Predictable Time-Based Auth Token Leading to Authentication Bypass | Sokudo
Predictable Time-Based Auth Token Leading to Authentication Bypass | Sokudo
Overview
Sokudo is a typing speed platform that used a custom authentication mechanism for accessing authenticated functionality.
During testing, the application was found generating authorization tokens using predictable timestamp values derived from user login times.
Because the token generation logic relied on publicly exposed information, it became possible to recreate valid authentication tokens for other users, including the administrator account.
Objective
The objective was to analyze the application’s authentication mechanism and determine whether authorization tokens could be predicted or forged.
The assessment focused on:
- token structure
- insecure authentication logic
- exposed user metadata
- privilege escalation possibilities
Reconnaissance
After registering an account and browsing the application functionality, requests were intercepted using Burp Suite.
A request to the leaderboard API revealed a suspicious authorization token:
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GET /api/stats/leaderboard HTTP/2
Authorization: Bearer 20260501104846
The token format appeared structured rather than random:
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20260501104846
This immediately suggested the value might represent a timestamp.
Token Analysis
The leaderboard response exposed user activity metadata, including login timestamps:
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[
{
"username":"admin",
"last_login":"2026-05-01T10:57:47.833Z"
},
{
"username":"kelvin",
"last_login":"2026-05-01T10:48:46.204Z"
}
]
Comparing the exposed timestamp with the authorization token revealed a direct relationship.
Example:
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2026-05-01 10:48:46
↓
20260501104846
The application appeared to generate tokens using:
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YYYYMMDDHHMMSS
derived directly from the user’s last login time.
This made authentication tokens entirely predictable.
Authentication Bypass
Using the exposed administrator login timestamp:
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2026-05-01T10:57:47.833Z
the administrator token was recreated:
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20260501105747
The token stored inside browser Local Storage was modified manually:
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Inspect → Storage → Local Storage → token
After replacing the existing value with the forged administrator token and refreshing the page, administrative functionality became accessible.
The administrator dashboard exposed the flag successfully:
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bug{....}
Proof of Exploitation
Attack Flow
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User registration
↓
Leaderboard API discovery
↓
Authorization token analysis
↓
Timestamp pattern identified
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User login timestamps exposed
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Admin token recreated
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Local Storage token modified
↓
Authentication bypass
Root Cause Analysis
The vulnerability existed because the application used predictable user metadata for authentication token generation.
The authentication logic relied on:
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token = formatted_last_login_timestamp
This introduced several critical weaknesses:
- predictable authentication tokens
- publicly derivable session values
- no cryptographic protection
- no server-side validation
- insecure trust in client-controlled tokens
Because login timestamps were exposed through the API, attackers could generate valid tokens for arbitrary users.
Impact
An attacker could potentially:
- impersonate arbitrary users
- bypass authentication entirely
- gain administrative access
- escalate privileges without credentials
- compromise sensitive application functionality
In real-world environments, predictable authentication mechanisms can lead to full platform compromise.
Mitigation
Use Cryptographically Secure Tokens
Authentication tokens must be:
- random
- unpredictable
- securely generated
- resistant to guessing
Never Derive Tokens from Public Data
Authentication mechanisms should never rely on:
- timestamps
- usernames
- email addresses
- predictable metadata
Enforce Server-Side Session Validation
The server should validate authenticated sessions instead of trusting client-generated token values.
Avoid Storing Sensitive Auth Logic Client-Side
Authentication state stored in:
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Local Storage
Session Storage
Cookies
should always be integrity protected and validated server-side.
Implement Signed Authentication Tokens
Authentication systems should use securely signed tokens or properly managed server-side sessions.
Real-World Insight
Many insecure authentication systems fail because developers confuse uniqueness with unpredictability.
A timestamp may be unique, but it is not secret.
If attackers can observe or derive the underlying value used to generate authentication tokens, account impersonation becomes trivial.
This lab demonstrates how exposing seemingly harmless metadata like login timestamps can completely undermine authentication security when token generation is poorly designed.