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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

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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.

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

Wiki Markup

However, this code suffers from a _Time of Check, Time of Use_ (or _TOCTOU_) vulnerability (see \[[Seacord 05|AA. C References#Seacord 05]\] 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

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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.

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/* ... */ 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

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Wiki Markup

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.

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/* ... */ 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

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POSIX)

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.

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