Application Security Engineer

PowerShell ships with Windows; bash ships with macOS and Linux. You already have everything you need.

1. 1

   Pick a real chore

   Not a toy problem — something that genuinely annoys you. Sorting a Downloads folder full of four hundred files. Renaming photos to a consistent format. Finding every file over 500MB. It has to be real, or the payoff at the end will not land.

2. 2

   Do it by hand once, and time it

   Actually time it. Write the number down. This is the figure you are going to beat, and it is also how you will one day justify automation work to a manager who wants to know why it is worth your afternoon.

3. 3

   Make a copy to work on

   Duplicate the folder. Everything from here runs against the copy. This is not caution for beginners — it is what people do professionally, because the cost of being wrong is asymmetric.

   Windows

   Copy-Item -Path .\Downloads -Destination .\Downloads-copy -Recurse

   macOS

   cp -R ~/Downloads ~/Downloads-copy

   Linux

   cp -R ~/Downloads ~/Downloads-copy

4. 4

   Ask what it WOULD do, before it does anything

   Run the operation in preview mode first. Nothing changes; it just tells you what it intends. Read that output properly. This habit is the single biggest difference between someone safe to give automation to and someone who is not.

   Windows

   Get-ChildItem .\Downloads-copy -File | Group-Object Extension | Format-Table Name, Count

   macOS

   find ~/Downloads-copy -type f | sed "s/.*\.//" | sort | uniq -c | sort -rn

   Linux

   find ~/Downloads-copy -type f | sed "s/.*\.//" | sort | uniq -c | sort -rn

5. 5

   Now let it run

   Sort every file into a folder named after its extension. Five lines or so. It will not work first time — read the error, change one thing, run it again. That loop, repeated, is the actual job.

   Windows

   Get-ChildItem .\Downloads-copy -File | ForEach-Object { $d = ".\Downloads-copy\" + $_.Extension.TrimStart("."); New-Item -ItemType Directory -Force -Path $d | Out-Null; Move-Item $_.FullName $d }

   macOS

   cd ~/Downloads-copy && for f in *.*; do d="${f##*.}"; mkdir -p "$d"; mv "$f" "$d"/; done

   Linux

   cd ~/Downloads-copy && for f in *.*; do d="${f##*.}"; mkdir -p "$d"; mv "$f" "$d"/; done

6. 6

   Compare it to your number

   You wrote down how long it took by hand. It now takes under a second, and it will take under a second every time from now on. Sit with that for a moment — that gap, multiplied across an organisation, is the entire economic argument for automation as a career.

Nmap is free and open source. One install, no account, available for every platform.

1. 1

   Establish what your own network is

   Find your own address and the range your network occupies. You need to know precisely what you own before you touch anything, and getting this wrong is how people accidentally scan their neighbour or their ISP.

   Windows

   ipconfig

   macOS

   ifconfig | grep "inet "

   Linux

   ip addr

2. 2

   Find out what is actually on it

   A ping sweep of your own range. Count the results. Almost everyone finds more than they expected — a thermostat, a TV, a printer they thought was off, a doorbell, a device they cannot identify at all. That last category is the interesting one.

   Any

   nmap -sn 192.168.1.0/24        # adjust to YOUR range from step 1

3. 3

   Identify the one you cannot name

   Pick the device you could not identify. Ask what it is — services, guessed operating system, vendor from its hardware address. Then go and physically find it in your home. This is device profiling, and it is precisely what a network access control system does automatically, all day, at enterprise scale.

   Any

   nmap -A 192.168.1.42          # the unknown device on YOUR network

4. 4

   Practise against the one legal target

   The Nmap Project runs scanme.nmap.org for exactly this purpose. Scan it once or twice — not repeatedly — and compare what a deliberately exposed host looks like against your own devices. Read the open ports and ask what each one is for.

   Any

   nmap -A scanme.nmap.org

5. 5

   Ask the question that actually matters

   You now have a list of open ports. A scanner will tell you what is open; it will never tell you whether it matters. For each finding ask: is it reachable from outside my house, what would someone get if they took it, and is there a reason it is on at all? That reasoning — exposure and impact, not severity scores — is the whole difference between running a tool and doing the job.

6. 6

   Write it up as if for someone else

   Three findings, ranked by what you would fix first, with a sentence each on why. Include one you decided NOT to worry about, and say why. Being able to justify what you deliberately ignored is what separates a useful report from a wall of noise nobody acts on.

A text editor and a browser. No install, no server, no account — the page runs from a file on your desktop.

1. 1

   Build something that works

   Save this as a file called app.html and open it in your browser. It is a greeting page that takes a name. Use it normally first — type your own name, see it work. Everything you build starts here: correct, for the input you imagined.

   Any

   <!doctype html>
   <input id="n" placeholder="your name">
   <button onclick="go()">Greet</button>
   <div id="out"></div>
   <script>
   function go() {
     const name = document.getElementById("n").value;
     document.getElementById("out").innerHTML = "Hello, " + name + "!";
   }
   </script>

2. 2

   Type something that is not a name

   Now put this in the box instead and press the button. Something will happen that the author did not intend and did not anticipate. You have just done the fundamental move of application security: supplying input the designer never pictured.

   Any

   <img src=x onerror="alert('this is now my page')">

3. 3

   Work out precisely why it worked

   The code never asked for code — it asked for a name. But it glued your text into the page as markup rather than as text, so the browser did what browsers do and executed it. Sit with that: the flaw is not the alert box, it is that data and instructions were allowed to occupy the same channel. Nearly every major vulnerability class is a variation of that one sentence.

4. 4

   Escalate it

   An alert box proves nothing to anyone. Change your payload so it reads something real from the page instead — the document title, a cookie you set yourself, the contents of another element. Proving actual impact rather than mere possibility is the difference between a finding a developer fixes and one they close as noise.

   Any

   <img src=x onerror="document.getElementById('out').textContent = document.title">

5. 5

   Fix it — the right way and the wrong way

   First fix it badly: filter out the word "script". Then defeat your own filter — the payload above never used that word. Now fix it properly by changing innerHTML to textContent, so the input is treated as text and can never be instructions. Try every payload again. This contrast, blocklist versus correct handling, is the single most useful lesson in the whole trial.

   Any

   document.getElementById("out").textContent = "Hello, " + name + "!";

6. 6

   Go looking for the same shape elsewhere

   Where else does something you have written take input and put it somewhere with meaning — a filename, a database query, a shell command, a URL? Write down three. You now have the pattern-recognition that application security work actually consists of, and it transfers to every language you will ever use.