My website is recording a lot of 404 errors, from numerous sources, for a file
crossdomain.xml. Does anyone know what this file is or why it's being requested?
crossdomain.xml file is requested to determine if Flash and Silverlight apps are "allowed" to access your website. Personally, I think it's a really dumb convention, but.. it's out there.
When calling a cross-domain service, Silverlight will check for the existence of clientaccesspolicy.xml first. This is the format defined by Silverlight and provides a pretty flexible way to define who can access what services. If not found, it will then default to look for crossdomain.xml, which is the file format implemented for Adobe Flash. It is important to note that this file will also still work for most public web services.
For Adobe Flash
Another change to the Flash Player 7 framework is the use of cross-domain policy files. A policy file is a simple XML file that gives the Flash Player permission to access data from a given domain without displaying a security dialog. When placed on a server, it tells the Flash Player to allow direct access to data on that server, without prompting the user to grant access.
Probably easiest to add this file if you don't want the 404s, and decide whether or not you want to allow Flash and Silverlight access to your website without a security prompt.
This file is requested any time a remote website script attempts to get resources from your site. See this question.
So, this means, someone is trying to leech your files - read referrers, if they are logged.