MBM7/protobuf-parsedicts-recursion-poc

protobuf — json_format.ParseDict() Uncontrolled Recursion (PoC)

Repo: MBM7/protobuf-parsedicts-recursion-poc
Status: Responsible disclosure — submitted to Huntr
Severity: High / CWE-674
Package: protobuf (PyPI) — google.protobuf

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Summary

A 7.4 KB crafted JSON payload with 200 nesting levels crashes any Python server calling ParseDict() with a RecursionError — unhandled, terminates the process.

Payload size	Depth	Result
7,443 bytes	200	RecursionError ✓
9,293 bytes	250	RecursionError ✓

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Root Cause

google/protobuf/json_format.py, _ConvertMessage():

def _ConvertMessage(value, message, path):
    for field_name, field_value in value.items():
        ...
        _ConvertMessage(field_value, sub_message, path)  # ← no depth check

ParseDict() → _ConvertMessage() recurses once per nesting level with no depth limit. Compare to the binary parser which has DEFAULT_RECURSION_LIMIT = 100.

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Asymmetry

Parser	Depth limit
ParseFromString() (binary)	DEFAULT_RECURSION_LIMIT = 100 ✅
ParseDict() (JSON→proto)	NONE ❌
MessageToJson()	NONE ❌
MessageToDict()	NONE ❌

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Attack scenario

# Server receives JSON from user, converts to proto:
user_json = json.loads(request.body)
proto_msg = MyMessage()
ParseDict(user_json, proto_msg)   # ← CRASH at depth 200

A 7.4 KB JSON payload with 200 nesting levels crashes the Python process.

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Reproduce

pip install protobuf
python poc_protobuf_recursive_json.py

Expected:

Nesting depth   : 200 levels
JSON payload    : 7,443 bytes
Result          : RecursionError — server CRASH ✓

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Suggested Fix

def _ConvertMessage(value, message, path, depth=0):
    if depth > _MAX_RECURSION_DEPTH:  # e.g. 100, matching binary parser
        raise ParseError('Message too deeply nested')
    for field_name, field_value in value.items():
        ...
        _ConvertMessage(field_value, sub_message, path, depth + 1)

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Environment

Package	Version
protobuf	5.29.6
Python	3.12