# 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:** JavaScript

- 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.

### JavaScript

Validate email addresses and strip CRLF characters from header values

## Warning Signs

- [HIGH] email header injection vulnerabilities where user input flows into
email headers (To, From, Subject,

## 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

### Javascript (1 rules)

- **Email Header Injection** [HIGH]: Detects email header injection vulnerabilities where user input flows into
email headers (To, From, Subject, Cc, Bcc) without validation. Attackers can
inject CRLF sequences (\r\n) to add arbitrary headers or body content.

Attack impact:
- Send spam/phishing emails via your server
- Add hidden recipients (Cc/Bcc injection)
- Modify email content
- Bypass spam filters using your domain reputation

Common vulnerable patterns:
- nodemailer with user-controlled options
- SendGrid/Mailgun APIs with user input
- Custom SMTP implementations
  - Remediation: Validate email addresses and remove CRLF from header values:

```javascript
const validator = require('validator');

function sanitizeHeader(value) {
  return value.replace(/[\r\n]/g, '');
}

if (!validator.isEmail(email)) {
  return res.status(400).json({ error: 'Invalid email' });
}

const safeSubject = sanitizeHeader(subject).slice(0, 200);
await transporter.sendMail({
  to: 'admin@example.com',
  subject: `Contact: ${safeSubject}`,
  text: message
});
```

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

### Typescript (1 rules)

- **Email Header Injection** [HIGH]: Detects email header injection vulnerabilities where user input flows into
email headers (To, From, Subject, Cc, Bcc) without validation. Attackers can
inject CRLF sequences (\r\n) to add arbitrary headers or body content.

Attack impact:
- Send spam/phishing emails via your server
- Add hidden recipients (Cc/Bcc injection)
- Modify email content
- Bypass spam filters using your domain reputation

Common vulnerable patterns:
- nodemailer with user-controlled options
- SendGrid/Mailgun APIs with user input
- Custom SMTP implementations
  - Remediation: Validate email addresses and remove CRLF from header values:

```javascript
const validator = require('validator');

function sanitizeHeader(value) {
  return value.replace(/[\r\n]/g, '');
}

if (!validator.isEmail(email)) {
  return res.status(400).json({ error: 'Invalid email' });
}

const safeSubject = sanitizeHeader(subject).slice(0, 200);
await transporter.sendMail({
  to: 'admin@example.com',
  subject: `Contact: ${safeSubject}`,
  text: message
});
```

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