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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.

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int fd = open(file_name, O_CREAT | O_WRONLY); /* access permissions are missing */
if (fd == -1){
  /* Handle Error */
}
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Access permissions for the newly created file should be specified in the call to open().

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

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

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File *fptr;
errno_t res = fopen_s(&fptr,file_name, "w");
if (res != 0) {
  /* Handle Error */
}
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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
• CVE-2006-1174