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The XML Security Relay Race

The risks associated with XML remain

A recent tweet about a free, Linux-based XML Security suite reminded me that we do not opine on the subject of XML security and its importance enough. SOA has certainly been dethroned as the technology darling du jour by cloud computing and virtualization and with that forced abdication has unfortunately also come a reduction in the focus on XML and security.

That’s particularly disturbing when you recognize that what’s replaced SOA – primarily WOA and RESTful APIs – exchange data primarily via one of two formats: XML and JSON. Whether you prefer one over the other is a debate I’d rather not have (again) at the moment. It is enough to note that XML is widely used by developers to exchange data between applications and between clients and servers, and that it is the core data format for Web Services used to manage many cloud computing environments, such as Amazon’s AWS.

It’s important, then, to remember that many – nay, most – of the security risks associated with SOA were actually due to its reliance on XML. SOA is an architecture and aside from certain standards proscribing the use of data formats – SOAP, WSDL, UDDI – it carried with it very few tangible security risks. XML, on the other hand, carries with it a wide variety of security risks that have not suddenly “gone away” simply because it’s being used to implement APIs in both RESTful and SOAPy environments.

The risks associated with XML remain and it is likely far more dangerous now than it ever was to ignore those risks because it is used far more often than it has been in the past and for a much broader audience. XML security issues didn’t disappear, SOA simply passed them right along to AJAX, REST, and cloud computing.


There are basically two types of XML-related attacks:

  1. Malicious content attacks, which try to force an endpoint (server/application) to do something malicious. This could be to retrieve data it shouldn’t or inject malicious data through SQL injection or poisoning of external files upon which XML relies – schemas for the most part.
  2. Operational attacks that produce a denial of service. These are typically associated with parser-related vulnerabilities that cause infinite loops in parsing through manipulation of XML as a self-defining format (see “A Billion More Laughs: The JavaScript hack that acts like an XML attack” for more details).

Other credential-based attacks are similar in nature to traditional web-based attacks in that they attempt to exploit authentication and authorization sources, but these are not peculiar to XML at all. XML security solutions are quite capable of protecting XML-based data in general, even though most often focus on SOA-related web services for purposes of evangelizing their solution. A solution capable of securing XML can secure SOA-related web services because all relevant SOA-related standards and protocols are based on XML. It’s really the underlying XML in most cases that is being inspected and secured, with a few exceptions.

The risks associated with XML are well-known and well-understood, and yet as noted above we’ve moved away from worrying about it even as we’ve increased where and how we leverage XML to access, publish, and share data between users, applications, and organizations. XML has become more prevalent even as we’ve apparently decided that XML security is passé. That’s dangerous, particularly as the use of XML as an operational standard for managing public cloud computing environments and as a data exchange format for Web 2.0 applications are now commonplace.

Just because it appears that the XML is “hidden” under the hood of an AJAX-related toolkit does not mean it’s not accessible to be manipulated for more nefarious purposes. In fact, the mixed use of XML and HTML in Web 2.0 applications creates an environment in which XML security is not enough, nor is simple HTTP/HTML-based security checks enough. But often only one or the other is used to validate user-generated content and this affords attackers an opportunity to exploit the relationship between XML and HTML in a way that could put applications and data at risk.


It is popular and common today to utilize AJAX as a means to submit comments and other user-generated data to Web 2.0 sites. Many AJAX-enabling toolkits (XAJAX, SAJAX, Prototype) utilize XML as the default method of exchanging data between the client and the browser. For some toolkits, XML is the only choice. In many cases developers aren’t aware – or don’t care – what the underlying format may be so they don’t bother with the options. The toolkit handles the dirty details of communicating with the server and exchanging the data, so it’s not necessary to worry about what’s “under the covers.” Basically if I write up some code I’ll simply hand the communication details off to the toolkit, and let it format it/send it and deconstruct it on the server-side as well.

This can turn out to be a Very Bad Idea. When user-generated content contains HTML it is often enclosed in CDATA tags within the XML document that is used to instruct the server-side application which function to call, it’s arguments, and of course the data itself. The problem is that many traditional web-based security solutions can’t parse the XML, and XML-security devices won’t parse the CDATA because the reason behind using CDATA is that it may contain markup that confuses parsers in the first place. So an attacker could use the input field to insert malicious code (see OWASP: XML Injection tests) that can result in an XSS (Cross-site scripting) or other traditional web application attack:

$HTMLCode = <![CDATA[<]]>script<![CDATA[>]]
   1: alert('XSS')


That means if a developer hasn’t properly sanitized the input…the application is at risk of exploitation. Sanitization may need to occur before and after parsing, just to be safe. If it’s data that’s coming via a toolkit it’s important to test it, as well, to ensure nothing can sneak through and exploit the inner-workings of the toolkit. Some web application security solutions can handle mixed environments containing both XML and HTML, but if they are not in place or the security/network administrators don’t know they need to be aware of the possibility, the application may not be as protected as it could be.


Even though SOA was prematurely declared dead, the security risks typically associated with it should not be assumed to be irrelevant. Almost all of the security risks associated with SOA were directly attributable to XML in general, and thus are just as relevant – if not more so – today. The increasing reliance on external frameworks for AJAX and JavaScript is a potential threat to the integrity of your data and the security of your applications. Make sure you understand the security implications of the data format being leveraged, and provide the means by which XML-related content can be secured – either within the application or externally in the infrastructure.

The need for thorough testing and a layered threat defense is paramount when deploying applications based on Web 2.0 and RIA principles, particularly those which take advantage of toolkits to handle the exchange of data between client (untrusted) and server (one hopes trusted) endpoints.

Read the original blog entry...

More Stories By Lori MacVittie

Lori MacVittie is responsible for education and evangelism of application services available across F5’s entire product suite. Her role includes authorship of technical materials and participation in a number of community-based forums and industry standards organizations, among other efforts. MacVittie has extensive programming experience as an application architect, as well as network and systems development and administration expertise. Prior to joining F5, MacVittie was an award-winning Senior Technology Editor at Network Computing Magazine, where she conducted product research and evaluation focused on integration with application and network architectures, and authored articles on a variety of topics aimed at IT professionals. Her most recent area of focus included SOA-related products and architectures. She holds a B.S. in Information and Computing Science from the University of Wisconsin at Green Bay, and an M.S. in Computer Science from Nova Southeastern University.

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