Broken Authentication – Predictable Timestamp Token Leads to Admin Account Takeover | Sokudo

Overview

Sokudo is a typing speed tracking web application. During testing, a critical flaw was identified in its authentication mechanism, where authorization tokens were generated using predictable timestamp values.

This allowed an attacker to forge valid tokens and impersonate other users, including the administrator, resulting in a complete account takeover.

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Objective

• Analyze authentication behavior
• Identify weaknesses in token generation
• Forge a valid admin token
• Achieve privilege escalation

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Reconnaissance

After registering an account, the application’s functionality was explored while intercepting HTTP traffic using Burp Suite.

While navigating the stats page, the following API request was observed:

GET /api/stats/leaderboard HTTP/2
Host: lab-1777632466593-oxprot.labs-app.bugforge.io
Authorization: Bearer 20260501104846

The Authorization token appeared unusual due to its structured numeric format.

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Analysis

The token value was:

20260501104846

At first glance, it did not resemble a secure session token or JWT, but rather a formatted value.

To investigate further, the API response was analyzed.

Response Inspection

The leaderboard endpoint returned user data including last_login timestamps:

[
  {
    "username": "admin",
    "last_login": "2026-05-01T10:57:47.833Z"
  },
  {
    "username": "kelvin",
    "last_login": "2026-05-01T10:48:46.204Z"
  }
]

A clear relationship was identified.

Token:

20260501104846

Corresponding timestamp:

2026-05-01T10:48:46

This confirmed that the application was generating tokens directly from the user’s last_login value.

Token Structure

The token follows a deterministic format:

YYYYMMDDHHMM

Part	Meaning
YYYY	Year
MM	Month
DD	Day
HH	Hour
MM	Minute

Because this value is derived from exposed data, it becomes fully predictable.

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Exploitation

Using this logic, a valid admin token can be forged.

Step 1: Extract Admin Timestamp

2026-05-01T10:57:47.833Z

Step 2: Convert to Token Format

20260501105747

Step 3: Replace Token in Browser

1. Open the application
2. Open Developer Tools
3. Navigate to: Application → Local Storage
4. Locate the stored token
5. Replace it with:

20260501105747

Step 4: Refresh

After refreshing the page, the session is authenticated as admin.

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Proof of Exploitation

Successful privilege escalation resulted in access to the admin dashboard and disclosure of the flag:

bug{....}

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Impact

• Full account takeover of any user
• Unauthorized admin access
• Exposure of sensitive functionality and data
• No need for brute force or credential compromise

This vulnerability enables instant privilege escalation with minimal effort.

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Root Cause

The issue stems from insecure authentication design:

• Tokens derived from predictable timestamps
• No randomness or entropy
• No cryptographic signing or validation
• Sensitive user metadata exposed via API

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Mitigation

To secure the application:

• Use cryptographically secure random tokens
• Implement signed tokens (e.g., JWT with strong secrets)
• Avoid deriving tokens from predictable values
• Limit exposure of sensitive fields like last_login
• Enforce proper server-side session validation
• Implement token expiration and rotation

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Real-World Insight

This vulnerability highlights a common mistake in custom authentication implementations.

Developers sometimes generate tokens using simple patterns such as timestamps or user identifiers, assuming uniqueness is sufficient. However, without unpredictability and cryptographic protection, these tokens can be trivially reconstructed.

In real-world scenarios, similar flaws have led to large-scale account takeovers, especially in poorly designed APIs.