XPath Injection (Part 2)

July 8, 2015 | Views: 2744

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The XML Example Document
We’ll use the following XML document in the examples below.

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

<bookstore>

<book>
  <title lang="eng">Harry Potter</title>
  <price>76.99</price>
</book>

<book>
  <title lang="eng">Learning XML</title>
  <price>22.95</price>
</book>

<book>
  <title lang="eng">Learning XPATH</title>
  <price>30.20</price>
</book>

<book>
  <title lang="eng">Learning Secrets of Injections</title>
  <price>50.99</price>
</book>

<book>
  <title lang="eng">Learning Programming</title>
  <price>53.45</price>
</book>

</bookstore>

 

Selecting Nodes

XPath uses path expressions to select nodes in an XML document. The node is selected by following a path or steps. The most useful path expressions are listed below:

Expression Description
nodename : Selects all nodes with the name “nodename”
/ : Selects from the root node
// : Selects nodes in the document from the current node that match the selection no matter where they are
. : Selects the current node
.. : Selects the parent of the current node
@ : Selects attributes

 

Some Basic Xpath Expressions

In the table below, we have listed some path expressions and the result of the expressions:

Path Expression Result
bookstore : Selects all nodes with the name “bookstore”
/bookstore : Selects the root element bookstore
Note: If the path starts with a slash ( / ) it always represents an absolute path to an element!
bookstore/book : Selects all book elements that are children of bookstore
//book : Selects all book elements no matter where they are in the document
bookstore//book : Selects all book elements that are descendant of the bookstore element, no matter where they are under the bookstore element
//@lang : Selects all attributes that are named lang

 

Predicates

Predicates are used to find a specific node or a node that contains a specific value.

Predicates are always embedded in square brackets.

In the table below, we have listed some path expressions with predicates and the results of the expressions (from w3schools):

 

Path Expression Result
/bookstore/book[1] : Selects the first book element that is the child of the bookstore element.
/bookstore/book[last()] : Selects the last book element that is the child of the bookstore element
/bookstore/book[last()-1] : Selects all the book elements except the last one that are children of the bookstore element
/bookstore/book[position()<3] : Selects the first two book elements that are children of the bookstore element
//title[@lang] : Selects all the title elements that have an attribute named lang
//title[@lang=’eng’] : Selects all the title elements that have an attribute named lang with a value of ‘eng’
/bookstore/book[price>35.00] : Selects all the book elements of the bookstore element that have a price element with a value greater than 35.00
/bookstore/book[price>35.00]/title : Selects all the title elements of the book elements of the bookstore element that have a price element with a value greater than 35.00

 

Selecting Unknown Nodes

XPath wildcards can be used to select unknown XML elements:

Wildcard Description
* Matches any element node
@* Matches any attribute node
node() Matches any node of any kind

 

In the table below, we have listed some path expressions and the results of the expressions:

Path Expression Result
/bookstore/* Selects all the child nodes of the bookstore element
//* Selects all elements in the document
//title[@*] Selects all title elements which have any attribute

 

Selecting Several Paths

By using the | operator in an XPath expression, you can select several paths.

In the table below, we have listed some path expressions and the results of the expressions:

Path Expression Result
//book/title | //book/price Selects all the title AND price elements of all book elements
//title | //price Selects all the title AND price elements in the document
/bookstore/book/title | //price Selects all the title elements of the book element of the bookstore element AND all the price elements in the document

 

Introduction to Injection in Xpath Query

If you read the above content, then let us, for example, take a page that takes some input – name and phone number of that user – if that user exists in the XML file. When injecting, we know that for a string type, either single quote or double quote will be used and that we can check by using ‘ ” or “”=” ‘ for double quote and we can use ‘ ‘ or ”=’ ‘ for single quote check. So, whichever works, we’ll come to know that it’s used internally in the query. Now, let’s assume a simple query.

/root/parent/something[username=’our_input_here’]/user

The username is extracted after the condition gets the username as input. We know that if we make the condition true using ‘ or ”=’, we’ll be able to see the first users details. But then, we want to enumerate with each user one by one. As we know, the position() function choose each node one by one. So, we can use it to enumerate each user one by one. Here we go:

/root/parent/something[username='' or position()=1 or '']/user
/root/parent/something[username='' or position()=2 or '']/user
/root/parent/something[username='' or position()=3 or '']/user
/root/parent/something[username='' or position()=4 or '']/user
/root/parent/something[username='' or position()=5 or '']/user

This is how we can enumerate each user one by one:

Previous: Part 1

* some examples used in Part 1 & Part 2 are from the w3schools website *

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2 Comments
  1. Oh, great! Thanks a lot!

  2. Thanks for this article. This is very Helpful.

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