# Race Condition in Purchase / Financial Transaction

> Detects financial transaction code that can be exploited via concurrent requests to exceed balance limits or purchase limits.

- Skill: `zakirkun/race-condition-in-purchase-financial-transaction` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add zakirkun/race-condition-in-purchase-financial-transaction`
- Raw SKILL.md: https://api.skillmd.com/api/skills/zakirkun/race-condition-in-purchase-financial-transaction/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: zakirkun (https://skillmd.com/u/zakirkun)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/zakirkun/race-condition-in-purchase-financial-transaction

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# Race Condition in Purchase / Financial Transaction

## Overview
A race condition in financial transactions allows attackers to send multiple concurrent requests to:
- Withdraw more funds than available balance
- Buy more items than stock allows
- Redeem the same voucher/coupon multiple times
- Use the same gift card balance repeatedly

This is a time-of-check/time-of-use (TOCTOU) vulnerability at the database level.

## Remediation
- Use database-level row locking (`SELECT ... FOR UPDATE`)
- Use optimistic locking with version counters
- Use atomic operations (`UPDATE balance SET balance = balance - amount WHERE balance >= amount`)
- Use idempotency keys for payment APIs