Injection Flaws in Grape with Basic Auth

Grape is a Ruby DSL for building REST-like APIs. When Basic Authentication is used, credentials are transmitted in an Authorization header as a base64-encoded string. While base64 is not encryption, the combination of predictable parameter handling and insufficient input validation in Grape endpoints can expose injection surfaces.

Consider an endpoint that accepts user-controlled input and passes it to system-level operations, such as command execution or database queries. If user input is concatenated into command strings or query fragments without sanitization, an attacker can inject shell metacharacters or query sublanguage syntax. In a Grape API protected only by Basic Auth, the presence of authentication may create a false sense of security, leading developers to skip strict validation. Attackers who obtain or guess valid credentials (or exploit exposed endpoints) can then manipulate parameter parsing to achieve command injection or query injection.

Two real-world patterns increase risk:

• Dynamic command construction using user input in system or backtick calls.
• Building database queries by interpolating parameters without using prepared statements or sanitization.

For example, a Grape resource that builds a shell command from a path parameter allows injection if the input is not strictly constrained:

class V1::Reports < Grape::API
  format :json
  helpers do
    def authenticate!
      # Basic Auth parsing logic assumed present
    end
  end

  before { authenticate! }

  get :export do
    path = params[:path]
    # Unsafe: user input directly concatenated into shell command
    result = `tar -czf /tmp/report #{path}`
    { output: result }
  end
end

An authenticated attacker could provide a path such as /tmp/report; cat /etc/passwd and cause unintended command execution. Similarly, database injection can occur when using raw Sequel or ActiveRecord calls with interpolated values:

resource :search do
  get do
    term = params[:q]
    # Unsafe: string interpolation in SQL
    results = DB[%{SELECT * FROM items WHERE name LIKE '%#{term}%'}]
    { results: results }
  end
end

Even with Basic Auth present, these injection flaws allow attackers to bypass authentication assumptions and manipulate backend behavior. The security checks in middleBrick evaluate these patterns by correlating authentication usage with input validation and output handling, highlighting cases where authenticated endpoints still accept unsafe input.

Remediation focuses on strict input validation, avoiding string interpolation for commands and queries, and using language-level protections. Do not rely on authentication alone to prevent injection.

1. Use parameterized shell commands via shellwords and avoid backticks for user input.

require 'shellwords'

class V1::Reports < Grape::API
  format :json
  helpers do
    def authenticate!
      # Basic Auth verification
    end
  end

  before { authenticate! }

  get :export do
    path = params[:path]
    # Safe: validate and sanitize path components
    sanitized = Shellwords.escape(path)
    result = `tar -czf /tmp/report #{sanitized}`
    { output: result }
  end
end

Even better, avoid shell execution entirely by using Ruby libraries for archive creation, which removes shell injection risk.

2. Use parameterized queries instead of interpolation.

resource :search do
  get do
    term = params[:q]
    # Safe: parameterized query
    results = DB[:items].where(Sequel.like(:name, "%#{term}%"))
    { results: results }
  end
end

If using raw SQL, prefer bind variables or placeholders supported by your adapter instead of manual string interpolation.

3. Apply strict allowlists for input formats.

VALID_PATH_SEGMENT = /^[
\w\-\.]+$/

helpers do
  def validate_path!(input)
    fail Grape::Exceptions::Validation, 'Invalid path' unless input.match?(VALID_PATH_SEGMENT)
  end
end

get :export do
  validate_path!(params[:path])
  # proceed safely
end

4. Use Grape validators to constrain parameters.

params do
  requires :path, type: String, values: { in: ["summary", "details", "overview"] }
  requires :q, type: String, length: 1..100
end

These validators reject unexpected or malformed input before it reaches business logic.

By combining authentication with rigorous input validation, parameterized queries, and avoidance of dynamic command construction, you reduce the attack surface even when Basic Auth is in use. middleBrick scans detect missing validations and dangerous patterns, providing remediation guidance aligned with secure coding practices.