Third review found the meta_path guard leaked exactly where it delegated to the
stdlib import system:
1. Non-.py submodules (CRITICAL). When a name had no dir and no .py, find_spec
returned None and the stdlib loaded a planted .so (ExtensionFileLoader) or a
sourceless .pyc unverified - an attacker deletes a signed .py and drops a
same-named .so with arbitrary init code, run on a normal request via core's
`from plugins.<name>.models import ...`. The guard now refuses any name for
which a non-source importable candidate (EXTENSION_SUFFIXES + BYTECODE_
SUFFIXES) exists on disk; None is reserved for genuinely-absent modules.
2. Top-level plugins/__init__.py (CRITICAL). It is in no plugin's provenance,
is attacker-writable, and Python runs it before any guarded submodule. The
guard now owns `plugins`: it execs an EMPTY package body (search points at
the plugins dir), so an overwritten plugins/__init__.py never runs.
Also: specs are built with spec_from_file_location so loaded modules get
__file__/__path__ (Flask blueprint root paths need it) while the loader still
execs the verified in-memory bytes - never re-reading the file.
Verified end to end: under PLUGIN_REQUIRE_SIGNED with all 13 bundled plugins
stamped, the app boots and loads every plugin through the guard; a tampered
plugin file is refused at load. 4 new guard tests (planted .so, sourceless
.pyc, absent-module defer, neutralized package root). Prior fixes #3/#4
confirmed still sound by the review. 1065 pass, naming green.
c59d2dab56