The ISO/IEC 9899-1999 C specification provides standard functions to manipulate files that are designed to avoid the details of the underlying system. However, file manipulation and file operations are inherently tied to the operating system. Many of the common vulnerabilities associated with file operations exist because the ISO/IEC 9899-1999 C specification lacks facilities to adequately interact with files and the file system, making it impossible to specify the correct behavior.
A better way to interact with files, in terms of security, is to use functions designed for the native system. Many implementation specific functions offer a level of control over file objects that the ISO/IEC 9899-1999 C specification does not.
Additionally, there are well-known recommendations for dealing with common file operations securely that use non-standard functions. This recommendation opens those options up to implementers of this standard.
Non-Compliant Example 1
The C99 standard function fopen() is typically used to open existing, and create new files. However, fopen() does not provide a way to test file existence potentially allowing a program to overwrite or access and unintended file.
In this example, a file name is supplied to fopen() to create and open for writing. Howerver, there is no gauruntee that the file referenced by file_name does not exist prior to calling fopen(). This may cause an unintended file to be overwritten.
...
FILE * fptr = fopen(file_name, "w");
if (!fptr) {
/* Handle Error */
}
...
Compliant Solution 1.
The open() function (defined in the POSIX standard) provides a a way to test for file existence . If the O_CREAT and O_EXCL flags are used together, the open() function will fail if the file file specified by file_name already exists.
...
int fd = open(file_name, O_CREAT | O_EXCL | O_WR_ONLY, 0600);
if (fd == -1) {
/* Handle Error */
}
...
References
- Seacord 05 Chapter 7, File I/O
- ISO/IEC 9899-1999 Sections 7.19.3, Files
- ISO/IEC 9899-1999 Sections 7.19.4, Operations on Files
- Open Group 04b