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POPreventing directory traversal with web-facing application - are regular expressions bullet-proof?
 

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    <p>I am in a situation where I need to allow a user to download a file dynamically determined from the URL. Before the download begins, I need to do some authentication, so the download has to run through a script first. All files would be stored outside of the web root to prevent manual downloading.</p> <p>For example, any of the following could be download links:</p> <ul> <li><a href="http://example.com/downloads/companyxyz/overview.pdf" rel="nofollow noreferrer">http://example.com/downloads/companyxyz/overview.pdf</a></li> <li><a href="http://example.com/downloads/companyxyz/images/logo.png" rel="nofollow noreferrer">http://example.com/downloads/companyxyz/images/logo.png</a></li> <li><a href="http://example.com/downloads/companyxyz/present/ppt/presentation.ppt" rel="nofollow noreferrer">http://example.com/downloads/companyxyz/present/ppt/presentation.ppt</a></li> </ul> <p>Basically, the folder depth can vary.</p> <p>To prevent a directory traversal, like say: <a href="http://example.com/downloads/../../../../etc/passwd" rel="nofollow noreferrer">http://example.com/downloads/../../../../etc/passwd</a> I need to obviously do some checking on the URI. (Note: I do not have the option of storing this info in a database, the URI must be used)</p> <p>Would the following regexp be bullet-proof in making sure that a user doesnt enter something fishy:</p> <pre><code>preg_match('/^\/([-_\w]+\/)*[-_\w]+\.(zip|gif|jpg|png|pdf|ppt|png)$/iD', $path) </code></pre> <p>What other options of making sure the URI is sane do I have? Possibly using realpath in PHP?</p>
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