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

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

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 the first Non-Compliant Code Example.

/* ... */
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

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 */
}
/* ... */

Care should be observed when using O_EXCL with remote file systems as it does not work with NFS version 2. NFS version 3 added support for O_EXCL mode in open(); see IETF RFC 1813 Callaghan 95, in particular the EXCLUSIVE value to the mode argument of CREATE.

Compliant Solution (POSIX)

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.

Examples of vulnerabilities resulting from the violation of this recommendation can be found on the CERT website.

References

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