By Aditya Sunny
Bug Bounty Hunter | Security Researcher | Ethical Hacker
📅 Reported: July 2025
🎯 Target: *.linktr.ee

— -

🔍 Summary

During security testing on Linktree’s user system, I discovered an input validation bypass that allowed registration of usernames with a leading space.

This tiny mistake introduced a serious impersonation vector, which could enable phishing, identity spoofing, and social engineering attacks — especially effective on mobile devices.

🔓 Vulnerability Overview

Component Details

Type Input Validation Bypass
CWE CWE-20: Improper Input Validation
Impact Account spoofing, phishing, trust abuse
Affected URL https://linktr.ee/admin/account
Exploit Register username with a space prefix

— -

🎯 Why This Matters

Linktree is widely used by:

Influencers

Brands

Public Figures

Creators

A malicious actor could register a deceptive username like:

https://linktr.ee/%20tech_iela

…which looks almost identical to:

https://linktr.ee/tech_iela

🔍 On mobile devices or embedded previews, the difference is nearly invisible.

— -

🔥 Real-World Risks

✅ Impersonation of public figures

✅ Phishing via malicious Linktree pages

✅ Reputation damage

✅ Link-based scams or malware distribution

— -

🧪 Proof of Concept (PoC)

✅ Steps to Reproduce:

1. Login to your Linktree account

2. Go to: Settings → Change Username

3. Try entering an existing username like tech_iela
👉 Error: “Username already taken”

4. Open Burp Suite and intercept the PATCH request

5. Modify the body like this:

{
“username”: “ tech_iela”
}

6. Forward the request — server accepts the change!

7. Now open:

https://linktr.ee/%20tech_iela

Compare with:

https://linktr.ee/tech_iela

Result: Most users won’t spot the difference.

— -

📹 Video PoC:

🎥 Watch here →

https://drive.google.com/file/d/1Dr7EbNv32K7vSJsxItiiLvF5ZNxf62Vd/view?usp=sharing

— -

🧬 Technical Breakdown

The frontend validates usernames using regex and blocks spaces.

But the backend fails to trim or sanitize input.

This causes client-server validation mismatch, a common flaw.

The space (%20) is URL-encoded and appears normal in most browsers.

⚠ Even invisible characters like %09 (tab), %0A (newline), or U+200B (zero-width space) can be tested in similar ways.

— -

🔐 Security Recommendations

🔧 For Linktree or similar platforms:

1. Sanitize input on the backend — trim spaces, remove invisible characters.

2. Mirror frontend validation on the server-side.

3. Normalize usernames before comparison or database lookups.

4. Disallow usernames that only differ by whitespace.

5. Audit past usernames for abuse of this trick.

— -

🗣 Final Thoughts

> Security isn’t only about complex attacks — it’s often about spotting small cracks before they become doors.

This bug is a perfect example of how low-severity issues can escalate into high-impact social engineering attacks.

🚨 A single space in a username can be used to impersonate, phish, and mislead thousands.

— -

👨‍💻 About the Researcher

Aditya Sunny
Cybersecurity Enthusiast | Ethical Hacker | Ex-Navodayan
📍 Reported bugs to Meta, Dell, Canva, Tesla, and more
🏆 Honored by Bajaj Finserv Security Heroes

🕸 Connect with me:

Twitter/X: @adityasunny06

GitHub: github.com/adityasunny06

LinkedIn: Aditya Sunny

— -

✅ Feel free to repost, share, or translate this writeup.
If you’re a developer — fix it.
If you’re a hacker — dig deeper.

One character can change everything.

— -

🔖 Tags: #BugBounty #Linktree #InputValidation #CWE20 #PoC #Impersonation