: If the database returns false (unused), the server calculates the discount and subtracts the balance from the checkout cart.

We need to run both scripts simultaneously. Open two terminal windows (or use & to background one process).

A race condition occurs when a system's behavior depends on the uncontrolled or sequence of multiple operations. In web security, this often manifests as a Time-of-Check to Time-of-Use (TOCTOU) flaw: the server checks if an action is valid (e.g., checking a file type) but then processes that action in a separate step. If an attacker can slip a malicious request into the tiny "race window" between the check and the use, they can bypass security controls. Breaking Down the HackViser Challenge

, these are explored as critical logic flaws that allow attackers to bypass intended business restrictions. The Mechanics of the "Race Window" The core of the vulnerability lies in a concept called the Race Window

The Hackviser training breaks down race conditions into several critical categories, helping students distinguish between different attack vectors:

: Rapidly clicking "Start Trial" or "Enable Feature" to trick the server into granting access before it validates your payment status.

How do developers prevent this?

Alex now has in digital goods while only ever starting with $100. The system "raced" to update the data, and Alex's dual-threat attack caused a collision that broke the logic. The Resolution: Securing the Vault

If we can make the program check one file but open another, we win.

A race condition hackviser fires 30 requests in 30ms. Standard rate limiting (e.g., 100 requests per minute) is a barn door against this sniper. You need at the load balancer level (e.g., max_connections_per_ip set to 2).

: Use SELECT ... FOR UPDATE in SQL. This locks the target row immediately when it is read, forcing all other concurrent threads to wait until the current transaction commits or rolls back.