Problem - Kevin Tafuro

Temporary files on Unix
The best solution for creating a temporary file on Unix is to use the mkstemp( ) function
in the standard C runtime library. This function generates a random filename, *
attempts to create it, and repeats the whole process until it is successful, thus guaranteeing
that a unique file is created. The file created by mkstemp( ) will be readable and
writable by the owner, but not by anyone else.
To help further ensure that the file cannot be accessed by any other process, and to
be sure that the file will not be left behind by your program if it should terminate
unexpectedly before being able to delete it, the file can be deleted by name while it is
open immediately after mkstemp( ) returns. Even though the file has been deleted, you
will still be able to read from and write to it because there is a valid descriptor for the
file. No other process will be able to open the file because a name will no longer be
associated with it. Once the last open descriptor to the file is closed, the file will no
longer be accessible.
66 | Chapter 2: Access Control
Between the time that a file is created with mkstemp( ) and the time
that unlink( ) is called to delete the file, a window of opportunity
exists where an attacker could open the file before it can be deleted.
The mkstemp( ) function works by specifying a template from which a random filename
can be generated. From the end of the template, “X” characters are replaced
with random characters. The template is modified in place, so the specified buffer
must be writable. The return value from mkstemp( ) is –1 if an error occurs; otherwise,
it is the file descriptor to the file that was created.
Temporary files on Windows
The Win32 API does not contain a functional equivalent of the standard C mkstemp( )
function. The Microsoft C Runtime implementation does not even provide support
for the function, although it does provide an implementation of mktemp( ). However,
we strongly advise against using that function on either Unix or Windows.
The Win32 API does provide a function, GetTempFileName( ), that will generate a
temporary filename, but that is all that it does; it does not open the file for you. Further,
if asked to generate a unique name itself, it will use the system time, which is
highly predictable.
Instead, we recommend using GetTempPath( ) to obtain the current user’s setting for
the location to place temporary files, and generating your own random filename
using CryptoAPI or some other cryptographically strong pseudo-random number
* The filename may not be strongly random. An attacker might be able to predict the filename, but that is generally
okay.
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