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Programmers frequently create temporary files in directories that are writable by everyone; examples include /tmp and /var/tmp on UNIX and C:\TEMP on Windows. Files in such directories may be purged regularly, for example, every night or during reboot. However, an attacker who has access to the local file system can exploit operations on files in shared directories when those files are created insecurely or remain accessible after use. For example, an attacker who can both predict the name of a temporary file and can change or replace that file, can exploit a time-of-check time-of-use (TOCTOU) condition to cause either a failure in creating the temporary file from within program code or operating on a file determined by the attacker. This exploit is particularly dangerous when the vulnerable program is running with elevated privileges. On multiuser systems, a user can be tricked by an attacker into unintentionally operating on their own files. Consequently, temporary file management must comply with FIO04 FIO00-J. Do not operate on files in shared directories.

Many programs that create temporary files attempt to give them unique and unpredictable file names. This is a common attempt at mitigating the risk of creating a file in an insecure or shared directory. If the filename is not sufficiently unique or random, an attacker can guess or predict the name of the file to be created, and create a file link with the same name, the final target of which is a file selected by the attacker. However, if a temporary file is created in a secure directory, an attacker cannot tamper with the file, and so the need for unpredictable names is eliminated.

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For this and subsequent code examples, we will assume that the files are automatically being created in a secure directory, to comply with rule FIO04 FIO00-J. Do not operate on files in shared directories. We will also assume the files are created with proper access permissions, to compy with FIO03-J. Create files with appropriate access permissions. Both requirements may be managed outside the JVM.

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