examine parents: the only parent is the category CWE-559 (Often Misused: Arguments and Parameters), which is intended to collect anything related to arguments and parameters. So, this item needs a new parent under CWE-1000.
identify abstraction level: the weakness is OS-specific (UNIX) and language-specific (C). Thus the abstraction level is Variant.
identify behavior: since CWE-560's name and description were not sufficiently clear, examine the context notes within CWE-560. This explains that the core error is that the programmer calls umask(), but the argument to umask is specified using a number that specifies permissions in chmod(). The same value has different interpretations in chmod() versus umask(), so the programmer is not specifying the argument to umask() correctly. This could cause the program to create files with insecure permissions.
identify chain: notice that the "incorrect specification of umask() argument" is primary to a later behavior, in which files are created with insecure permissions. Thus there is a chaining relationship; here, we will concentrate on the primary weakness, the incorrect specification of the argument.