\documentclass[a4paper]{article}
\usepackage[ngerman]{babel}
\usepackage[utf8]{inputenc}
\usepackage{multicol}
\usepackage{calc}
\usepackage{ifthen}
\usepackage[landscape]{geometry}
\usepackage{amsmath,amsthm,amsfonts,amssymb}
\usepackage{color,graphicx,overpic}
\usepackage{xcolor, listings}
\usepackage[compact]{titlesec} %less space for headers
\usepackage{mdwlist} %less space for lists
\usepackage{pdflscape}
\usepackage{verbatim}
\usepackage[most]{tcolorbox}
\usepackage[hidelinks,pdfencoding=auto]{hyperref}
\usepackage{bussproofs}
\usepackage{fancyhdr}
\usepackage{lastpage}
\pagestyle{fancy}
\fancyhf{}
\fancyhead[L]{Network Security}
\fancyfoot[L]{\thepage/\pageref{LastPage}}
\renewcommand{\headrulewidth}{0pt} %obere Trennlinie
\renewcommand{\footrulewidth}{0pt} %untere Trennlinie

\pdfinfo{
 /Title (Network Security - Cheatsheet)
 /Creator (TeX)
 /Producer (pdfTeX 1.40.0)
 /Author (Robert Jeutter)
 /Subject ()
}

%%% Code Listings
\definecolor{codegreen}{rgb}{0,0.6,0}
\definecolor{codegray}{rgb}{0.5,0.5,0.5}
\definecolor{codepurple}{rgb}{0.58,0,0.82}
\definecolor{backcolour}{rgb}{0.95,0.95,0.92}
\lstdefinestyle{mystyle}{
 backgroundcolor=\color{backcolour}, 
 commentstyle=\color{codegreen},
 keywordstyle=\color{magenta},
 numberstyle=\tiny\color{codegray},
 stringstyle=\color{codepurple},
 basicstyle=\ttfamily,
 breakatwhitespace=false, 
}
\lstset{style=mystyle, upquote=true}

%textmarker style from colorbox doc
\tcbset{textmarker/.style={%
 enhanced,
 parbox=false,boxrule=0mm,boxsep=0mm,arc=0mm,
 outer arc=0mm,left=2mm,right=2mm,top=3pt,bottom=3pt,
 toptitle=1mm,bottomtitle=1mm,oversize}}

% define new colorboxes
\newtcolorbox{hintBox}{textmarker,
 borderline west={6pt}{0pt}{yellow},
 colback=yellow!10!white}
\newtcolorbox{importantBox}{textmarker,
 borderline west={6pt}{0pt}{red},
 colback=red!10!white}
\newtcolorbox{noteBox}{textmarker,
 borderline west={3pt}{0pt}{green},
 colback=green!10!white}

% define commands for easy access
\renewcommand{\note}[2]{\begin{noteBox} \textbf{#1} #2 \end{noteBox}}
\newcommand{\warning}[1]{\begin{hintBox} \textbf{Warning:} #1 \end{hintBox}}
\newcommand{\important}[1]{\begin{importantBox} \textbf{Important:} #1 \end{importantBox}}

% This sets page margins to .5 inch if using letter paper, and to 1cm
% if using A4 paper. (This probably isn't strictly necessary.)
% If using another size paper, use default 1cm margins.
\ifthenelse{\lengthtest { \paperwidth = 11in}}
 { \geometry{top=.5in,left=.5in,right=.5in,bottom=.5in} }
 {\ifthenelse{ \lengthtest{ \paperwidth = 297mm}}
 {\geometry{top=1.3cm,left=1cm,right=1cm,bottom=1.2cm} }
 {\geometry{top=1.3cm,left=1cm,right=1cm,bottom=1.2cm} }
 }

% Redefine section commands to use less space
\makeatletter
\renewcommand{\section}{\@startsection{section}{1}{0mm}%
 {-1ex plus -.5ex minus -.2ex}%
 {0.5ex plus .2ex}%x
 {\normalfont\large\bfseries}}
\renewcommand{\subsection}{\@startsection{subsection}{2}{0mm}%
 {-1explus -.5ex minus -.2ex}%
 {0.5ex plus .2ex}%
 {\normalfont\normalsize\bfseries}}
\renewcommand{\subsubsection}{\@startsection{subsubsection}{3}{0mm}%
 {-1ex plus -.5ex minus -.2ex}%
 {1ex plus .2ex}%
 {\normalfont\small\bfseries}}
\makeatother

% Don't print section numbers
\setcounter{secnumdepth}{0}

\setlength{\parindent}{0pt}
\setlength{\parskip}{0pt plus 0.5ex} 
% compress space
\setlength\abovedisplayskip{0pt}
\setlength{\parskip}{0pt}
\setlength{\parsep}{0pt}
\setlength{\topskip}{0pt}
\setlength{\topsep}{0pt}
\setlength{\partopsep}{0pt}
\linespread{0.5}
\titlespacing{\section}{0pt}{*0}{*0}
\titlespacing{\subsection}{0pt}{*0}{*0}
\titlespacing{\subsubsection}{0pt}{*0}{*0}

\begin{document}

\raggedright
\begin{multicols}{3}\scriptsize
    % multicol parameters
    % These lengths are set only within the two main columns
    %\setlength{\columnseprule}{0.25pt}
    \setlength{\premulticols}{1pt}
    \setlength{\postmulticols}{1pt}
    \setlength{\multicolsep}{1pt}
    \setlength{\columnsep}{2pt}
start
\end{multicols}
\end{document}