If (file.getCanonicalPath().Due to the Exchange vulnerability I've been running an updated version of the MSERT scan every evening. It should verify that the canonicalized path starts with the expected base directory.īelow is an example of some simple Java code to validate the canonical path of a file based on user input:įile file = new File(BASE_DIRECTORY, userInput) If that isn't possible for the required functionality, then the validation should verify that the input contains only permitted content, such as purely alphanumeric characters.Īfter validating the supplied input, the application should append the input to the base directory and use a platform filesystem API to canonicalize the path. Ideally, the validation should compare against a whitelist of permitted values. The application should validate the user input before processing it. If it is considered unavoidable to pass user-supplied input to filesystem APIs, then two layers of defense should be used together to prevent attacks: Many application functions that do this can be rewritten to deliver the same behavior in a safer way. The most effective way to prevent file path traversal vulnerabilities is to avoid passing user-supplied input to filesystem APIs altogether. PRACTITIONER File path traversal, validation of file extension with null byte bypass How to prevent a directory traversal attack
\ are valid directory traversal sequences, and an equivalent attack to retrieve a standard operating system file would be: On Unix-based operating systems, this is a standard file containing details of the users that are registered on the server. sequences step up from /var/www/images/ to the filesystem root, and so the file that is actually read is: is valid within a file path, and means to step up one level in the directory structure. This causes the application to read from the following file path: The application implements no defenses against directory traversal attacks, so an attacker can request the following URL to retrieve an arbitrary file from the server's filesystem: The image files themselves are stored on disk in the location /var/In the above case, the application reads from the following file path: The loadImage URL takes a filename parameter and returns the contents of the specified file. Images are loaded via some HTML like the following:
Reading arbitrary files via directory traversalĬonsider a shopping application that displays images of items for sale.
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