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Some small edits.

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Maybe I need to get an editor.
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Dhananjay Balan 2019-08-09 15:13:31 +02:00
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blog/2019-08-06-trouble-with-ocsp.markdown
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@ -6,30 +6,32 @@ comments: true
tags: ocsp, tor, security, privacy
---
> This is a post about side channel information leakage that was present in OnionBrowser in some OCSP flows. This post omits a lot of details about OCSP protocol.
> This is a post about side channel information leak present in OnionBrowser OCSP requests. This post omits a lot of details about OCSP protocol.
Digital certificates are issued for a longer timespan. It reduces maintenance overhead. Most of the cases CAs issue a certificate and they are valid until the expiration time.
Digital certificates usually have long expiration, to reduce
maintenance overhead. Most of the cases CAs issue a certificate for
few years.
But what are we to do when a certificate is compromised? We can
re-issue a certificate, but the old one is still in the wind. That's
where Online Certificate Status Protocol comes in. It defines a way to
check validity of a certificate in a timely[^1] manner.
re-issue a certificate, but compromised certificate is still in the
wind. Online Certificate Status Protocol (OCSP) is designed to help
with this exact situation. It defines a way to check validity of a
certificate in a timely[^1] manner.
OCSP[^2] works roughly as follows in an https connection:
1. Client looks up the OCSP responder server from `AuthorityInfoAccess`section in the certificate.
2. Client crafts a OCSP request and sends it to OCSP responder.
1. Client looks up the OCSP responder server from `AuthorityInfoAccess` section in the certificate.
2. Client crafts a OCSP request and sends it to OCSP responder (server provided by the CA).
3. Responder returns the current status of the certificate, one of `good`, `revoked` or `unknown`
There are many other interactions defined in the OCSP ecosystem. Maybe
the most important one is [OCSP
Stapling](https://en.wikipedia.org/wiki/OCSP_stapling). In stapling
the original request server sends back OCSP validation message with
the certificate itself, removing the need for another seperate
request.
> Note: There are many other interactions defined in the OCSP ecosystem. Maybe
> the most important one is [OCSP Stapling](https://en.wikipedia.org/wiki/OCSP_stapling). In stapling
> the original request server sends back OCSP validation message with
> the certificate itself, removing the need for another seperate
> request.
## Dissecting an OCSP Request
If the request is <255 bytes, OCSP allows it to be passed as a GET path. A typical request looks like this
If the request is less than `255 bytes`, OCSP allows it to be passed as a GET path. A typical request looks like this
```bash
GET http://ocsp.int-x3.letsencrypt.org/MFgwVqADAgEAME8wTTBLMAkGBSsOAwIaBQAEFH7maudymrP8%2BKIgZGwWoS1gcQhdBBSoSmpjBH3duubRObemRWXv86jsoQISA6D%2BPqgUVCy3wtolHIxq%2Bk0e
@ -76,10 +78,14 @@ curl https://crt.sh/?serial=03a0fe3ea814542cb7c2da251c8c6afa4d1e
## Privacy Takes a Backseat.
Careful examination of above workflow will reveal that the OCSP flow
is happening over HTTP. Most issuers seem to stick to http; possibly
to avoid cyclical dependencies. This means man-in-the-middle leakage
of certificates a user is validating is happening, and by extension
leakage of websites user is accessing.
is happening over HTTP. With the above dissecting example, an attacker
will know that it is highly probable that user is accessing
<https://check.torproject.org>.
Most issuers seem to stick to http; possibly to avoid cyclical
dependencies. This means man-in-the-middle leakage of certificates a
user is validating is happening, and by extension leakage of websites
user is accessing.
## Onion Browser
iOS has inbuilt mechanism for checking revocations. They seem to be
@ -94,6 +100,14 @@ Whenever Onion Browser accesses a website with EV cert, (for e.g
<https://check.torproject.com>), the OCSP request is routed via
**regular** transport, not via onion network as one would assume.
An attacker observing network can see any of the OCSP requests, and
thus know a subset of sites user is visiting, even if they are on Tor.
A weird detail is that the [**`.onion` endpoint of NYTimes has
an EV certificate**](https://crt.sh/?id=939411753). The ocsp request
for it is send via plain transport and gets leaked, if you were using
OnionBrowser to access it.
I stumbled upon this accidently while inspecting requests from my
iPhone with [mitmproxy](https://mitmproxy.org/). The bug was reported
to Onion Browser team and [they have a nice write up of the