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

On POSIX compliant systems the permissions may be restricted by the value of the POSIX {{umask()}} function \[[Open Group 04|AA. C References#Open Group 04]\].  

The operating system modifies the access permissions by computing the intersection of the inverse of the umask and the permissions requested by the process \[[Viega 03||AA. C References#Viega 03]\]. For example, if the variable {{requested_permissions}} contained the permissions passed to the operating system to create a new file, the variable {{actual_permissions}} would be the actual permissions that the operating system would use to create the file:

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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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John Viega and Matt Messier also provide the following advice \[[Viega 03|AA. C References#Viega 03]\]:

Do not rely on setting the umask to a "secure" value once at the beginning of the program and then calling all file or directory creation functions with overly permissive file modes. Explicitly set the mode of the file at the point of creation. There are two reasons to do this. First, it makes the code clear; your intent concerning permissions is obvious. Second, if an attacker managed to somehow reset the umask between your adjustment of the umask and any of your file creation calls, you could potentially create sensitive files with wide-open permissions.

Risk Assessment

Creating files with weak access permissions may allow unintended access to those files.

References

\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 7.19.5.3, The fopen function

\[[Open Group 04|AA. C References#Open Group 04]\] "The open

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 function," "The umask function"

\[[ISO/IEC TR 24731-2006|AA. C References#SO/IEC TR 24731-2006]\] Section 

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6.5.2.1, "The fopen_s function"

\[[CVE-2006-1174|http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2006-1174]\]

\[[Viega 03|AA. C References#Viega 03]\] Section 2.7 Restricting Access Permissions for New Files on Unix