Thick Client Security Testing on Linux: A Beginner’s Guide to AppImage Applications

Your roadmap for this journey !

1. Introduction
2. What is a Thick Client?
3. What is an AppImage?
4. Setting Up Your Testing Environment
5. Static Analysis of AppImage
6. Dynamic Analysis of AppImage
7. Common Vulnerabilities in AppImage Applications
8. Conclusion

1. Introduction

Let’s be honest — most of us start security testing with web applications.

Open browser → intercept requests → find bugs → life feels sorted.

But then suddenly… someone hands you a .AppImage file and says:

“Please perform security testing on this.”

And your brain goes:

“Wait… what is this thing? And how do I even start?” 🤯

Don’t worry — you’re not alone.

In this guide, we’ll break everything down in a simple and practical way — so you can go from “What is this file?” to “Let’s test this like a pro.”

2. What is a Thick Client?

A thick client is just an application that runs on your system instead of inside a browser.

Simple comparison:

Web App → Runs in Browser Thick Client → Runs on Your System

Real-world examples:

🔹 .exe (Windows) 🔹 .dmg (Mac) 🔹 .AppImage (Linux)

Why should you care?

Because thick clients:

• Can access your local files
• May store sensitive data
• Talk directly to the OS

In short… they have more power → more risk → more fun for testers 😏

3. What is an AppImage?

AppImage is basically the “plug-and-play” version of Linux apps.

No installation. No dependency headaches.

Download → Give Permission → Run → Done

Example:

chmod +x target.AppImage
./target.AppImage

Makes the AppImage executable and runs it.

That’s it.

How AppImage Works Internally

Before breaking things, we need to understand what we’re breaking 😉

How an AppImage is created

An Electron app (built using JavaScript, HTML, and Node.js) is packaged into a Linux application and then converted into a single portable file called an AppImage.

What’s inside an AppImage?

An AppImage file contains everything inside it, including The main executable (binary), app code (app.asar, Where the actual app logic lives), and required libraries to run independently on Linux.

4. Setting Up Your Testing Environment

OS:

🔹 Kali Linux / Ubuntu

Tools:

🔹 Burp Suite → For intercepting traffic

🔹 strings → To find hidden secrets

🔹 ldd → To check dependencies

🔹 netstat / ss → To see open ports

🔹 strace → To see what the app is doing behind your back 👀

5. Static Analysis of AppImage

Step 1 — Extract

The following command unpacks the AppImage into a squashfs-root/ directory, giving you access to all internal application files for analysis.

./target.AppImage - appimage-extract

Boom 💥

You now have:

squashfs-root/

Step 2 — Explore

cd squashfs-root/
 ls

Step 3 — Extract Electron Code

The following command unpacks app.asar so you can see the actual app code.

asar extract app.asar extracted_app

Step 4 — Find Secrets

strings target.AppImage | grep -Ei "token|password|api"

Hidden Folders (Where secrets hide 🕵️‍♀️)

🔹 AppImage apps love hiding data in places like ~/.config and ~/.local/share — basically their secret stash.

🔹 Once you run and login, these folders may quietly store goodies like tokens, cookies, and session data.

🔹 Electron apps behave like mini browsers, so yes… cookies and local storage = locally saved 👀

🔹 Files like Cookies (SQLite DB) can literally spill session info if you peek inside using sqlite3.

~/.config
 ~/.cache
 ~/.local/share
 /tmp

Can’t see them? Press: Ctrl + H Or directly use terminal as shown in below PoCs:

look for, example:

grep -Rni "token"
grep -Rni "auth"

What you’re looking for:

• Hardcoded passwords
• API keys
• Internal URLs
• Debug configs

Basically… developer mistakes 😅

6. Dynamic Analysis of AppImage

Step 1 — Run

./target.AppImage

Step 2 — Check processes

ps aux | grep -i target

Step 3 — Check ports

netstat -tulnp

Step 4 — Intercept traffic

Burp Suite Setup (Yes, we need this)

🔹Download

Go to - https://portswigger.net/burp/communitydownload

Download the Linux version.

