Extensible Markup Language (XML) is a markup language used for encoding documents in a format that is both easy to handle for machines and human-readable. However, this commonly used format includes multiple security flaws. In this first XML related blog post, I will explain the basics of handling XML documents securely by using a schema.

OWASP divides the different vulnerabilities related to XML and XML schemas in two categories.

Malformed XML documents

Malformed XML documents are documents that do not follow the W3C XML specifications. Some examples that result in a malformed document are the removing of an ending tag, changing the order of different elements or the use of forbidden characters. All of these errors should result in a fatal error and the document should not undergo any additional processing.

In order to avoid vulnerabilities caused by malformed documents, you should use a well-tested XML parser that follows W3C specifications and does not take significantly longer to process malformed documents.

Invalid XML documents

Invalid XML documents are well formed but contain unexpected values. Here an attacker may take advantage of applications that do not properly define an XML schema to identify whether documents are valid. Below you can find a simple example of a document that, if not validated correctly, might have unintended consequences.

A web store which stores its transactions in XML data:

<purchase>

    <id>123</id>

    <price>200</price>

</purchase>

And the user only has control over the <id> value. It is then possible, without the right counter measures, for an attacker to input something like this:

<purchase>

    <id>123</id><price>0</price><id></id>

    <price>200</price>

</purchase>

If the parser that processes this document only reads the first instance of the <id> and <price> tags this will lead to unwanted results. 

It is also possible that the schema is not restrictive enough or that other input validation is insufficient, so that negative numbers, special decimals (like NaN or Infinity) or exceedingly big values can be entered where they are not expected, leading to similar unintended behavior. 

Avoiding vulnerabilities related to invalid XML documents should be done by defining a precise and restrictive XML Schema to avoid problems of improper data validation. 

Next blog post we will go into some more advanced attacks on XML documents such as Jumbo Payloads and the feared OWASP Top Ten number four, XXE.

In the meantime you can hone or challenge your skills on XML input validation on our portal.