FIO03-C. Do not make assumptions about fopen() and file creation

The C fopen() function is used to open an existing file or create a new one. However, in C, fopen() does not indicate whether 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.

Noncompliant Code Example (fopen())

In this noncompliant code example, the 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;

/* initialize file_name */

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

Noncompliant Code Example (fopen_s(), ISO/IEC TR 24731-1)

The ISO/IEC TR 24731-1 fopen_s() function is designed to improve the security of the fopen() function [ISO/IEC TR 24731-1:2007]. 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.

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(), C11)

The C Standard provides a new flag to address this problem. Section 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() to fail rather than open an existing file.

char *file_name;

/* initialize file_name */

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

Use of this option allows for the easy remediation of legacy code.

Compliant Solution (open(), POSIX)

The open() function, as defined in the Open Group Base Specifications Issue 6 [Open Group 2004], 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 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 error */
}

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 defines the EXCLUSIVE value to the mode argument of CREATE [Callaghan 1995]:

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.

For examples on 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 [Open Group 2004].

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 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 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