fopen() function is used to open an existing file or create a new one [[ISO/IEC 9899:1999]]. 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.
For examples on how to just check for the existence of a file without actually opening it, please see FIO10-A. Take care when using the rename() function.
Non-Compliant Code Example:
In this non-compliant coding example, the file referenced by
file_name is opened for writing. This example is non-compliant, however, if the programmer's intent was to create a new file, but the reference file already exists.
Non-Compliant Code Example:
fopen_s() (ISO/IEC TR 24731-1)
fopen_s() function defined in ISO/IEC TR 24731-1:2007 is designed to improve the security of the
fopen() function. However, like
fopen_s() provides no mechanism to determine if an existing file has been opened for writing or a new file has been created.
open() function as defined in the Open Group Base Specifications Issue 6 [[Open Group 04]] is available on many platforms and provides the control that
fopen() does not provide. If the
O_EXCL flags are used together, the
open() function fails if the file specified by
file_name already exists.
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(). IETF RFC 1813 defines the
EXCLUSIVE value to the
mode argument of
CREATE [[Callaghan 95]].
EXCLUSIVEspecifies 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.
Section 12.3 of the GNU C Library says: [[Loosemore 07]]
The GNU C library defines one additional character for use in
opentype: the character '
x' insists on creating a new fileâ”if a file
fopenfails rather than opening it. If you use '
x' you are guaranteed that you will not clobber an existing file. This is equivalent to the
O_EXCLoption to the
Use of this non portable extension can allow for easy remediation of legacy code.
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 04]].
The ability to determine if an existing file has been opened or a new file has been created provides greater assurance that the intended file is accessed, or perhaps more importantly, a file other than the intended file is not acted upon.
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
[[ISO/IEC 9899:1999]] Section 7.19.3, "Files," and Section 7.19.4, "Operations on Files"
[[ISO/IEC TR 24731-1:2007]] Section 126.96.36.199, "The
[[Loosemore 07]] Section 12.3, "Opening Streams"
[[Open Group 04]]
[[Seacord 05a]] Chapter 7, "File I/O"