RSTR-DES-003 — Python `marshal.loads` on untrusted input

Summary

marshal is a Python-internal serializer used by the compiler for .pyc files. It is not a general-purpose data format and the docs explicitly warn against using it on untrusted input — marshal.loads can construct code objects which exec will then execute.

Severity

Critical.

Languages

Python.

What rastray flags

import marshal
obj = marshal.loads(request.data)                  # ← flagged
obj = marshal.load(open('payload.bin', 'rb'))      # ← flagged

What rastray deliberately does not flag

Reads of .pyc files by the import system (handled internally; the rule fires on user-level marshal.load(s) calls).
json.loads, tomllib.loads, msgpack.unpackb.

How to fix it

Use a data format. JSON for short data, MessagePack or CBOR for binary, Protocol Buffers or Cap'n Proto for schemas. None of those can construct executable Python objects.

import json
obj = json.loads(request.data)

There is no safe-mode marshal. If you find marshal in production code outside the import machinery, treat the discovery as a P0 and replace.

rastray rules