How To Limit File Upload Sizes

There was some discussion a while back on a possible vulnerability in CFMX. I was worried that a denial of service attack would be possible by uploading a single very large file. My fear was that the ColdFusion server was buffering files in RAM before writing them out to the file system, however I was relieved when two people from the ColdFusion team confirmed that the files were written directly to disk and not buffered in RAM. In other words, the input stream from the request is written directly to a file output stream (without being buffered more than just a few K), which means you cannot cause the JVM to run out of memory by uploading a file. Very good implementation.

The next question, however, was whether or not it was possible to create a DOS attack by making the server run out of disk space. Could someone upload enough gigabytes of data to fill an entire partition? Shouldn’t there be a way to guard against this type of issue?

Fortunately, there is. On the advice of Laurent Rouquette, I confirmed this morning that you can use the variable cgi.CONTENT_LENGTH to decide if you want to allow a file to be uploaded or not. The code below only allows files less than 25,000 bytes to be saved:

<cfif cgi.CONTENT_LENGTH lt 25000>
<cffile action="upload"
nameConflict="overwrite" />

Note that the file will still be uploaded and saved in a temporary location, but if the cffile tag does not get executed, the temp file will simply be deleted after the upload is complete.

I recommend that anyone who has an application that allows files to be uploaded use this technique as an extra level of security. Remember, though, that the content length of the request contains all the information in the request, so allow a little extra space for other data, as well.