[tbb-dev] A Proposal for Double-Checking self-signed certificates
tom at ritter.vg
Thu Mar 7 05:23:55 UTC 2019
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.
-------------- next part --------------
Title: Protecting Against Malicious Exit Nodes Performing TLS Interception
Author: Tom Ritter
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
In the period of time between the nodes being identified and being
blocklisted, users are put at risk from these nodes.
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
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
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
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 showing
the malicious exit node, and prompting the user to submit the information.
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.
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
More information about the tbb-dev