🔓 Abusing API Key Leakage in Frontend Frameworks (2025 Edition)

How to Hunt, Decode, and Exploit Secrets in Chunked JavaScript from Next.js, Angular, and React Apps.

🧠 Why This Topic Matters (and Pays Big)

As modern frontend frameworks like Next.js, Angular, and React shift toward chunked builds and server-side hydration, developers are embedding more logic and config into public-facing JavaScript than ever before.

The result?

🧵 API keys, tokens, and secrets leaking quietly into production builds — right under WAFs and scanners.

💰 These leaks have led to $500–$10,000+ bounties in the wild… and most bug hunters aren’t even checking for them.

🎯 What You'll Learn

✅ How to automate parsing and deobfuscating chunked JS files

✅ How to extract and validate leaked API keys from modern web apps

✅ How to pivot into real-world vulnerabilities from leaked secrets

✅ 2025-specific TTPs that bypass modern build tooling and obfuscation

✅ How I found real bugs in React + Next.js and got paid

🔍 Phase 1: Recon – Finding the Juice

Modern JavaScript apps are broken into dozens or hundreds of chunked files, often named like:

/_next/static/chunks/34289-dfj29sd0.js  
/static/js/main.a8d8fc9d.chunk.js  
/dist/vendor~app~checkout.98x21a.js        

These chunks are browser-accessible and often not indexed or scanned by WAFs or SCA tools.

🛠️ Tools for Recon:

• 🔎 getJS, JSFinder, subjs — Pull JavaScript endpoints from websites
• 🕸️ LinkFinder or SecretFinder — Parse files for endpoints + secrets
• 🔥 Custom regex scripts — To target secrets exposed via common config patterns

🤖 Bonus: Automating Discovery

Use this simple Python snippet to extract JS chunks from a target page:

import re, requests
from bs4 import BeautifulSoup

url = "https://target.com"
resp = requests.get(url)
soup = BeautifulSoup(resp.text, "html.parser")

scripts = [script.get("src") for script in soup.find_all("script") if script.get("src")]
chunks = [src for src in scripts if "/chunks/" in src or src.endswith(".js")]

print("\n".join(chunks))        

🔓 Phase 2: Extraction – Hunting for Secrets

Once you’ve got the files, your job is to find secrets inside them. Here's what to look for:

🧪 Common API Key Patterns

Regexes that still work in 2025:

(?i)(api[_-]?key|access[_-]?token|secret|auth[_-]?token)[\"'\s:=]+[\"']?[a-z0-9]{32,64}[\"']?        

But in modern frameworks, secrets are often URL-encoded, base64’d, or embedded in process.env.* configs.

🔍 Framework-Specific Patterns

🔹 Next.js:

Look for:

• NEXT_PUBLIC_* env vars
• Embedded config objects:

__NEXT_DATA__ = { env: { NEXT_PUBLIC_API_KEY: "abc123" } }        

🔹 React:

• process.env.REACT_APP_*
• Custom global window variables:

window.config = { apiKey: "sk_live_abc123" }        

🔹 Angular:

• Angular’s environment.ts files often get bundled:

export const environment = {
  production: true,
  apiKey: 'abcd1234',
  endpoint: 'https://api.thirdparty.com'
};        

🧪 Phase 3: Decoding, Validating, and Exploiting

Now that you’ve extracted a key, let’s figure out what it’s for and how to abuse it.

✅ Step 1: Identify the Service

Check prefixes for common services:

• sk_live_ (Stripe Live Secret Key)
• AIza (Google API Key)
• SG. (SendGrid Token)
• ghp_ (GitHub Personal Token)
• pk_test_ (Payment Processor (Test))

If it’s custom, look at surrounding JS for endpoint patterns.

✅ Step 2: Validate the Key

Use Postman, curl, or custom scripts to test:

curl -H "Authorization: Bearer <KEY>" https://api.thirdparty.com/v1/user/me        

Watch for:

• 200 OK responses
• Email / PII exposure
• Admin-only functionality

✅ Step 3: Exploit & Report (Examples)

🔥 Exploit Example #1: SendGrid API Key

Found in main.4234a.chunk.js:

window.config = { sendgridKey: "SG.xxxxxx" }        

✅ Used key to list marketing campaigns and exfiltrate all subscriber email addresses.

💰 Reported → $500 bounty.

🔥 Exploit Example #2: Firebase Key with Misconfigured Rules

Found in Angular build:

apiKey: "AIzaSyD..."        

✅ Authenticated to Firebase backend, read all Firestore documents (public read perms).

💰 Reported → $1,000 bounty.

🔥 Exploit Example #3: Stripe Secret Key (Critical)

Found in Next.js:

NEXT_PUBLIC_STRIPE_SECRET: "sk_live_123..."        

✅ Used to create unlimited transactions + refund abuse via backend API.

💰 Reported → $3,000 bounty.

🚀 Pivot Opportunities from Leaked Keys

If you extract a key, try:

• 🔄 Hitting private endpoints with elevated scopes
• 📁 Accessing cloud storage buckets or CI/CD configs
• 🔗 Chaining with IDOR / BOLA for lateral movement
• 🔒 Using OAuth tokens to impersonate other users or access APIs with delegated scopes

🧠 Key Takeaways for Bug Bounty Hunters

• Frontend frameworks are shipping more client-side JS than ever, making them a goldmine for recon and secrets discovery
• Most developers don’t realize build-time secrets are bundled, especially in modern SPA/SSR setups
• Regex automation, decoding, and manual review are still critical for finding gold
• Combine leaked keys with endpoint fuzzing, IDOR, and cloud misconfigs for impact

💬 Found This Article Useful?

If you’re an advanced bug hunter aiming to hit more $500–$10K+ bounties in 2025, this is one of the least competitive, highest ROI attack surfaces right now.

🧠 Like, comment, and share this post with your crew — and drop your best JS parsing win in the replies!

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