Which vulnerability arises when software checks a resource and uses it later, potentially after changes?

Which vulnerability arises when software checks a resource and uses it later, potentially after changes?

• A. Race condition
• B. Buffer overflow
• C. Hardening
• D. TOCTOU

Answer: (D)

The idea being tested is the time gap between checking a resource and using it, which creates a TOCTOU vulnerability. TOCTOU stands for time-of-check-to-time-of-use, and it occurs when a program first verifies something about a resource (like existence, type, or permissions) and then, after that check, uses the resource. If the resource’s state can change in that short window, an attacker or another process could exploit the momentary inconsistency. A classic example is a program that checks a file’s permissions and then opens or reads it. If another process swaps that file for a malicious one during the interval between the check and the use, the program ends up operating on something it wasn’t supposed to trust. This is a specific kind of race condition, where the outcome depends on the timing between two operations. Mitigation centers on making the check and the use atomic, or otherwise eliminating the window where changes can occur—using atomic system calls, proper locking, or secure file handling patterns that perform the verification and use in a single, indivisible step.

The idea being tested is the time gap between checking a resource and using it, which creates a TOCTOU vulnerability. TOCTOU stands for time-of-check-to-time-of-use, and it occurs when a program first verifies something about a resource (like existence, type, or permissions) and then, after that check, uses the resource. If the resource’s state can change in that short window, an attacker or another process could exploit the momentary inconsistency.

A classic example is a program that checks a file’s permissions and then opens or reads it. If another process swaps that file for a malicious one during the interval between the check and the use, the program ends up operating on something it wasn’t supposed to trust. This is a specific kind of race condition, where the outcome depends on the timing between two operations.

Mitigation centers on making the check and the use atomic, or otherwise eliminating the window where changes can occur—using atomic system calls, proper locking, or secure file handling patterns that perform the verification and use in a single, indivisible step.