XQuery Injection Mea Culpa

Dec 19, 2011    

There was a paper at the 2011 Balisage about XQuery Injection attacks.  The paper focuses on attacks against eXist but got me thinking.

Usually when we talk about Injection attacks at ML, we focus on xdmp:eval() and xdmp:value() and making sure that the input string is not derived from user supplied data.

With injection attacks in mind, I turned my attention to source code I wrote for a project going live this month and found the following:

let $uri := xdmp:get-request-field($cfg:URI_PARAM, ())
let $doc := fn:doc($uri)/element()
   v:render( $doc )

So the web client clicks on a link  something like

<a href="/page/render?uri=%2Fdoc%2F1.xml">link</a>

and gets an XHTML rendering of the document at URI “/doc/1.xml”.   The only things in the content database of this HTTP server are   XML documents which 100% of the user base are allowed to see and XML audit logs, which are really meant only for admins.  It may not the biggest attack vulnerability but a user could URL-BASH to see other user’s audit logs.    Making the following change solves the problem:

let $doc := fn:doc($uri)/doc:doc

I then got paranoid and put in 2 or 3 other safeguards, but you get my point.  Here are some tips I have compiled:

  • Watch out for passing user contributed strings into fn:doc(), fn:collection() as well as xdmp:eval() and xdmp:value() if you aren’t using document permissions for security.
  • Prefer XPath to fn:doc() when selecting documents for update.
  • Never store a user’s password as plaintext in XML.  If you aren’t using the security database users, 1-way hash+salt those passwords or database hipsters will call you names.
  • Throw in an extra xdmp:request-timestamp() value assert to force read-only transactions where appropriate.  This is most likely to help protect you down the road in O&M when others are quickly modifying the code base to solve problems or add features.


van der Vlist, Eric. “XQuery Injection: Easy to exploit, easy to prevent….” Presented at Balisage: The Markup Conference 2011, Montréal, Canada, August 2 – 5, 2011. In Proceedings of Balisage: The Markup Conference 2011. Balisage Series on Markup Technologies, vol. 7 (2011). doi:10.4242/BalisageVol7.Vlist02.