# Detecting Off By One

> Detects off-by-one errors by identifying incorrect loop conditions, array indexing mistakes, and boundary condition problems. Use when analyzing loops, array access, or investigating fencepost errors.

- Skill: `majiayu000/detecting-off-by-one` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds add majiayu000/detecting-off-by-one`
- Raw SKILL.md: https://api.skillmd.com/api/skills/majiayu000/detecting-off-by-one/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: majiayu000 (https://skillmd.com/u/majiayu000)
- Updated: 2026-09-09
- Page: https://skillmd.com/skills/majiayu000/detecting-off-by-one

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# Off-by-One Detection

## Detection Workflow

1. **Identify array operations**: Find all array accesses, loop iterations, buffer allocations, string operations
2. **Analyze boundary conditions**: Check loop termination conditions, array index ranges, buffer size calculations
3. **Check edge cases**: Test boundary conditions, verify fencepost cases, assess null terminator handling
4. **Assess impact**: Can off-by-one cause overflow/underflow? What's the security impact?

## Key Patterns

- Loop bound errors: using <= instead of <, or < instead of <=
- Array index errors: accessing array[size] instead of array[size-1]
- String handling errors: missing null terminator, incorrect buffer size
- Allocation errors: allocating size instead of size+1

## Output Format

Report with: id, type, subtype, severity, confidence, location, vulnerability, loop condition, array access, array size, error type, exploitability, attack scenario, impact, mitigation.

## Severity Guidelines

- **HIGH**: Off-by-one causing buffer overflow
- **MEDIUM**: Off-by-one causing information disclosure
- **LOW**: Off-by-one with minor impact

## See Also

- `patterns.md` - Detailed detection patterns and exploitation scenarios
- `examples.md` - Example analysis cases and code samples
- `references.md` - CWE references and mitigation strategies