[tbb-dev] A Proposal for Double-Checking self-signed certificates

[Tom Ritter]

Attached is one of three user safety proposals I'm trying to write.

This one in particular deals with self-signed and similar certificate
errors, and basically double checks the cert over a new circuit. If
they match, the user can bypass the certificate error page. If they
don't, they can't bypass it.

-tom
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Filename: xxx-selfsigned-user-safety.txt
Title: Protecting Against Malicious Exit Nodes Performing TLS Interception
Author: Tom Ritter
Created: 06-Mar-2019
Status: Open

1. Motivation

  Sometimes, exit nodes are malicious and perform TLS Interception using self-
  signed or otherwise invalid TLS certificates. Tor Project and volunteers
  scan and report malicious exit relays where-upon they are given the BadExit
  flag.

  In the period of time between the nodes being identified and being
  blocklisted, users are put at risk from these nodes.

2. Proposal

2.1. Classifying TLS Certificate Errors

  First we classify TLS Certificate Errors into two categories. We will use
  these classifications later.

  Class 1: Suspicious Certificate Errors

   - A self-signed Certificate
   - A certificate signed by a Trust Anchor but for a different hostname
   - A certificate that appears to be signed by a Trust Anchor, but is
     missing an intermediate allowing a full path to be built

  Class 2: Unsuspicious Certificate Errors

   - An expired certificate signed by a Trust Anchor
   - A certificate that requires an OCSP staple, but the staple is not
     present

2.2. Browser Logic

  If the browser encounters an invalid TLS Certificate when connecting to a
  hostname, and the type of invalidness is a Suspicious Certificate Error,
  the browser will not _immediately_ allow the user to bypass the error and
  add an exception.

  Instead it will create a new circuit through a new exit node (making sure
  to check the Family of the nodes), begin a TLS handshake, and obtain the
  certificate offered.

  If the certificate is the same as the one offered through the initial
  circuit, the user is allowed to add an exception and continue. If the
  certificate is different, the user not allowed to bypass the error.

2.3. Optional Extension

  If a certificate mismatch occurs, the browser could prompt the user to
  send a report to Tor Project.

  The simple version of this feature could open an email message with
  details prepopulated and addressed to badrelays at .

  The more advanced version could submit the information to an onion
  service operated by Tor Project. On the backend, we could build an
  automatic verification process as well.

  The details would include the hostname visited, time, exit nodes, and
  certificates received over which exit nodes.

3. False Positives

  It is possible, although I suspect uncommon, that a server may have
  geographic or other load balancing that presents different self-signed
  certificates to different exit nodes.

  If we receive reports of such occurances, we could either relax protects
  for such domains we hardcode into the browser, or perform the new-circuit
  verification choosing an exit node in the same country.

4. User Interface/Experience

  While the certificate is being verified over another circuit, it would be
  best to provide feedback to the user.

  a. The button can appear disabled and say something like 
     'Pending (Verifying Certificate)'
  b. A small progress bar can appear under the button that tracks the progress
     of creating and extending the circuit, sending the request and getting
     the reply.
  c. A small 'Retry' underlined, clickable link could sit by the progress
     bar to retry the circuit in case it gets stalled.

  If the certificate comes back and is a mismatch, we could replace the entire
  error page with more information, including a cloudflare-style diagram[0] showing
  the malicious exit node, and prompting the user to submit the information.

  [0] https://external-preview.redd.it/S65-yhtC6IAqpzS6AMhMnrrFwvtyRA6WjuM_hQpJLg0.png?auto=webp&s=285e86af8e638df6ecc143a52af024f006389151

  If the certificate comes back with a match, we could add some text noting that
  some amount of verification has been performed. However it seems bad to
  automatically accept the certificate or relax the warning too much, since it
  is still possible a TLS attack is occuring (just not inside the Tor network.)
  Alternately, we could not change the warning page at all.

5. Concerns

  An exit node who observes an aborted TLS handshake will learn that a user
  encountered a self-signed certificate error for this server on another circuit.
  What would this tell them? It leaks a user's browsing activity. It also leaks
  the prescence of a malicious exit node on the network (assuming the exit node
  observes a valid TLS certificate.)

  Exit nodes who lie about their family have a chance to successfully attack the
  user.