Tor Project Signing Key Poisoning and Ubuntu's torbrowser-launcher package

Posted: 2019-07-09 12:45:23 by Alasdair Keyes

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I started up the Tor browser yesterday and noticed that it didn't start in it's usual time frame, 10 minutes later the browser had still not opened.

Checking top, I saw that a GPG process was using 100% CPU.

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND                                                                                                                                                                                             
19330 username  20   0   78364  47020   4448 R  99.7  0.6   0:16.43 gpg                                                                                                                                                                                                 
 3145 username  20   0 3458164 139712  63512 R  12.6  1.7  18:43.51 cinnamon                                                                                                                                                                                            

I'd read recently about an attack on GPG where keys were being poisoned with a large number of signatures to exploit a GPG bug and corrupt GPG installs https://threatpost.com/pgp-ecosystem-targeted-in-poisoning-attacks/146240/, I wondered if this is what was occuring.

I checked what the GGP process was running.

$ ps aux | grep 19330
username 19330 64.6  0.6  82192 50980 ?        RL   10:51   0:31 /usr/bin/gpg --status-fd 2 --homedir /home/username/.local/share/torbrowser/gnupg_homedir --keyserver hkps://hkps.pool.sks-keyservers.net --keyserver-options ca-cert-file /usr/share/torbrowser-launcher/sks-keyservers.netCA.pem include-revoked no-honor-keyserver-url no-honor-pka-record --refresh-keys

It seemed to be running --refresh-keys which requests updates to keys from the key servers. I ran the following to see what keys were being refreshed.

$ /usr/bin/gpg --homedir /home/username/.local/share/torbrowser/gnupg_homedir --list-keys
/home/username/.local/share/torbrowser/gnupg_homedir/pubring.kbx
----------------------------------------------------------------
pub   rsa4096 2014-12-15 [C] [expires: 2020-08-24]
      EF6E286DDA85EA2A4BA7DE684E2C6E8793298290
uid           [ unknown] Tor Browser Developers (signing key) <torbrowser@torproject.org>
sub   rsa4096 2018-05-26 [S] [expires: 2020-09-12]

I checked the key servers entry for the key EF6E286DDA85EA2A4BA7DE684E2C6E8793298290 at http://pgp.mit.edu/pks/lookup?op=vindex&search=0x4E2C6E8793298290 and saw the key had received a large number of signatures on 2019-06-30, it does indeed look like it has been poisoned with excessive signatures.

I downloaded the latest Tor Browser for Linux directly from https://www.torproject.org/ and didn't receive this issue during startup which is good news.

However, my tor install is through the torbrowser-launcher provided by the Linux Mint repos (originally provided by Ubuntu).

Because the torbrowser-launcher doesn't contain the TOR Browser itself (as the name suggests, it's just a launcher), it is a python environment that will download the latest Tor Browser directly from Tor project. To do this, it uses the Tor Project's public GPG Key to verify the downloaded files are legitimate, during this process it does a refresh from the key servers and hits the poisoning issue.

It seems if you are affected by this, you're best off downloading tor direct from the Tor Project itself. Unfortunately, verification that the file you download from the website requires gpg, you can certainly try and ensure that the key that created the signature is correct...

$ gpg --verify tor-browser-linux64-8.5.3_en-US.tar.xz.asc Downloads/tor-browser-linux64-8.5.3_en-US.tar.xz
gpg: Signature made Fri 21 Jun 2019 02:30:51 PM CEST
gpg:                using RSA key EB774491D9FF06E2
gpg: Can't check signature: No public key

That key EB774491D9FF06E2 matches the key listed at https://2019.www.torproject.org/docs/verifying-signatures.html.en and is a subkey for the Tor Project Signing key, but without the key in your keyring, this check isn't as secure as it should be.


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