# Inefficient Regular Expression Complexity (CWE-1333)

The product uses a regular expression with an inefficient, possibly exponential worst-case computational complexity that consumes excessive CPU cycles.

**Stack:** Python

- Prevalence: 中 3 言語をカバー
- Impact: ハイ 1 件の重大度ハイのルール
- Prevention: 文書化済み 3 件の修正例

**OWASP:** Injection (A03:2021-Injection) - #3

## Description

Certain regular expression patterns can take exponential time to evaluate on certain inputs (ReDoS). Attackers can craft inputs that cause the regex engine to consume excessive CPU time, leading to denial of service.

## Prevention

1 件の Shoulder 検出ルールに基づく ReDoS の予防策。

### Key Practices

- Use exponential time complexity when matching certain inputs

### Python

Replace nested quantifiers with simple patterns and bounded repetition

## Warning Signs

- [MEDIUM] regular expressions with catastrophic backtracking patterns that can
cause exponential time complexi

## Consequences

- DoS

## Mitigations

- 正規表現でネストした量指定子や重なり合う選択肢を避ける
- 正規表現にタイムアウト機構を導入する
- バックトラッキングを行わない正規表現エンジンの利用を検討する

## Detection

- Total rules: 3
- Languages: go, javascript, typescript, python

## Rules by Language

### Python (1 rules)

- **Regular Expression Denial of Service (ReDoS)** [MEDIUM]: Detects regular expressions with catastrophic backtracking patterns that can
cause exponential time complexity when matching certain inputs. Attackers can
exploit this to cause denial of service. Use simpler patterns or set timeouts.
  - Remediation: Avoid nested quantifiers like (a+)+. Use simple patterns with bounded quantifiers.

```python
import re

# Safe: simple character class with bounded quantifiers
pattern = re.compile(r'^[a-zA-Z0-9_]{3,20}$')

if not pattern.match(username):
    raise ValueError('Invalid username')
```

Learn more: https://shoulder.dev/learn/python/cwe-1333/redos