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The C fopen() function is used to open an existing file or create a new one. The C11 version of the fopen() and fopen_s() functions function 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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char *file_name;
FILE *fp;

/* Initialize file_name */

fp = fopen(file_name, "w");
if (!fp) {
  /* Handle error */
}

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Compliant Solution (fopen

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("x"), C11

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)

The C11 Annex K fopen_s() function is designed to improve the security of Starting in C11 a new mode suffix ("x") was added to 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  function which causes fopen() to return NULL if the file already exists or cannot be created [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;
FILE *fp;

/* Initialize file_name */

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

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Compliant Solution (open(), POSIX)

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The ability to determine 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 is not acted upon.

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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