Privacy & Security

🔍 How to Verify Your Data Never Leaves Your Browser

📅 Published: November 9, 2025 ⏱️ 10 min read 🛠️ Technical Level: Beginner

"Trust us, your data is private."

You've heard that phrase a thousand times. But should you trust it? Absolutely not—at least not without verification.

The good news: you don't need to be a developer to verify if an online tool is truly private. Your browser comes with built-in tools that let you see exactly what data is being sent where. This guide will show you how to use them.

⚠️ Why This Matters: A tool claiming to be "privacy-first" means nothing if you can't verify it. Many tools claim client-side processing while secretly uploading your data to servers for "analytics" or "improvements." Don't take anyone's word for it—verify it yourself.

What You'll Learn

By the end of this guide, you'll be able to:

✅ Open and navigate browser developer tools
✅ Monitor network traffic in real-time
✅ Identify when data is sent to external servers
✅ Distinguish between legitimate requests and data exfiltration
✅ Audit any online tool for privacy yourself

Step 1: Open Browser Developer Tools

1 Access Developer Tools

Every modern browser has built-in developer tools. Here's how to open them:

Windows/Linux: Press F12 or Ctrl + Shift + I
Mac: Press Cmd + Option + I
Alternative: Right-click anywhere on the page → Select "Inspect" or "Inspect Element"

A panel will open at the bottom or side of your browser window. This is the Developer Console—your window into what's happening behind the scenes.

Step 2: Navigate to the Network Tab

2 Switch to Network Monitoring

In the developer tools panel, you'll see several tabs at the top:

Elements - Shows HTML structure
Console - Shows JavaScript logs and errors
Network - This is what you need
Other tabs (Sources, Performance, etc.)

Click on the "Network" tab. This tab shows every HTTP request your browser makes—loading images, stylesheets, scripts, and most importantly, sending data to servers.

Step 3: Clear Previous Requests

3 Start Fresh

The Network tab likely shows requests from loading the page (CSS files, JavaScript, images, etc.). Before testing the tool, clear these requests so you only see what happens when you use the tool.

Look for the clear icon (🚫 or a circle with a line through it) in the Network tab's toolbar. Click it to clear all existing logs.

Tip: Check the "Preserve log" checkbox if you want to keep logs across page reloads (useful for multi-step processes).

Step 4: Use the Online Tool

4 Perform the Action

Now, use the tool normally. For example:

Paste text into a Base64 encoder
Upload an image to an image resizer
Format JSON in a JSON formatter
Generate a hash with a hash generator

Watch the Network tab as you interact with the tool. Every time your browser sends data to a server, a new row will appear.

Step 5: Inspect Network Requests

5 Analyze What Was Sent

Here's what to look for in the Network tab:

🟢 Normal/Expected Requests:

Type: GET - Usually loading resources (images, CSS, fonts)
analytics.js, ga.js, gtag.js - Google Analytics (anonymous usage tracking)
cdn.example.com - Content Delivery Network files (libraries, fonts)

🔴 Suspicious Requests (Data Exfiltration):

Type: POST or PUT - Sending data to a server
Payload contains your input - Click the request → "Payload" or "Request" tab to see what was sent
Third-party domains - Data sent to domains other than the tool's own domain

Example: Verifying AutoFemto's Base64 Encoder

Let's walk through a real example using our Base64 Encoder/Decoder:

Open the tool: Visit https://autofemto.com/tools/base64.html
Open Developer Tools: Press F12
Go to Network tab: Click "Network"
Clear logs: Click the clear icon
Enter sensitive text: Type something like "My Social Security Number is 123-45-6789"
Click "Encode"
Check Network tab: You'll see... nothing (or only analytics requests)

✅ Result: No POST requests. No data payloads containing your text. Your sensitive data was processed entirely in your browser using JavaScript. It never left your device.

Example: Catching a Non-Private Tool

Now let's see what a non-private tool looks like:

Open a typical server-side PDF converter
Follow steps 1-4 above
Upload a PDF file
Check Network tab

⚠️ What You'll See:

POST request to /upload or /convert
Payload tab shows your file being uploaded
Server response includes a processed file URL

Translation: Your file was uploaded to their server, processed there, and sent back to you. That file is now on their infrastructure, potentially stored indefinitely, potentially analyzed, and definitely vulnerable to breaches.

Advanced: Inspecting Request Payloads

If you see a POST request and want to know exactly what was sent:

Click on the request in the Network tab
Look for tabs labeled "Payload," "Request," or "Headers"
Check the "Request Payload" or "Form Data" section

You'll see exactly what data was transmitted. If it contains your input data—red flag.

Example: What a Privacy-Violating Payload Looks Like

POST /api/convert HTTP/1.1
Host: suspicious-tool.com
Content-Type: application/json

{
  "userInput": "My Social Security Number is 123-45-6789",
  "userId": "anonymous-user-12345",
  "timestamp": "2025-11-09T14:23:00Z"
}
            

Translation: This tool just sent your sensitive data (Social Security Number) to their server, along with a user ID for tracking. Not good.

Example: What a Clean Network Log Looks Like

GET /tools/base64.html HTTP/1.1
Host: autofemto.com

GET /theme.css HTTP/1.1
Host: autofemto.com

GET /analytics.js HTTP/1.1
Host: www.google-analytics.com
            

Translation: Only GET requests for loading the page itself and analytics. No POST requests. No data payloads. Your input was processed locally.

Common Questions

Q: What about analytics requests?

A: Google Analytics (gtag.js, analytics.js) will show up in the Network tab. These requests send anonymized page view data (which page you visited, how long you stayed), but not your input data. Click on the analytics request and check the Payload—you'll see page URLs and timestamps, not your document contents.

Q: I see requests to CDNs (cdn.jsdelivr.net, cdnjs.cloudflare.com). Is that bad?

A: No. These are GET requests loading JavaScript libraries (like Chart.js, jQuery, etc.) from Content Delivery Networks. They're not sending your data anywhere—they're just loading code to make the tool work.

Q: Can a tool hide requests from the Network tab?

A: No. The Network tab shows all HTTP/HTTPS traffic your browser sends. There's no way for a website to "hide" network requests from browser developer tools. If data is being transmitted, you'll see it.

Q: What if the tool uses WebSockets or other protocols?

A: WebSocket connections also appear in the Network tab (filter by "WS"). The same principles apply—click on the WebSocket connection and inspect messages sent/received.

Tools to Verify Right Now

Use the technique above to verify any of AutoFemto's tools:

Base64 Encoder/Decoder - Try encoding sensitive text
JSON Formatter - Paste API keys or credentials (safely!)
Hash Generator - Hash passwords
Image Resizer - Upload personal photos
CSV to JSON Converter - Convert customer data

You'll see the same result every time: zero POST requests containing your data. Because we build tools that respect your privacy.

Need Help Auditing a Tool?

Not sure if a tool is leaking your data? Contact us and we'll help you verify it—or build you a privacy-first alternative.

Get Help

Conclusion: Don't Trust, Verify

Privacy claims mean nothing without verification. Now you have the skills to audit any online tool yourself:

Open Developer Tools (F12)
Go to Network tab
Clear logs
Use the tool
Check for POST requests with your data

If you see POST requests containing your input data—don't use that tool. Your data is being sent to their servers, stored, and potentially misused.

With AutoFemto tools, you'll never see those suspicious POST requests. Because your data never leaves your browser. Don't take our word for it—verify it yourself.