108 lines
4.5 KiB
Markdown
108 lines
4.5 KiB
Markdown
---
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layout: post
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title: "Trouble with OCSP"
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date: 2019-08-06
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comments: true
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tags: ocsp, tor, security, privacy
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---
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# Trouble with OCSP
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> This is a post about side channel information leakage that was present in OnionBrowser in some OCSP flows. This is not a exhaustive documentation of OCSP protocol.
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Digital certificates are issued for a longer timespan. It reduces maintaince overhead. Most of the cases CAs issue a certificate and they are valid untill the expiration time.
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But what are we to do when a certificate is compromised? We can
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re-issue a certificate, but the old one is still in the wind. Thats
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where Online Certificate Status Protocol comes in. It defines a way to
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check validity of a certificate in a timely[^1] manner.
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OCSP[^2] works roughly as follows in an https connection:
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1. Client looks up the OCSP responder server from `AuthorityInfoAccess`section in the certificate.
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2. Client crafts a OCSP request and sends it to OCSP responder.
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3. Responder returns the current status of the certificate, one of `good`, `revoked` or `unknown`
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There are many other interactions defined in the OCSP ecosystem. Maybe
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the most important one is [OCSP
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Stapling](https://en.wikipedia.org/wiki/OCSP_stapling). In stapling
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the original request server sends back OCSP validation message with
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the certificate itself, removing the need for another seperate
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request.
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## Dissecting an OCSP Request
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If the request is <255 bytes, OCSP allows it to be passed as a GET path. So a typical request looks like this
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```bash
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GET http://ocsp.int-x3.letsencrypt.org/MFgwVqADAgEAME8wTTBLMAkGBSsOAwIaBQAEFH7maudymrP8%2BKIgZGwWoS1gcQhdBBSoSmpjBH3duubRObemRWXv86jsoQISA6D%2BPqgUVCy3wtolHIxq%2Bk0e
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```
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The request is `URL-Encoded > BASE64 encoded > ASN-1` data. After decoding, one can use `ocsptool` from [GnuTLS](https://www.gnutls.org/) to read it.
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```bash
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echo -n "MFgwVqADAgEAME8wTTBLMAkGBSsOAwIaBQAEFH7maudymrP8%2BKIgZGwWoS1gcQhdBBSoSmpjBH3duubRObemRWXv86jsoQISA6D%2BPqgUVCy3wtolHIxq%2Bk0e" \
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| python -c "import sys, urllib.parse as ul; print(ul.unquote(sys.stdin.read()));" \
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| base64 -d \
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| ocsptool -i
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OCSP Request Information:
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Version: 1
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Request List:
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Certificate ID:
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Hash Algorithm: SHA1
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Issuer Name Hash: 7ee66ae7729ab3fcf8a220646c16a12d6071085d
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Issuer Key Hash: a84a6a63047dddbae6d139b7a64565eff3a8eca1
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Serial Number: 03a0fe3ea814542cb7c2da251c8c6afa4d1e
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```
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.. and then lookup the cert parameters
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```bash
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curl https://crt.sh/?serial=03a0fe3ea814542cb7c2da251c8c6afa4d1e
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```
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```bash
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...
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Issuer: (CA ID: 16418)
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commonName = Let's Encrypt Authority X3
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organizationName = Let's Encrypt
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countryName = US
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Validity
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Not Before: Mar 29 00:08:51 2019 GMT
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Not After : Jun 27 00:08:51 2019 GMT
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Subject:
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commonName = check.torproject.org
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...
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```
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## Privacy Takes a Backseat.
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Careful examination of above workflow will reveal that the OCSP flow
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is happening over HTTP. Most issuers seems to stick to http; possibly
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to avoid cyclical dependencies. This means man-in-the-middle leakage
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of certificates a user is validating is happening, and by extension
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leakage of websites user is accessing.
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## Onion Browser
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iOS has inbuilt mechanism for checking revocations. They seem to be
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only enforced for
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[EV](https://en.wikipedia.org/wiki/Extended_Validation_Certificate)
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certificates though. This is not a bad thing at all, but it creates a
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weird situation where it leaks urls with EV certs from [Onion
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Browser](https://apps.apple.com/us/app/onion-browser/id519296448). Semi-Official
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(as far as I can tell) Tor browser on iOS.
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Whenever Onion Browser accesses a website with EV cert, (for e.g
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<https://check.torproject.com>), the OCSP request is routed via
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**regular** transport, not via onion network as one would assume.
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I stumbled upon this accidently while inspecting requests from my
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iPhone with [mitmproxy](https://mitmproxy.org/). The bug was reported
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to Onion Browser team and [they have a better write up of the
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situation](https://github.com/OnionBrowser/OnionBrowser/wiki/2019-178:-sites-with-EV-HTTPS-certificates-leak-information-via-OCSP). Unfortunatly
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it is really hard to fix. :-(
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[^1]: Opposed to checking againist a [Certificate Revocation List](https://en.wikipedia.org/wiki/Certificate_revocation_list).
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[^2]: Familier readers will note that this is plain OCSP, the non-stapling kind.
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