How a Default Password Let Me Log Into Almost Anyone’s Account

A story about how one shared default password, combined with a simple user-enumeration flaw, turned into a critical account takeover vulnerability.

While testing a web platform that I'll call REDACTED.COM for responsible disclosure, I came across an authentication flaw that initially looked pretty ordinary.

The platform allowed users to manage their profile, vehicle, purchase, and other account-related information. The application itself didn't immediately look unusual.

Then I noticed something interesting in the registration flow.

Every newly created account was being assigned the same default password.

That single observation led me down a rabbit hole that eventually uncovered a complete account takeover chain.

The First Clue: "Does This User Exist?"

I started by looking at the registration and account-validation functionality.

There was an API endpoint designed to check whether a phone number was already registered.

When I submitted a registered number, the server responded with something along the lines of:

User already exists.

From an application's perspective, this makes sense. The frontend needs to know whether it should continue with registration or direct the user elsewhere.

From a security perspective, however, this creates a user-enumeration vulnerability.

An attacker could potentially submit phone numbers and distinguish between:

Registered

and

Not registered

The endpoint effectively became an account-existence oracle.

On its own, this wasn't enough to take over an account.

But it was the first piece of the puzzle.

The Second Clue: Everyone Gets the Same Password

While looking at the registration flow, I noticed something far more interesting.

When a new account was created, the platform sent login credentials to the user via SMS.

The username was based on the user's mobile number.

The password, however, wasn't unique.

It was a shared default password.

Conceptually, the credentials looked like this:

Username: <Mobile Number>
Password: <Shared Default Password>

Username: <Mobile Number>
Password: <Shared Default Password>

The important question became:

What happens if a user never changes that password?

If the default password remains valid after registration, then every account that still uses it is potentially vulnerable.

And unlike a leaked password belonging to one person, a shared default password can potentially affect an entire population of users.

Connecting the Dots

At this point, the attack chain became clear.

The combination of the two issues was much more serious than either issue individually.

The flow was essentially:

User Enumeration
       ↓
Identify Registered Account
       ↓
Predictable Username
       ↓
Shared Default Password
       ↓
Successful Authentication
       ↓
Access Customer Account
       ↓
Password Change
       ↓
Potential Account Takeover

User Enumeration
       ↓
Identify Registered Account
       ↓
Predictable Username
       ↓
Shared Default Password
       ↓
Successful Authentication
       ↓
Access Customer Account
       ↓
Password Change
       ↓
Potential Account Takeover

I tested the credential behavior within the scope of authorized security testing.

The default credentials were accepted for accounts that had not changed their initial password.

Once authenticated, the account exposed information such as:

• Full name
• Email address
• Mobile number
• User/account identifiers
• Vehicle information
• Buyer information
• Transaction-related details
• Vehicle ownership information

At that point, this was clearly more than a weak-password finding.

It was an account takeover vulnerability with access to sensitive customer information.

The Password Change Made It Worse

I then looked at what an authenticated user could do with the account.

The application provided a password-change functionality.

Because the attacker already knew the existing password, they could authenticate normally and then replace the password with one controlled by the attacker.

That changes the situation from:

"I can access the account."

to:

"I can potentially take permanent control of the account."

The legitimate user could potentially be locked out, while the attacker retained access using the newly changed password.

This is an important distinction when assessing account takeover vulnerabilities.

Why This Was a Systemic Issue

One of the most important observations during the testing was that this wasn't limited to a single account.

The behavior could be reproduced against multiple existing accounts within the authorized testing scope.

That suggested the problem wasn't simply:

"One user chose a weak password."

Instead, the underlying issue was in the platform's authentication design.

Several individually small weaknesses were interacting with each other:

1. Predictable usernames

The username was based on a mobile number.

2. Shared default credentials

New accounts received the same default password.

3. Default credentials remained usable

Users weren't sufficiently protected if they never changed the initial password.

4. User enumeration

The application disclosed whether a phone number was already registered.

5. Account modification after authentication

An authenticated attacker could change the account password.

Individually, some of these issues might receive limited severity.

Together, they created a critical attack chain.

The Interesting Part: No Brute Force Required

What made this vulnerability particularly interesting was that it didn't depend on traditional password brute forcing.

There was no need to guess thousands of random passwords.

There was no need to exploit a complicated authentication bypass.

There was no need for malware or phishing.

The attack relied on understanding how the application itself handled account creation and authentication.

The password wasn't something an attacker had to discover through brute force.

It was a value the application itself was distributing.

That completely changes the threat model.

How I Would Fix It

The remediation doesn't require rebuilding the entire application.

A few architectural changes can eliminate the attack chain.

Remove shared default passwords

Every account should have a unique credential or, preferably, use a secure account-activation flow instead of assigning a shared password.

Make temporary credentials single-use

If temporary credentials are necessary, they should be:

• Random
• Unique
• Short-lived
• Single-use
• Invalidated after activation

Force account activation

A newly created account shouldn't remain permanently accessible using an initial temporary credential.

Prevent user enumeration

Instead of explicitly returning:

User already exists

User already exists

the application should provide a generic response that doesn't reveal whether an account exists.

Protect authentication endpoints

Implement appropriate:

• Rate limiting
• Credential-stuffing detection
• Abuse monitoring
• Login anomaly detection
• MFA/OTP where appropriate

Protect password changes

Password changes should require strong authentication and should invalidate existing sessions where appropriate.

Responsible Disclosure

I reported the vulnerability through the platform's responsible disclosure/bug bounty program.

The finding was classified as P1 / Critical, with a CVSS score of 9.0.

The vulnerability was subsequently marked Closed & Rewarded.

To respect the disclosure process and protect users, I have intentionally redacted the platform name, real endpoints, credentials, customer information, and other sensitive details from this article.

Final Takeaway

This finding reminded me of something I see repeatedly in security testing:

The most dangerous vulnerabilities aren't always complicated.

Sometimes they're created by a few seemingly harmless design decisions.

A shared default password doesn't necessarily look catastrophic.

User enumeration doesn't necessarily look catastrophic.

A password-change feature doesn't necessarily look dangerous.

But when you connect them:

Enumeration
     +
Predictable Username
     +
Shared Default Password
     +
No Forced Credential Rotation
     +
Password Change
     =
Account Takeover

Enumeration
     +
Predictable Username
     +
Shared Default Password
     +
No Forced Credential Rotation
     +
Password Change
     =
Account Takeover

That's where the real impact appears.

The lesson for developers is simple:

Never assume a temporary credential will remain temporary.

Always design for the user who never changes the default password, never enables MFA, and never revisits their security settings.

Because attackers will.

And sometimes, all they need is one predictable password.

This vulnerability was discovered during authorized security research and responsibly disclosed through the platform's bug bounty program. The platform name and sensitive technical details have been redacted to respect the disclosure process.