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CSP-D203: Deserialization with marshal

Vulnerability Category: Deserialization

Severity: MEDIUM

Description

This rule flags the use of marshal.load() and marshal.loads(). The marshal module is primarily used internally by Python to read and write the compiled .pyc files. The Python documentation explicitly warns that the marshal format is not secure and should not be used with untrusted data.

While not as straightforward to exploit as pickle, a malicious marshal payload can crash the Python interpreter, cause a denial of service, or potentially lead to code execution in some versions of Python.

From the official Python documentation:

Warning: The marshal module is not intended to be secure against erroneous or maliciously constructed data. Never unmarshal data received from an untrusted or unauthenticated source.

Vulnerable Code Example

import marshal

# Assume 'data_from_network' is received from an untrusted source.
# A malformed or malicious payload can crash the program.
try:
    obj = marshal.loads(data_from_network)
except ValueError:
    print("Could not unmarshal the data.")

Safe Code Example

For serializing Python objects, especially for data interchange, use a secure, standardized format like JSON.

import json

my_data = {"key": "value", "number": 42}

# Serialize with JSON
json_string = json.dumps(my_data)

# --- Send over network / save to file ---

# Deserialize with JSON
# This is safe and robust against malformed data.
try:
    received_data = json.loads(json_string)
    print(received_data)
except json.JSONDecodeError:
    print("Invalid JSON received.")

How to Suppress a Finding

The marshal module should almost never be used for data persistence or interchange. Its use is typically restricted to Python's internal workings. If you have a specific, internal use case where you control the data being unmarshalled, you can suppress the finding.

import marshal

# This data is from a trusted, internal source.
# ignore
internal_obj = marshal.loads(trusted_data)

Or, for this specific rule:

# ignore: CSP-D203
internal_obj = marshal.loads(trusted_data)