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The ISO/IEC 9899-1999 C standard function fopen() function 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 The C11 version of the fopen() and fopen_s() functions provides a mode flag, x, that provides the mechanism needed to determine if the file that is to be opened exists. Not using this mode flag can lead to a program overwriting or accessing an unintended file.

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Noncompliant Code Example

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(fopen())

In this noncompliant code example, an attempt is made to check whether a file exists before opening it for writing by trying to open the file for readingthe file referenced by file_name is opened for writing. This example is noncompliant if the programmer's intent was to create a new file, but the referenced file already exists.

char *file_name;

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

/* Initialize file_name */

fp = fopen("foo.txt",file_name, "w");
  ...
  fclose(fp);
} elseif (!fp) {
   /* fileHandle existserror */
  fclose(fp);
}
...

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.

Compliant Solution 1

}

Noncompliant Code Example (fopen_s(), C11 Annex K)

The C11 Annex K fopen_s() function is designed to improve the security of the fopen() function. Like the fopen() function, fopen_s() provides a mechanism to determine whether the file exists. See below for use of the exclusive mode flag.

char *file_name;
FILE *fp;

/* Initialize file_name */
errno_t res = fopen_s(&fp, file_name, "w");
if (res != 0) {
  /* Handle error */
}

Compliant Solution (fopen_s(), C11 Annex K)

The C Standard provides a new flag to address this problem. Subclause 7.21.5.3, paragraph 5 [ISO/IEC 9899:2011], states:

Opening a file with exclusive mode ('x' as the last character in the mode argument) fails if the file already exists or cannot be created. Otherwise, the file is created with exclusive (also known as non-shared) access to the extent that the underlying system supports exclusive access.

This option is also provided by the GNU C library [Loosemore 2007].

This compliant solution uses the x mode character to instruct fopen_s() to fail rather than open an existing file:

char *file_name;

/* Initialize file_name */
FILE *fp;
errno_t res = fopen_s(&fp, file_name, "wx");
if (res != 0) {
  /* Handle error */
}

Use of this option allows for the easy remediation of legacy code. However, note that Microsoft Visual Studio 2012 and earlier do not support the x mode character [MSDN].

Compliant Solution (open(), POSIX)

The open() function, as defined in the Standard for Information Technology—Portable Operating System Interface (POSIX®), Base Specifications, Issue 7 [IEEE Std 1003.1:2013], is available on many platforms and provides finer control than fopen(). In particular, open() accepts the O_CREAT and O_EXCL flags. When used together, these flags instruct the open() function to fail if the file specified by file_name already Wiki MarkupThe {{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.

char *file_name;
int new_file_mode;

/* Initialize file_name and new_file_mode */

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

Care should be taken when using O_EXCL with remote file systems because it does not work with NFS version 2. NFS version 3 added support for O_EXCL mode in open(). IETF RFC 1813 [Callaghan 1995] defines the EXCLUSIVE value to the mode argument of CREATE:

EXCLUSIVE specifies that the server is to follow exclusive creation semantics, using the verifier to ensure exclusive creation of the target. No attributes may be provided in this case, since the server may use the target file metadata to store the createverf3 verifier.

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For examples of how to check for the existence of a file without opening it, see recommendation FIO10-C. Take care when using the rename() function.

Compliant Solution (fdopen(), POSIX)

For code that operates on FILE pointers and not file descriptors, the POSIX fdopen() function can be used to associate an open stream with the file descriptor returned by open(), as shown in this compliant solution [IEEE Std 1003.1:2013]:

Compliant Solution 2

Wiki MarkupThe 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.

...char *file_name;
int new_file_mode;
FILE *fp;
int fd;

/* Initialize file_name and new_file_mode */

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

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

Compliant Solution (Windows)

The Win32 API CreateFile() allows a programmer to create or open a file depending on the flags passed in. Passing in the CREATE_NEW flag ensures the call fails if the file already exists. This compliant solution demonstrates how to open a file for reading and writing without sharing access to the file such that the call fails if the file already exists.

TCHAR *file_name;
HANDLE hFile = CreateFile(file_name, GENERIC_READ | GENERIC_WRITE, 0, 0, 
                          CREATE_NEW, FILE_ATTRIBUTE_NORMAL, 0);
if (INVALID_HANDLE_VALUE == hFile) {
  DWORD err = GetLastError();
  if (ERROR_FILE_EXISTS == err) {
    /* Handle file exists error */
  } else {
    /* Handle other error */
  }
}

Risk Assessment

The ability to determine if whether an existing file has been opened , or a new file has been created provides greater assurance that a file other than the intended file accessed is the one that was intendedis not acted upon.

References

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Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography

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