Home Assistant does not correctly validate SSL for outgoing requests in core and used libs

Summary

Problem: Potential man-in-the-middle attacks due to missing SSL certificate verification in the project codebase and used third-party libraries.

Details

In the past, aiohttp-session/request had the parameter verify_ssl to control SSL certificate verification. This was a boolean value. In aiohttp 3.0, this parameter was deprecated in favor of the ssl parameter. Only when ssl is set to None or provided with a correct configured SSL context the standard SSL certificate verification will happen.

When migrating integrations in Home Assistant and libraries used by Home Assistant, in some cases the verify_ssl parameter value was just moved to the new ssl parameter. This resulted in these integrations and 3rd party libraries using request.ssl = True, which unintentionally turned off SSL certificate verification and opened up a man-in-the-middle attack vector.

Example:
https://github.com/home-assistant/core/blob/c4411914c2e906105b765c00af5740bd0880e946/homeassistant/components/discord/notify.py#L84

When you scan the libraries used by the integrations in Home Assistant, you will find more issues like this.

The general handling in Home Assistant looks good, as homeassistant.helpers.aoihttp_client._async_get_connector handles it correctly.

PoC

1. Check that expired.badssl.com:443 gives an SSL error in when connecting with curl or browser.
2. Add the integration adguard with the setting host=expired.badssl.com, port=443, use-ssl=true, verify-ssl=true.
3. Check the logs - you get a HTTP 403 response.

Expected behavior:

1. The integration log shows an ssl.SSLCertVerificationError.

The following code shows the problem with ssl=True. No exception is raised when ssl=True (Python 3.11.6).

import asyncio
from ssl import SSLCertVerificationError

import aiohttp

BAD_URL = "https://expired.badssl.com/"


async def run_request(verify_ssl, result_placeholder: str):
    async with aiohttp.ClientSession() as session:
        exception_fired: bool = False
        try:
            await session.request("OPTIONS", BAD_URL, ssl=verify_ssl)
        except SSLCertVerificationError:
            exception_fired = True
        except Exception as error:
            print(error)
        else:
            exception_fired = False
        print(result_placeholder.format(exception_result=exception_fired))


# Case 1: ssl=False --> expected result: No exception
asyncio.run(run_request(False, "Test case 1: expected result: False - result: {exception_result}"))

# Case 2: ssl=None --> expected result: Exception
asyncio.run(run_request(None, "Test case 2: expected result: True - result: {exception_result}"))

# Case 3: ssl=True --> expected result: No Exception
asyncio.run(run_request(True, "Test case 3: expected result: False - result: {exception_result}"))