Crlf Injection in Sinatra

CRLF injection in Sinatra applications typically occurs when user-controlled input is incorporated into HTTP headers without proper sanitization. This vulnerability allows attackers to inject carriage return (CR) and line feed (LF) characters, enabling them to manipulate HTTP responses, create new headers, or perform response splitting attacks.

Consider this common Sinatra pattern:

get '/set-cookie' do
  cookie_name = params[:name]
  cookie_value = params[:value]
  response.set_cookie(cookie_name, cookie_value)
  "Cookie set!"
end

If an attacker requests /set-cookie?name=session&value=abc%0D%0A%20Secure%3D%22false%22, the %0D%0A sequence (CR+LF) allows them to inject additional cookie attributes. The resulting Set-Cookie header becomes:

Set-Cookie: session=abc
 Secure="false"

This disables the Secure flag, potentially exposing the cookie over HTTP.

Another manifestation occurs in redirect handling:

get '/redirect' do
  redirect params[:url]
end

An attacker could supply a URL containing CRLF sequences to manipulate the Location header or inject additional headers entirely.

Header manipulation is particularly dangerous in Sinatra because the framework's simplicity can lead to insufficient validation. For example:

get '/custom-header' do
  header_name = params[:name]
  header_value = params[:value]
  response[header_name] = header_value
  "Header set!"
end

This endpoint is vulnerable to any header injection attack, including CRLF injection, since it directly uses user input as header names and values.

Detecting CRLF injection in Sinatra applications requires both static analysis and dynamic testing. The most effective approach combines code review with automated scanning tools.

Static analysis should focus on these Sinatra patterns:

# Vulnerable patterns to flag
response.set_cookie(params[:name], params[:value])
redirect params[:url]
response[params[:header_name]] = params[:header_value]
headers 'Content-Type' => params[:type]

Dynamic testing with middleBrick specifically targets these vulnerabilities through black-box scanning. The tool automatically tests for CRLF injection by sending payloads containing %0D%0A sequences to endpoints that handle cookies, redirects, and custom headers.

middleBrick's detection methodology for Sinatra applications includes:

• Testing cookie-setting endpoints with CRLF payloads to verify header injection
• Scanning redirect handlers for location header manipulation
• Analyzing custom header endpoints for header injection vulnerabilities
• Checking for improper Content-Type handling that could lead to response splitting

The scanner runs 12 parallel security checks, including authentication bypass attempts and input validation testing, all without requiring credentials or configuration. For Sinatra applications, this means you can simply provide the base URL and receive a comprehensive security assessment within 5-15 seconds.

middleBrick also analyzes OpenAPI specifications if provided, cross-referencing documented endpoints with runtime findings to identify discrepancies between documented and actual behavior.

Remediating CRLF injection in Sinatra requires input validation and proper header handling. The most effective approach combines sanitization with Sinatra's built-in features.

For cookie handling, always validate and sanitize inputs:

get '/set-cookie' do
  cookie_name = params[:name].to_s.gsub(/[
]/, '')
  cookie_value = params[:value].to_s.gsub(/[
]/, '')
  response.set_cookie(cookie_name, cookie_value)
  "Cookie set!"
end

This removes any CR or LF characters before setting the cookie. For more robust validation, use a whitelist approach:

VALID_COOKIE_NAME = /[a-zA-Z0-9_.-]+/
VALID_COOKIE_VALUE = /[ -~]+/

For redirects, validate URLs against a whitelist or use Sinatra's built-in helpers:

get '/redirect' do
  url = params[:url]
  # Validate against allowed domains
  if url.start_with?('https://example.com/') || url.start_with?('/')
    redirect url
  else
    halt 400, 'Invalid redirect URL'
  end
end

For custom headers, sanitize both names and values:

get '/custom-header' do
  header_name = params[:name].to_s.gsub(/[^a-zA-Z0-9-]/, '')
  header_value = params[:value].to_s.gsub(/[
]/, '')
  response[header_name] = header_value
  "Header set!"
end

middleBrick's CLI tool can help verify your remediation:

npm install -g middlebrick
middlebrick scan https://your-sinatra-app.com

The tool will test your endpoints with CRLF payloads and provide a security score with specific findings. For CI/CD integration, you can fail builds if the security score drops below your threshold:

# .github/workflows/security.yml
name: Security Scan
on: [push, pull_request]
jobs:
  scan:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v3
      - name: Run middleBrick scan
        run: |
          npm install -g middlebrick
          middlebrick scan https://staging.your-app.com --threshold B

This ensures CRLF injection vulnerabilities are caught before deployment. The Pro plan includes continuous monitoring that can scan your APIs on a configurable schedule and alert you if new vulnerabilities are detected.