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Client-side JavaScript is no longer just a recon artifact — it’s an active attack surface.
While static analysis can reveal endpoints, secrets, and tokens, the real vulnerabilities often appear when the code runs. That’s where runtime analysis comes in — opening doors to logic flaws, role manipulation, DOM-based XSS, and internal function abuse directly from your browser.
In this advanced walkthrough, we’ll dissect how to move beyond js-beautify
and dive into runtime debugging, obfuscation unpacking, local overrides, and DevTools exploitation techniques that help turn recon into actionable bugs.
🧪 Want to practice these techniques hands-on?
Check out the full GitHub lab here:
👉 https://github.com/amrelsagaei/JavaScript-Analysis-Masterclass-Lab
Table of contents [Show]
You’ve got the JS. Now let’s watch it breathe.
Open DevTools (F12
) — this is your entry into the live heartbeat of any web app.
click
)isAdmin = true
Want changes to persist across refreshes?
Chrome makes it possible with Local Overrides.
DevTools → Sources → Overrides
Now you can:
Some JS files don’t want to be understood.
Signs of obfuscation:
_0xabc123
eval()
or new Function()
These break static tools — but not you.
First step: Make it readable.
Now you’ve got indentation and logical flow — the first step to live debugging or decryption.
Ever seen this?
This is live reverse engineering — in the browser.
Don’t underestimate AI. Use it against itself.
Ask:
AI is your pair programmer now — especially against obfuscated JS.
Sometimes devs leave internal helpers like:
Example:
Boom — you've impersonated an admin, bypassed auth logic, or triggered a hidden feature.
This is where XSS, logic bugs, IDORs all live.
Found something cool? Validate before you report.
Then use:
httpx
to test endpointscurl
to replay requestsffuf
/ arjun
to fuzz parametersnuclei
to scan for known issuesThis builds credibility and avoids false positives.
Found something? Now make it shine.
Good writing = Faster triage = Bigger bounties.
Mastering JavaScript runtime analysis means more than pulling files and parsing code — it’s about understanding how the app behaves under pressure, and how logic unravels when user input meets insecure execution.
Whether you're a bug bounty hunter, red teamer, or appsec researcher, this skill separates those who read JS from those who break it.
Dive into the full lab on GitHub, try these techniques, and apply them on real-world apps.
Stay sharp, hack smart, and keep pushing boundaries.
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