Creation of Temporary File With Insecure Permissions

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

In the following code examples a temporary file is created and written to. After using the temporary file, the file is closed and deleted from the file system.

FILE *stream;
if( (stream = tmpfile()) == NULL ) {


  perror("Could not open new temporary file\n");
  return (-1);

}
// write data to tmp file
...
// remove tmp file
rmtmp();

However, within this C/C++ code the method tmpfile() is used to create and open the temp file. The tmpfile() method works the same way as the fopen() method would with read/write permission, allowing attackers to read potentially sensitive information contained in the temp file or modify the contents of the file.

try {
  File temp = File.createTempFile("pattern", ".suffix");
  temp.deleteOnExit();
  BufferedWriter out = new BufferedWriter(new FileWriter(temp));
  out.write("aString");
  out.close();
}
catch (IOException e) {
}

Similarly, the createTempFile() method used in the Java code creates a temp file that may be readable and writable to all users.

Additionally both methods used above place the file into a default directory. On UNIX systems the default directory is usually "/tmp" or "/var/tmp" and on Windows systems the default directory is usually "C:\\Windows\\Temp", which may be easily accessible to attackers, possibly enabling them to read and modify the contents of the temp file.

See Also