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 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, a file name is supplied to fopen() to create and open for writing. However, there is no guarantee that the file referenced by file_name does not exist prior to calling fopen(). This may cause an unintended file to be accessed or overwritten.

...
FILE * fptr = fopen(file_name, "w");
if (!fptr) {
  /* Handle Error */
}
...

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 \[[Open Group 04|AA. C References#Open Group 04]\] 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()}} \[[Open Group 04|AA. C References#Open Group 05]\] 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.

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

FIO03-A

3 (high)

2 (probable)

1 (high)

P6

L2

References