An example of an injection attack using Javascript

Delegates sometimes ask me what an "injection attack" is, and for examples. I came across a really good example this morning on (oops!) one of our own pages - it's now fixed, but I'm documenting here and showing you what was happening so that you can learn from it.

Please Try Later

Visiting one of the most popular pages on our web site this morning, all the expected content came up, but then this popup came on top of the page. It had me rather worried - the Image upload, store on database, retrieve, send back to browser demonstration is one of the busiest on our web site, and it had suddenly started coming up with this message. I tried it several times, and it was consistent - so it wasn't a genuine "database busy" message.

What had happened? We have a "review this page" facility on may of the pages on our web site - it encourages visitors to add comments. The comments don't get directly posted, but come to me for approval - this means that I can eliminate adverts for Viagra. And I had just approved a series of posts. Several of them were technical - included code - and I had overlooked the need to add in a call to htmlspecialchars. One of the visitor comments included:

die ("<html><script language='JavaScript'>alert('Unable to connect to database! Please try again later.'),history.go(-1)</script></html>");

which - alas - caused the script (which was being sent out to the browser) to run the Javascript ... outputting to the screen, then reverting to the previous page if the database was missing.

Cure? Simply adding an htmlspecialchars on each comment, problem fixed. I'm reluctant to call an "injection attack" as I don't think that the commenter was actually malicious - he was helpful, but he just caught me out. But what a great example of the sort of user input you need to be so careful to protect against!
(written 2010-02-08 17:24:53)

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