This article is inspired by the “X marks the spot” challenge in picoCTF 2021. For the solution to the challenge, skip to the ‘Exploitation’ section.

WhileSQL injections are one of the most common web application vulnerabilities, its less notorious twin can be equally, if not more dangerous.

XPath?

XPath is a query language that locates elements in an XML document. Conceptually, it is similar to SQL. Most web applications use relational databases and SQL to store and query large amounts of data. Yet, in some use cases, especially those where data needs to be extracted and transferred between systems easily, XML databases have become much more appealing.

It is thus increasingly common for web applications to use XML data on the backend, using XPath the same way as SQL is traditionally used.

XML Documents

We can think of XML documents as a tree structure.

The above tree would correspond to the following XML document:

<bookstore><bookcategory="cooking"><titlelang="en">Everyday Italian</title><author>Giada De Laurentiis</author><year>2005</year><price>30.00</price></book> ...</bookstore>

XPath Syntax

Basic XPath queries consist of path expressions./ will select from the root node, while// will select nodes no matter where they are in the document.

For instance,bookstore/book will select allbook elements that are children ofbookstore.//book on the other hand, will select allbook elements no matter where they are in the document.

Same Same, But Different

Much like SQL injections, XPath injections occur when user-supplied data is embedded in the XPath query in an unsafe manner.

In SQL, access control is implemented with user-level security — each user is restricted to certain resources. However, when using XPath, there areno access controls and it is possible to access any part of the XML document.

Therefore, an XPath injection attack can be much more dangerous and devastating than an SQL injection attack.

Exploitation

In this challenge, we are given a simple login page. There are two POST parameters, name and pass.

The Basics

First, we can try to imagine the source code that constructs the query. It would look something like this:

StringFindUserXPath;FindUserXPath="//user[username/text()='"+Request("name")+"' And password/text()='"+Request("pass")+"']";

A basic payload would be:

name=' or 1=1 or 'a'='a&pass=test

which would translate to the following query:

//user[username/text()='' or 1=1 or 'a'='a' And password/text()='test']

Importantly, note the order of operations in boolean algebra:AND comes beforeOR. Therefore, as long as the first part of the query

username/text()='' or 1=1

evaluates to True, the entire query is True. This will be useful later on.

Using this payload, we get the message “You’re on the right path”. Note that this is ablind injection since we do not get any actual data from the XML document (we only have a boolean indicator telling us whether or not our query evaluated to True or False). This mirrors the real-world scenario where a successful login means our query returned True, while a failed login means our query returned False.

Booleanization

In blind injection attacks, the key is to focus on getting one piece of information at a time by using a series of boolean conditions.

Remember the order of operations we discussed above? We can tweak our previous query to the following:

//user[username/text()='' or BOOLEAN_CONDITION or 'a'='a' And password/text()='test']

This will only evaluate to True ifBOOLEAN_CONDITION is True, allowing us to test any condition.

Using XPath Functions

We can use XPath functions with booleanization to extract information about the XML document. For instance,count() returns the number of nodes in a node-set.

The following payload evaluates to False, telling us that the number of user nodes in the XML document isnot 1.

name=' or count(//user)=1  or '1'='1&pass=test

Then, we change the count to 2, then 3, and so on… until we get a payload that evaluates to True:

name=' or count(//user)=3  or '1'='1&pass=test

This tells us that there are 3 user nodes in the document.

The same logic can be applied to getting the number of child nodes.

name=' or count(//user[position()=1]/child::node())=5  or '1'='1&pass=test

evaluates to True, telling us that for the first user node, there are 5 child nodes. The same can be checked for all 3 users.

Getting Node Values

To get the node values, there are two steps:

1. Get the value length usingstring-length(). The payload for this would be something like:

name=' or string-length(//user[position()=USER_POSITION]/child::node()[position()=NODE_POSITION])=LENGTH or ''='&pass=test

whereUSER_POSITION andNODE_POSITION refer to the position of the user and child node respectively (ifUSER_POSITION = n, thenth user is selected) andLENGTH refers to the length we want to test for.

1. Get the value, character by character usingsubstring(). The payload for this would be something like:

name=' or substring((//user[position()=USER_POSITION]/child::node()[position()=NODE_POSITION]),INDEX,1)=CHARACTER or ''='&pass=test

This will test forCHARACTER at indexINDEX (starting from 1) of the string.

There are a few ways to automate this. Burpsuite’s Intruder allows us to load a list of payloads, which can be a list of numbers in this instance. Alternatively, we could write a simple script.

The following script will take two arguments,USER_POSITION andNODE_POSITION, find the length of the node value, then finds the ASCII characters at each position.

To get child node 2 of user 1:

We can repeat this process for the other two users, and find their usernames (“bob” and “admin” respectively).

Let’s continue getting the other child node values. After some trial and error, child node 4 looked promising:

If we run the script with arguments 3 and 4 (to get the admin user’s 4th child node value), we are handed the flag.

Prevention

Now that we know how XPath injections work, how can we prevent them? The solutions are quite similar to those of preventing SQL injections but may be overlooked due to the lack of built-in APIs.

Parameterized XPath

Similar to SQL prepared statements, the idea is to ensure that user-specified data is never interpreted as executable content (and always interpreted as only a parameter).

However, this likely requires the use of an XQuery processor such as Saxon, and the use of the corresponding external APIs. Bare-bones implementations of parameterized XPath queries can be rather cumbersome and tricky for platforms such as .NET and Java SE.

Input Validation / Sanitization

Never trust user-provided data. Input validation / sanitization should be treated as thebare minimum, not a panacea.

It may be impractical to implement an overly-strict filter. For instance, is a password that consists of letters only secure?

// Restrict the username and password to letters onlyif(!Regex.IsMatch(user,"^[a-zA-Z]+$")||!Regex.IsMatch(pass,"^[a-zA-Z]+$")){returnBadRequest();}Stringexpression="/users/user[@name='"+user+"' and @pass='"+pass+"']";returnContent(doc.SelectSingleNode(expression)!=null?"success":"fail");

Yet, an overly-loose filter will leave gaps (filter-bypass techniques can be borrowed from SQL injections).

Conclusion

Yay, you made it to the end! We’ve looked at how XPath injections work, and how they can be prevented. I hope you’ve learnt something new.

Thanks for reading!