXE exploitation we'll have to understand XML properly.
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?
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.
Ex:
<?xml version="1.0" encoding="UTF-8"?><mail> <to>falcon</to> <from>feast</from> <subject>About XXE</subject> <text>Teach about XXE</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.
Ex:<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.
Before we move on to start learning about XXE we'll have to understand what is DTD in XML.
DTD stands for Document Type Definition. A DTD defines the structure and the legal elements and attributes of an XML document.
Let us try to understand this with the help of an example. Say we have a file named note.dtd with the following content:
<!DOCTYPE note [ <!ELEMENT note (to,from,heading,body)> <!ELEMENT to (#PCDATA)> <!ELEMENT from (#PCDATA)> <!ELEMENT heading (#PCDATA)> <!ELEMENT body (#PCDATA)> ]>
Now we can use this DTD to validate the information of some XML document and make sure that the XML file conforms to the rules of that DTD.
Ex: Below is given an XML document that uses note.dtd
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE note SYSTEM "note.dtd">
<note>
<to>falcon</to>
<from>feast</from>
<heading>hacking</heading>
<body>XXE attack</body>
</note>So now let's understand how that DTD validates the XML. Here's what all those terms used in note.dtd mean
to element to be of type "#PCDATA"from element to be of type "#PCDATA"heading element to be of type "#PCDATA"body element to be of type "#PCDATA"NOTE: #PCDATA means parseable character data.
Now we'll see some XXE payload and see how they are working.
1) The first payload we'll see is very simple. If you've read the previous task properly then you'll understand this payload very easily.<!DOCTYPE replace [<!ENTITY name "feast"> ]> <userInfo> <firstName>falcon</firstName> <lastName>&name;</lastName> </userInfo>
As we can see we are defining a ENTITY called name and assigning it a value feast. Later we are using that ENTITY in our code.
2) We can also use XXE to read some file from the system by defining an ENTITY and having it use the SYSTEM keyword<?xml version="1.0"?><!DOCTYPE root [<!ENTITY read SYSTEM 'file:///etc/passwd'>]><root>&read;</root>
Here again, we are defining an ENTITY with the name read but the difference is that we are setting it value to `SYSTEM` and path of the file.
If we use this payload then a website vulnerable to XXE(normally) would display the content of the file /etc/passwd.
In a similar manner, we can use this kind of payload to read other files but a lot of times you can fail to read files in this manner or the reason for failure could be the file you are trying to read.