🔹 Install

cd ~/Downloads
 chmod +x burpsuite_community_linux*.sh
 ./burpsuite_community_linux*.sh

Click → Next → Next → Install (classic ritual 😄)

🔹Check Proxy

Proxy → Options → 127.0.0.1:8080

🔹 Run App with Proxy

./target.AppImage - proxy-server=127.0.0.1:8080

Now everything goes through Burp.

🔹 HTTPS not working?

Open - http://burp

Download certificate → Install it → Done.

What to check:

• Business logic vulnerabilities
• Sensitive data in responses
• Weak headers
• CORS issues
• Missing cookie flags etc.

Basically… everything that shouldn’t be there but usually is 😄

7. Common Vulnerabilities in AppImage Applications

Few example PoCs are below so you don’t end up running commands in the wrong place and questioning your life choices 😄

Commands you must know (quick checklist):

1. *chmod +x target.AppImage && ./target.AppImage* → Makes the file executable and runs the app to observe initial behavior.
2. *./target.AppImage --appimage-extract → Extracts the AppImage into `squashfs-root/`* for internal access.
3. *cd squashfs-root/resources/ → Navigates to app resources where `app.asar`* is usually present.
4. *asar extract app.asar extracted_app* → Extracts Electron source code (JS/HTML) for full visibility.
5. *grep -Rni "nodeIntegration" . && grep -Rni "contextIsolation"* . → Checks for insecure Electron configs (XSS → RCE risk).
6. *grep -Rni "enableRemoteModule" . && grep -Rni "sandbox" *. → Identifies disabled protections leading to possible RCE.
7. preload.js (manual review) → Analyze exposed APIs that may give sensitive access.
8. review fs / exec usage → Look for dangerous APIs that allow file or command execution.
9. *grep -Rni "child_process" . && grep -Rni "exec("* . → Finds command execution points (possible OS injection).
10. *grep -RniE "token|password|apikey"* . → Searches for hardcoded secrets in source code.
11. *strings target.AppImage | grep -Ei "token|secret|api"* → Extracts hidden credentials from binary.
12. CTRL + SHIFT + I → Opens DevTools to inspect and debug the app.
13. *./target.AppImage --proxy-server=127.0.0.1:8080* → Runs app through proxy to capture network traffic.
14. Open http://burp → Install Burp certificate to intercept HTTPS traffic.
15. *grep -RniE "certificate-error|ignore-certificate-errors" squashfs-root* → Checks if SSL validation is disabled (MITM risk).
16. *ps aux | grep -i target* → Monitors running processes for suspicious activity.
17. *lsof | grep -i target* → Checks which files the app is accessing.
18. netstat -tulnp → Lists open ports/services exposed by the app.
19. file:///etc/passwd → Tests if local file access is possible (data exposure).
20. *~/.config/ & ~/.local/share/* → Inspect stored data like tokens, sessions, logs.
21. *find ~/.config/"Target" -type f | grep -Ei "db|sqlite|ldb"* → Finds local databases storing user data.
22. *grep -Rni "token" ~/.config* → Searches for stored credentials (account takeover risk).
23. *grep -Rni "ipcMain" . && grep -Rni "ipcRenderer"* . → Analyzes IPC communication for unsafe data flow.
24. *grep -Rni "protocol"* . → Checks custom URI handlers (injection risk).
25. *grep -RniE "update|updater|download" squashfs-root* → Reviews update mechanism for supply chain risks.
26. *ldd <binary>* → Checks library dependencies for possible hijacking (code execution).

8. Conclusion

AppImage makes life easy for users…

But for security testers? It opens up a whole new playground.

Once you understand:

• How it works
• Where to look
• What to test

You’ll start seeing vulnerabilities everywhere.