Magic Login Link Bug Causes Account Takeover.

Magic Login Links: $500 Bug Bounty Explained Simply​

Magic login links are everywhere today . They’re fast, easy, and password-free. But when they’re implemented the wrong way, they can turn into a serious account takeover (ATO) risk - and that’s exactly what happened here.

A security researcher earned $500 after discovering a flaw that allowed full account takeover without phishing or user mistakes. Let’s break it down in a simple, human way

TL;DR - What’s the Issue?​

A mobile app failed to secure its magic login deep links.
Because of weak verification, another app on the same Android device could intercept the login token and hijack the account.
No passwords
No phishing
No social engineering
Just bad deep link handling

---

​

What Exactly Went Wrong?​

Here’s what the app did:

• Used magic login links instead of passwords
• Sent login links through email
• Opened links using deep linking (Branch.io)
• Embedded a login token inside the URL

Here’s the problem The app did not properly verify which app was allowed to open that link.
So what happened?

• Any malicious Android app listening for the same URL pattern could catch it
• That app could steal the login token
• The attacker could log in as the victim instantly

Full account takeover.

---

​

Simple Real-Life Example​

Imagine this

1. Alice types her email in the app
2. She gets a magic login link by email
3. The link should open only in the real app
4. But another app on her phone also listens for that link
5. That app grabs the login token silently
6. The attacker logs in as Alice

No password stolen. No clicks tricked. Just a deep link misconfiguration.

---

​

Why This Is Dangerous​

Magic links are safe only if done right If not:

• Attackers get instant access
• Personal data is exposed
• Messages and sessions are hijacked
• Anything the user can do, the attacker can do

A convenience feature becomes a security nightmare

---

​

🛠 Technical Insight (Easy Version)​

• Magic Link / Deep Link
  A URL that opens directly inside the mobile app
• Branch.io
  A popular deep linking service (not insecure by default)
• The real issue
  Missing or weak Android App Link verification

On Android, any app can register the same URL unless strict checks are enforced.

---

​

Example: Secure Android App Links​

Here’s a simple example of how developers should lock this down

<intent-filter android:autoVerify="true">
    <action android:name="android.intent.action.VIEW" />
    <category android:name="android.intent.category.DEFAULT" />
    <category android:name="android.intent.category.BROWSABLE" />
    <data
        android:scheme="https"
        android:host="example.com"
        android:pathPrefix="/login" />
</intent-filter>

This ensures only the verified app can open the login link.

---

​

Lessons for Hunters & Developers​

Bug Bounty Hunters​

Always check:

• How login tokens are passed
• Whether deep links are verified
• If another app can intercept them

Developers​

Always enforce:

• Android App Link verification
• Token binding to app/device
• Server-side token validation

Security must be enforced server-side, not just in the app.

---

​

Final Thoughts​

This was a real, repeatable, high-impact vulnerability - not clickbait.
A small mistake in deep link handling turned a helpful feature into a full account takeover bug, earning the researcher a $500 bounty and saving users from real risk.