"Never trust user input" can only be a starting point for thinking about the input processing problem. Considering every input as a word written in a language. Programs exact some language as input and fail deeply, if this input turns out to not comply to this language. Long story short: Every time a program reads input or produces output that leaves the type system of the program language a definition of the input and output language is required to automatically ensure the correctness of the read/written data. I recommend langsec.org for a brainwash about how input processing really works.
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"Never trust user input" can only be a starting point for thinking about the input processing problem. Considering every input as a word written in a language. Programs exact some language as input and fail deeply, if this input turns out to not comply to this language. Long story short: Every time a program reads input or produces output that leaves the type system of the program language a definition of the input and output language is required to automatically ensure the correctness of the read/written data. I recommend langsec.org for a brainwash about how input processing really works.