FIO03-A. Do not make assumptions about fopen() and file creation

The ISO/IEC 9899-1999 C standard function fopen() is typically used to open an existing file or create a new one. However, fopen() does not indicate if an existing file has been opened for writing or a new file has been created. This may lead to a program overwriting or accessing an unintended file.

Non-Compliant Code Example 1

In this example, an attempt is made to check whether a file exists before opening it for writing by trying to open the file for reading.

...
FILE *fp = fopen("foo.txt","r");
if( !fp ) { /* file does not exist */
  fp = fopen("foo.txt","w");
  ...
  fclose(fp);
} else {
   /* file exists */
  fclose(fp);
}
...

However, this code suffers from a Time of Check, Time of Use (or TOCTOU) vulnerability (see [[Seacord 05]] Section 7.2). On a shared multitasking system there is a window of opportunity between the first call of fopen() and the second call for a malicious attacker to, for example, create a link with the given filename to an existing file, so that the existing file is overwritten by the second call of fopen() and the subsequent writing to the file.

Non-Compliant Code Example 2

The fopen_s() function defined in ISO/IEC TR 24731-2006 is designed to improve the security of the fopen() function. However, like fopen(), fopen_s() provides no mechanism to determine if an existing file has been opened for writing or a new file has been created. The code below contains the same TOCTOU race condition as in Non-Compliant Code Example 1.

...
FILE *fptr;
errno_t res = fopen_s(&fptr,"foo.txt", "r");
if (res != 0) { /* file does not exist */
  res = fopen_s(&fptr,"foo.txt", "w");
  ...
  fclose(fptr);
} else {
  fclose(fptr);
}
...

Compliant Solution 1

The fopen() function does not indicate if an existing file has been opened for writing or a new file has been created. However, the open() function as defined in the Open Group Base Specifications Issue 6 [[Open Group 04]] is available on many platforms and provides such a mechanism. If the O_CREAT and O_EXCL flags are used together, the open() function fails when the file specified by file_name already exists.

...
int fd = open(file_name, O_CREAT | O_EXCL | O_WRONLY, new_file_mode);
if (fd == -1) {
  /* Handle Error */
}
...

Compliant Solution 2

The function fdopen() [[Open Group 04]] can be used in conjunction with open() to determine if a file is opened or created, and then associate a stream with the file descriptor.

...
FILE *fp;
int fd;

fd = open(file_name, O_CREAT | O_EXCL | O_WRONLY, new_file_mode);
if (fd == -1) {
  /* Handle Error */
}

fp = fdopen(fd,"w");
if (fp == NULL) {
  /* Handle Error */
}
...

Risk Assessment

The ability to determine if an existing file has been opened, or a new file has been created provides greater assurance that the file accessed is the one that was intended.

References

• Seacord 05 Chapter 7, "File I/O"
• ISO/IEC 9899-1999 Sections 7.19.3, "Files"; 7.19.4, "Operations on Files"
• ISO/IEC TR 24731-2006 Section 6.5.2.1, "The fopen_s function"
• Open Group 04