Wednesday, May 12, 2010

Dynamically-Generated PDFs

Many web applications will generate PDF files for users so they can view nicely-formatted statements, reports, etc. These dynamically-generated PDFs are typically accessed by users in two different ways.

1) The application creates an actual PDF file on the server and redirects the user's browser to the file (via 302 response code).

2) The application streams the bytes for the PDF directly to the user's browser as part of the HTTP response.

This second method is much more secure. The reason is that a PDF file sitting on a server can probably be accessed by anyone. The only information needed is the directory and the file name. That said, I have seen the first method implemented securely, but two important mitigating factors were employed. First, the application used randomized file names. A globally-unique identifier (GUID) is great for this. Second, the files were deleted within about 10 seconds of them being created. These two factors, when combined, made it almost impossible to gain access to another user's PDF file.

Testing for this vulnerability is simple. If you run across an application that allows you to download a dynamic PDF, make note of the URL when viewing the PDF. Is it a direct link to the PDF file? If so, copy the link, open a different browser (not another window of the same browser), and paste the URL. Is the PDF loaded? If so, then you have a problem, especially if the file name is guessable or follows a pattern. You might also check to see if the PDF is deleted from the server after a time.

This topic applies to other static files that your application might generate dynamically too, such as Excel files. PDFs just seem to be the most common.

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