# Improper Neutralization of CRLF Sequences ('CRLF Injection') (CWE-93)

The product uses CRLF (carriage return line feed) as a special element, e.g. to separate headers or records, but it does not neutralize or incorrectly neutralizes CRLF sequences from inputs.

**Stack:** Python

- Prevalence: Medium 3 languages covered
- Impact: High 3 high-severity rules
- Prevention: Documented 3 fix examples

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

## Description

CRLF injection can be used to inject malicious headers in HTTP responses (HTTP response splitting), forge log entries, or manipulate other protocols that use CRLF as a delimiter.

## Prevention

Prevention strategies for CRLF Injection based on 1 Shoulder detection rules.

### Python

Strip newline characters from email headers before use

## Warning Signs

- [HIGH] user input used in email headers without newline sanitization

## Consequences

- Modify Application Data
- Execute Unauthorized Code
- Hide Activities

## Mitigations

- Strip or encode CRLF sequences from all input used in headers or logs
- Use frameworks that automatically handle header encoding
- Validate that inputs do not contain unexpected control characters

## Detection

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

## Rules by Language

### Python (1 rules)

- **Email Header Injection** [HIGH]: Detects user input used in email headers without newline sanitization.
  - Remediation: Remove newlines from email headers before use.

```python
safe_subject = subject.replace('\r', '').replace('\n', '')
```

Learn more: https://shoulder.dev/learn/python/cwe-93/email-injection