# Integer Overflow / Underflow

> Detects arithmetic operations that can overflow or underflow integer bounds, leading to heap overflows, logic bypasses, or unexpected behavior.

- Skill: `zakirkun/integer-overflow-underflow` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add zakirkun/integer-overflow-underflow`
- Raw SKILL.md: https://api.skillmd.com/api/skills/zakirkun/integer-overflow-underflow/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/integer-overflow-underflow

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# Integer Overflow / Underflow

## Overview
Integer overflow occurs when arithmetic results exceed the data type's maximum value. In C/C++:
- Signed integer overflow is **undefined behavior** (UB) — compilers can optimize it away
- Unsigned overflow wraps around (e.g., `0 - 1 = UINT_MAX`)

Common exploitable scenarios:
- **Heap overflow**: `malloc(user_len + 4)` — if `user_len = UINT_MAX - 3`, result wraps to small allocation
- **Loop condition bypass**: `for (i = 0; i <= MAX_INT; i++)` — wraps to 0
- **Signed/unsigned mismatch**: `if (len < MAX)` where `len` is unsigned and `MAX` is signed

## Remediation
- Validate input sizes before arithmetic operations
- Use safe integer libraries (`SafeInt`, `checked_arithmetic`)
- Use compiler sanitizers (UBSan, ASan) to detect overflow