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Difficulty: HardApplication and Software Vulnerabilities

An application security engineer analyzes transaction execution traces and thread dumps from a multi-threaded microservice responsible for handling account withdrawals. The application verifies an account's available funds prior to deducting the balance and completing the transfer. During high-concurrency peak load testing, automated monitoring detects instances where account balances drop below zero despite validation checks executing successfully without failure. Code analysis confirms that the check and the update operations are executed as non-atomic statements across separate database connections. Which of the following vulnerabilities is demonstrated in this scenario?

  1. Time-of-check to time-of-use (TOCTOU) race conditionAnswer
  2. B
    Integer overflow vulnerability
  3. C
    Server-side request forgery (SSRF)
  4. D
    Insecure direct object reference (IDOR)

Answer

Time-of-check to time-of-use (TOCTOU) race condition
The correct option correctly identifies the vulnerability as a Time-of-Check to Time-of-Use (TOCTOU) race condition. TOCTOU occurs when a program checks the state of a resource (e.g., account balance) and then acts upon that resource (e.g., executing a withdrawal), but the state changes in the time window between checking and acting due to concurrent execution threads.

Step-by-Step Solution

1
Analyze the observed system behavior described in the scenario logs.
Identified that validation passes before execution, but state changes occur concurrently under high thread volume.
Understanding why checks pass despite invalid final state points directly to timing gaps.
2
Evaluate the relationship between the validation check and the execution operation.
Determined that the verification (time-of-check) and modification (time-of-use) are non-atomic and exposed to race conditions.
Non-atomic operations across asynchronous or concurrent threads allow state manipulation in the window between check and execution.
3
Select the vulnerability classification matching timing-dependent state flaws.
Confirmed the flaw as a Time-of-Check to Time-of-Use (TOCTOU) race condition.
TOCTOU explicitly describes vulnerability windows created between system condition verification and resource access.

Key Concept

Time-of-Check to Time-of-Use (TOCTOU) and Concurrency Vulnerabilities
Estimated Time:2m 0s
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Topics

Application and Software Vulnerabilities

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