Security hardening: --python_out emits unvalidated filename-derived import statements (CWE-94)

Summary

The protoc Python code generator (--python_out) derives module names and
import aliases directly from .proto filenames and writes them verbatim into
generated *_pb2.py files as Python import statements, without validating
that the result is a legal Python identifier.

Affected File

src/google/protobuf/compiler/python/helpers.cc (ModuleName())
src/google/protobuf/compiler/python/generator.cc (PrintImports())

Current Behavior

ModuleName() performs only -→_ and /→. substitution on the filename
before appending _pb2. Characters with syntactic meaning in Python
(;, #, quotes, parentheses) are not rejected or escaped.

PrintImports() interpolates the unvalidated module name directly into a
Python import statement written to generated source.

Impact

A .proto filename containing ; or # characters causes the generated
_pb2.py to contain injected Python statements that execute on import.

Example: a file named os;payload#.proto causes generated code like:

import os;payload#_pb2 as os;payload#__pb2

Related Issues

This is the same bug class as:

• #26569 — Ruby/PHP/C# codegen option injection
• #26833 / #26940 — Java java_package/class name injection

This report covers the --python_out sink, which is distinct and not
covered by the above fixes.

Suggested Fix

After substitution in ModuleName(), validate the result matches
[A-Za-z0-9_.]+ and reject filenames that produce invalid identifiers.
Apply the same guard to the .pyi stub generation path in
pyi_generator.cc.

Note: This was reported privately to Google VRP and they suggested opening
a public GitHub issue for tracking.

Start with ModuleName() in src/google/protobuf/compiler/python/helpers.cc and follow how PrintImports() in generator.cc writes the derived name into generated Python. Review the .pyi stub generation path in pyi_generator.cc as well. Done means invalid filename-derived names are rejected or safely handled in both generated-source paths so they cannot inject Python statements.