Why cookies are unreliable for identifying users

(and what we used instead)

Cookies have been the default way to identify users on the web for years.

But if you’ve built anything related to authentication, fraud prevention, or abuse detection, you’ve probably noticed the cracks.

Here’s what kept breaking for us — and how we approached the problem differently.

🚫 The problems with cookies

In theory, cookies are simple.

In practice, they fail in more situations than people expect.

Some common issues we kept running into:

-Users clear cookies (often automatically)

-Private / incognito mode resets identity

-ITP & browser restrictions shorten cookie lifetimes

-Ad blockers & privacy tools interfere silently

-Multiple accounts from the same device look unrelated

For analytics, this is annoying.

For fraud detection or duplicate prevention, it’s a real problem.

What we actually needed

Our use case wasn’t tracking users across the web.

We needed something much simpler:

-Identify the same device across sessions

-Work without cookies

-Be lightweight

-Avoid creepy cross-site tracking

-Be reasonable from a privacy perspective

Cookies alone clearly weren’t enough.

Alternatives we considered

Before jumping to a solution, we explored a few options:

1. LocalStorage / IndexedDB

Slightly better than cookies — but still easy to wipe or block.

2. Login-only solutions

Only works after signup.

Doesn’t help with abuse before account creation.

3. IP-based detection

Unreliable with VPNs, mobile networks, shared offices.

None of these solved the core problem.

Device fingerprinting (with trade-offs)

Eventually, we landed on device fingerprinting.

Not the invasive, cross-site kind — but a focused, first-party approach.

The idea is simple:

-Collect a small set of stable browser/device signals

-Combine them into a fingerprint

-Use it only within your own application

This helps answer questions like:

-“Have we seen this device before?”

-“Is this likely the same user creating multiple accounts?”

-“Does this login look suspicious?”

It’s not perfect — and that’s important to admit.

Privacy & limitations (important!)

Device fingerprinting has real trade-offs:

-It should never be used for cross-site tracking

-It must be transparent and purpose-limited

-It can change when users update browsers or devices

Used responsibly, it’s a signal, not a source of absolute truth.

That mindset shaped how we built our solution.

What we ended up building

After dealing with this repeatedly, we built a small internal tool — which later became deviceprint.io.

Our goals were intentionally modest:

-Lightweight integration

-No cookies required

-First-party only

-Designed for developers, not marketers

We now use it mainly for:

-Fraud prevention

-Duplicate account detection

-Security-sensitive workflows

Final thoughts

If cookies work for your use case — great.

But if you’re dealing with abuse, fraud, or edge cases, they’re often not enough on their own.

Device fingerprinting isn’t a silver bullet, but used carefully, it can fill an important gap.

If you’ve run into similar problems, I’d love to hear how you approached them.

Project: https://deviceprint.io

Happy to answer technical or privacy-related questions in the comments 👋