---
tags:
  - security
  - vulnerability
  - poc
  - protobuf
  - google
  - uncontrolled-recursion
  - cwe-674
  - dos
license: mit
---

# 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

---

## 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 ✓ |

---

## Root Cause

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

```python
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`.

---

## Asymmetry

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

---

## Attack scenario

```python
# 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.

---

## Reproduce

```bash
pip install protobuf
python poc_protobuf_recursive_json.py
```

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

---

## Suggested Fix

```python
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)
```

---

## Environment

| Package  | Version |
|----------|---------|
| protobuf | 5.29.6  |
| Python   | 3.12    |