test: mechanical security floor for the server library

test/security_test.exs

@@ -0,0 +1,45 @@
+defmodule Terminalwire.SecurityTest do
+  use ExUnit.Case, async: true
+
+  # A mechanical security floor. This server runs untrusted input: a hostile client
+  # can send arbitrary frames, and the CLI it serves runs argv the client controls.
+  # None of the primitives below — code execution, shell-out, unsafe deserialization,
+  # or untrusted-string→atom conversion (atom-table exhaustion) — may be even one grep
+  # away in the library. If you ever need one deliberately, you have to delete it from
+  # this list on purpose, which is the point: it can't creep in silently.
+  @forbidden [
+    # arbitrary code execution
+    "Code.eval",
+    "eval_string",
+    "eval_quoted",
+    "EEx.",
+    # shell / OS command execution
+    "System.cmd",
+    ":os.cmd",
+    "Port.open",
+    "open_port",
+    # unsafe deserialization (RCE + atom creation from bytes)
+    "binary_to_term",
+    # atom-table exhaustion from untrusted strings (note: the *_to_existing_atom
+    # variants are bounded and intentionally NOT forbidden)
+    "String.to_atom",
+    "binary_to_atom",
+    "list_to_atom"
+  ]
+
+  test "the library never reaches for code-exec, shell-out, unsafe deserialization, or atom creation" do
+    files = Path.wildcard("lib/**/*.ex")
+    assert files != [], "expected to find library sources under lib/"
+
+    offenders =
+      for path <- files,
+          source = File.read!(path),
+          token <- @forbidden,
+          String.contains?(source, token),
+          do: "  #{path}: #{token}"
+
+    assert offenders == [],
+           "Forbidden security-sensitive primitives found in lib/:\n" <>
+             Enum.join(offenders, "\n")
+  end
+end