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File sharing often refers to the exchange of files (such as MP3 files)
over peer-to-peer networks, but this is not what we are concerned with
here. Our concern is the sharing of files through a central Web
repository, e.g. for the purpose of online collaboration. Centralized
file-sharing may seem easier to secure than peer-to-peer file sharing,
but it is exposed to a security threat that is often overlooked.
Some of the files that Web users may wish to share by uploading them
to, and downloading them from, a Web repository are HTML files. HTML
files may contain Javascript code or other active content, which is
executed as the file is rendered by the browser, and which may be
malicious. Files of various types other than HTML files may also
rendered by the browser and carry malicious active content.
Malicious code may of course be found anywhere on the Web, but
malicious code in shared files poses a special problem because, if no
precautions are taken, it may execute in a trusted context. The
problem is similar to the threat posed by cross-site scripting, but
more difficult to address. Providers of Web applications that have a
file-sharing feature often take no precautions against attacks
through HTML shared files.
The white paper
describes a range of attacks through shared files, including
cross-user attacks, attacks by former users, and cross-instance
attacks, and a countermeasure against such attacks.
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