# 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: Mittel 3 Sprachen abgedeckt
- Impact: Hoch 1 Regeln mit hohem Schweregrad
- Prevention: Dokumentiert 3 Fix-Beispiele

**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

Präventionsstrategien für ReDoS basierend auf 1 Shoulder-Erkennungsregeln.

### 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

- Verschachtelte Quantifizierer und überlappende Alternationen in Regex vermeiden
- Timeout-Mechanismen für Regex verwenden
- Den Einsatz nicht-backtrackender Regex-Engines erwägen

## 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