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.
Specifically:
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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