Fix Logic Flow Bypass in Roda

Logic flow bypasses in Roda typically manifest when authentication or authorization guards are improperly scoped within the routing tree. Because Roda executes code as it traverses the tree, placing a security check inside a specific leaf (like a GET block) while omitting it from sibling leaves (like POST or PUT) allows an attacker to manipulate state without authorization.

The Vulnerable Pattern

class App < Roda
  route do |r|
    r.on "account" do
      r.get "settings" do
        check_auth! # Only protects the view
        render("settings")
      end
  r.post "update" do
    # VULNERABILITY: No check_auth! call here.
    # Attacker can bypass UI and POST directly to this endpoint.
    Account.update(r.params["id"], r.params["data"])
    r.redirect "/account/settings"
  end
end
end
end

The Secure Implementation

In the vulnerable example, the security guard `check_auth!` is scoped only to the `r.get` block. Roda's routing tree is a procedural execution path; if the request is a POST, the GET block is skipped entirely, hitting the unprotected update logic. The fix involves hoisting the authorization guard to the top of the `r.on` branch. This ensures every sub-route, regardless of the HTTP verb or specific path suffix, inherits the security context. Additionally, always use session-bound identifiers (like `current_user.id`) rather than user-supplied parameters to prevent IDOR within the logic flow.

class App < Roda
  route do |r|
    r.on "account" do
      check_auth! # Guard the entire branch
  r.get "settings" do
    render("settings")
  end

  r.post "update" do
    Account.update(current_user.id, r.params["data"])
    r.redirect "/account/settings"
  end
end
end
end