Crlf Injection in Strapi

CRLF injection in Strapi occurs when untrusted user input containing carriage return ( ) and line feed ( ) characters is incorporated into HTTP headers or responses without proper sanitization. In Strapi's architecture, this vulnerability typically manifests in several specific contexts:

Custom Controllers and Services: When developers create custom endpoints or modify Strapi's default behavior, they often handle user input directly. For example, a custom controller that sets response headers based on user input:

// Vulnerable Strapi controller
module.exports = {
  async customEndpoint(ctx) {
    const { headerName, headerValue } = ctx.request.body;
    ctx.set(headerName, headerValue); // CRLF injection possible here
    ctx.send({ message: 'Header set' });
  }
};

If an attacker submits headerName as Content-Type: text/html Set-Cookie: session=malicious, the response will include unintended headers, potentially leading to session fixation or XSS.

Plugin Development: Strapi's plugin system allows developers to extend functionality, but custom plugins often mishandle user input in response headers. The strapi-plugin-email has historically been vulnerable to CRLF injection when constructing email headers from user input:

// Vulnerable email plugin code
const email = {
  to: ctx.request.body.to,
  subject: ctx.request.body.subject,
  text: ctx.request.body.text,
  headers: {
    'X-Custom-Header': ctx.request.body.customHeader // CRLF injection here
  }
};

Content-Type Manipulation: Strapi's content-type system allows defining custom fields and relationships. When these fields are used in API responses without proper validation, attackers can inject CRLF sequences to manipulate response structure:

{
  "title": "Normal Article",
  "content": "This is fine.\r\nContent-Type: text/html\r\n\r\nX-Powered-By: PHP/7.4.3"
}

This payload, when rendered in certain Strapi configurations, could alter the HTTP response headers or introduce unexpected content types.

Detecting CRLF injection in Strapi requires both manual testing and automated scanning. middleBrick's API security scanner includes specific checks for this vulnerability in Strapi applications:

Automated Scanning with middleBrick: The scanner tests for CRLF injection by submitting payloads containing various CRLF sequences to all endpoints in your Strapi application. It specifically looks for:

• Header injection attempts in response headers
• Content-Type manipulation in API responses
• Authentication bypass through header manipulation
• Response splitting vulnerabilities

middleBrick's CRLF detection includes 27 regex patterns that identify potential injection attempts across different response formats. For Strapi specifically, it tests against the framework's common response patterns and header manipulation points.

Manual Testing Methodology: To manually test your Strapi application for CRLF injection:

# Test with curl - look for unexpected headers
curl -v -X POST http://localhost:1337/api/custom-endpoint \
  -H "Content-Type: application/json" \
  -d '{"headerName": "X-Injected: value\r\nX-Second-Injected: second", "headerValue": "test"}'

# Check response headers for injected values
# Look for headers like:
# X-Injected: value
# X-Second-Injected: second

Strapi-Specific Indicators: In Strapi applications, watch for these signs of potential CRLF vulnerabilities:

• Custom controllers that use ctx.set() or ctx.response.set() with user input
• Plugin code that constructs email headers or HTTP responses dynamically
• Middleware that modifies response headers based on request parameters
• Services that generate CSV or other text-based exports with user-controlled content

middleBrick CLI Integration: For continuous security testing, integrate middleBrick into your Strapi development workflow:

# Install middleBrick CLI
npm install -g middlebrick

# Scan your Strapi API
middlebrick scan http://localhost:1337/api

# Run as part of your CI/CD pipeline
middlebrick scan --fail-below B http://staging.yourstrapiapp.com

The scanner provides detailed reports showing exactly which endpoints are vulnerable to CRLF injection and what specific payloads triggered the vulnerability.

Remediating CRLF injection in Strapi requires a multi-layered approach. Here are Strapi-specific fixes for common vulnerability patterns:

Input Sanitization in Controllers: Always sanitize user input that might affect headers or responses:

// Secure Strapi controller
const sanitizeCRLF = (input) => {
  return input.replace(/[
]/g, '');
};

module.exports = {
  async customEndpoint(ctx) {
    const { headerName, headerValue } = ctx.request.body;
    
    // Sanitize inputs
    const safeHeaderName = sanitizeCRLF(headerName);
    const safeHeaderValue = sanitizeCRLF(headerValue);
    
    ctx.set(safeHeaderName, safeHeaderValue);
    ctx.send({ message: 'Header set' });
  }
};

Using Strapi's Built-in Validation: Leverage Strapi's validation system to prevent malicious input:

// In your controller's validation schema
const Joi = require('joi');

const schema = Joi.object({
  headerName: Joi.string()
    .pattern(/^[a-zA-Z0-9-_]+$/) // Only allow safe characters
    .required(),
  headerValue: Joi.string()
    .max(200)
    .required()
});

module.exports = {
  async customEndpoint(ctx) {
    const { error, value } = schema.validate(ctx.request.body);
    if (error) {
      ctx.status = 400;
      ctx.send({ error: error.details[0].message });
      return;
    }
    
    ctx.set(value.headerName, value.headerValue);
    ctx.send({ message: 'Header set' });
  }
};

Middleware for Global Protection: Create Strapi middleware to automatically sanitize all responses:

// config/middleware.js
module.exports = ({ env }) => ({
  crlfProtection: {
    enabled: true,
    priority: 1000,
    init: (params) => {
      return async (ctx, next) => {
        await next();
        
        // Sanitize response body if it's text
        if (typeof ctx.body === 'string') {
          ctx.body = ctx.body.replace(/\r\n/g, '\n');
        }
      };
    }
  }
});

Plugin Security Best Practices: When developing Strapi plugins, always validate and sanitize:

// Secure plugin email service
const sanitizeEmailHeader = (header) => {
  // Remove any CRLF characters and control characters
  return header.replace(/[\r\n\x00-\x1F\x7F]/g, '');
};

module.exports = {
  async sendEmail(ctx) {
    const { to, subject, body, customHeader } = ctx.request.body;
    
    const safeEmail = {
      to,
      subject: sanitizeEmailHeader(subject),
      text: body,
      headers: {
        'X-Custom-Header': sanitizeEmailHeader(customHeader || '')
      }
    };
    
    // Send email logic here
  }
};

Content-Type Security Headers: Add security headers to prevent response manipulation:

// config/policies.js
module.exports = {
  securityHeaders: {
    enabled: true,
    priority: 500,
    init: (params) => {
      return async (ctx, next) => {
        ctx.set('X-Content-Type-Options', 'nosniff');
        ctx.set('X-Frame-Options', 'DENY');
        await next();
      };
    }
  }
};

Testing Your Fixes: After implementing these fixes, use middleBrick to verify the vulnerabilities are resolved:

middlebrick scan --test-crlf http://localhost:1337/api

This command specifically tests for CRLF injection vulnerabilities and provides detailed feedback on whether your Strapi application is now secure against this attack vector.