Programmers frequently create temporary files. Temporary file directories are writable by everyone and include /tmp, /var/tmp, and C:\TEMP and may be purged regularly (for example, every night or during reboot).
When two or more users, or a group of users, have write permission to a directory, the potential for sharing and deception is far greater than it is for shared access to a few files. The vulnerabilities that result from malicious restructuring via hard and symbolic links suggest that it is best to avoid shared directories.
Securely creating temporary files in a shared directory is error prone and dependent on the version of the C runtime library used, the operating system, and the file system. Code that works for a locally mounted file system, for example, may be vulnerable when used with a remotely mounted file system.
Several rules apply to creating temporary files in shared directories including this one: temporary files must have unique names. Privileged programs that create files in world-writable directories can overwrite protected system files. An attacker who can predict the name of a file created by a privileged program can create a symbolic link (with the same name as the file used by the program) to point to a protected system file. Unless the privileged program is coded securely, the program will follow the symbolic link instead of opening or creating the file that it is supposed to be using. As a result, the protected system file referenced by the symbolic link can be overwritten when the program is executed.
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