FIO06-A. Create files with appropriate access permissions

Files should be created with appropriate access permissions. Creating a file with insufficient file access permissions may allow unintended access to program-critical files. File permissions are heavily dependent on the underlying operating system.  This recommendation offers three examples of how to specify access permissions for newly created files using standard C and POSIX functions.

Non-compliant Code Example (Standard C)

The fopen() function does not provide a mechanism to specify file access permissions. In the example below, if the call to fopen() creates a new file, the access permissions for that file will be implementation defined. Note that on POSIX compliant systems the permissions may be influenced by the value of umask().

...
FILE * fptr = fopen(file_name, "w");
if (!fptr){
  /* Handle Error */
}
...

Compliant Solution (Standard C)

The fopen_s() function defined in ISO/IEC TR 24731-2006 provides some control over file access permissions. Specifically, the report states: "If the file is being created, and the first character of the mode string is not 'u', to the extent that the underlying system supports it, the file shall have a file permission that prevents other users on the system from accessing the file."

...
File *fptr;
errno_t res = fopen_s(&fptr,file_name, "w");
if (res != 0) {
  /* Handle Error */
}
...

Non-compliant Code Example (POSIX)

Using the POSIX function open() to create a file but failing to provide access permissions for that file may cause that file to be created with unintended access permissions. This omission has been known to lead to vulnerabilities; for instance, CVE-2006-1174.

...
int fd = open(file_name, O_CREAT | O_WRONLY); /* access permissions are missing */
if (fd == -1){
  /* Handle Error */
}
...

Compliant Solution (POSIX)

Access permissions for the newly created file should be specified in the call to open(). Again, the permissions may be influenced by the value of umask().

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

Risk Assessment

Creating files without appropriate access permissions may allow unintended access to those files.

References

• ISO/IEC 9899-1999 Section 7.19.5.3, The fopen function
• Open Group 04 The open function
• ISO/IEC TR 24731-2006 Section 6.5.2.1, The fopen_s function
• CreateFile CreateFile
• Creating and Opening Files Creating and Opening Files
• CVE-2006-1174