On 8/2/19 1:24 PM, reddit....@vfemail.net wrote:
In Qubes, is it possible to set up a VM that can receive email, but not send information out, via email or otherwise?

The motivation is: Many online accounts rely on an email address to reset passwords. However, the VM that handles inbound emails, processes a lot of untrusted input. If the VM gets compromised by an attacker, the attacker can then send password reset emails and read them. So to defend against this, I want to prevent the compromised VM from communicating out the contents of these password reset emails.

1. Assume the VM is compromised (can't rely on in-VM enforcement mechanisms).
2. Assume the email provider is not compromised

To further illustrate the problem, here are example setups and why they don't work:

Setup 1: Use qubes firewall to restrict to the email provider's server and IMAP port. Block UDP requests using qvm-firewall. Why it doesn't work: Attacker can create an account on the same email provider and connect to their account (the firewall rules will not prevent this). They can then sync emails containing any data, to their account.

Setup 2: Like Setup 1, but use POP3.
Why it doesn't work: Attacker creates account at provider, transmits data via POP3 delete operations.

How about setup the firewall to black hole the entire IP range of the email service company, then set up a proxy on the firewall which you then control, and you set their email program to use the proxy. If need be you can black hole all the pop/smtp/imap ports for all Internet traffic forcing them to use the proxy for any email no matter what email program or provider they use. When they try to send any email the proxy simply closes that connection.

Controlling HTTP/s traffic might be more difficult, but if necessary you can proxy all that as well. If its just one service provider you care about then the black hole IP trick should do the job.

You put any custom logic for your specific requirements into the proxy which then controls their access accordingly. Basically its a default deny gateway which needs to match on the permitted rules before they are ever granted access. The downside is you will likely need to write your own proxy for this.

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