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JavaScript files are more than just scripts powering a web application — they’re often a direct window into an app’s logic, structure, and sometimes even its vulnerabilities.
In this guide, we’re diving deep into how to analyze JavaScript files for security bugs, covering static analysis techniques, collection strategies, and how to extract real signals from buried code. This is your full walkthrough — from recon to secrets, tokens, and hidden endpoints.
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JavaScript runs in the browser, which means you, the hacker, have access to all its logic. If it’s in the JS, it’s yours to read — and potentially abuse.
Inside .js
files, you’ll commonly find:
innerHTML
, eval()
, document.write()
The first step is to pull in as many JS files related to your target as possible. Here are the methods you should be using:
script
and extension .js
js-burp.txt
This pulls archived JS files, even if they’re no longer linked.
##install it
go install github.com/tomnomnom/waybackurls@latest
##run it
echo 'example.com' | waybackurls | grep '\.js$' | sort -u > js-wayback.txt
curl -G "https://web.archive.org/cdx/search/cdx" \
--data-urlencode "url=*.example.com/*" \
--data-urlencode "output=text" \
--data-urlencode "fl=original" | grep '\.js$' > js-cdx.txt
Then validate:
cat js-cdx.txt | httpx -mc 200 -o js-live.txt
Run a grep loop to look for common leaked variables:
cat js.txt | while read url; do
echo "[+] Scanning: $url"
curl -s "$url" | grep -Eio 'apikey|token|auth|secret|password|jwt' && echo "--- Found in $url ---"
done
Extract hidden endpoints:
##install it
git clone https://github.com/GerbenJavado/LinkFinder.git
cd LinkFinder
pip3 install -r requirements.txt
#run it
python3 linkfinder.py -i https://example.com/1.js -o cli
Grab secrets, keys, and AWS creds:
##install it
go install github.com/0xTeles/jsleak/v2/jsleak@latest
##run it
cat live-js.txt | jsleaks -s -l -k
Use DevTools > Application > LocalStorage or run:
localStorage.getItem('token')
sessionStorage.getItem('auth')
Look for: token
, jwt
, role
, user_info
, etc.
If you spot this:
const token = jwt.sign(payload, 'hardcoded-secret');
You may be able to forge your own token, escalate privileges, or access another user’s data.
nuclei -l js.txt -t ~/nuclei-templates/exposures/ -o js_bugs.txt
Or use a loop to download and scan offline:
while IFS= read -r link; do
wget "$link"
done < js.txt
grep -r -E "aws_access_key|apikey|secret|token|auth|config|admin" *.js
Tool | Purpose |
---|---|
BurpSuite | Manual JS harvesting |
Wayback | Historical assets |
CDX API | Deep archive extraction |
LinkFinder | Endpoint discovery |
jsleak | Secret/token extraction |
Grep | Manual confirmation |
Nuclei | Automated exposure scans |
And there you have it — the full breakdown of how to hunt down vulnerabilities hiding in JavaScript files.
From passive recon to active file scraping… from manual grep to full-blown secret and endpoint extraction — this is the recon phase real hunters use to build leads that pay off.
You’re not just scanning files — you’re reading logic, reverse-engineering patterns, and building the map no one else sees.
In Part 2, we’re going deeper into the code.
Live debugging. Logic tampering. Obfuscation. Exploits.
You’ll see how JavaScript really breaks.
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