gpg key shadowing

[Jamie Strandboge]

Georgi Guninski reported on http://seclists.org/fulldisclosure/2012/Jun/267 (slightly modified for language):
"While wasting my time with apt-key noticed strange behaviour with
colliding subkeys.

Out of paranoia ubuntu disallows importing certain trusted keyids.
This is trivial to circumvent by making a collision with subkey.

Attached is a key with subkey keyid colliding with
Ubuntu Archive Master Signing Key <ftpmaster () ubuntu com>.

By emulating apt-key netupdate, noticed that the order of the keyrings
is important. If the master keyring is first, the colliding key with
correct signature fails validation (probably because the other key is used).
If the colliding keyring is first, everything is ok (modulo reporting wrong
signer).

Probably this may lead to gpg abuse.

colliding first:

$gpg --ignore-time-conflict --no-options --no-default-keyring --secret-keyring /etc/apt/secring.gpg --keyring /tmp/sec1
--keyring /usr/share/keyrings/ubuntu-master-keyring.gpg --check-sigs
/tmp/sec1
---------
pub 1024R/76A4410F 2012-06-13
uid f...31 (f) <f () f>
sig!3 76A4410F 2012-06-13 f...31 (f) <f () f>
sig! 3F272F5B 2012-06-13 f...31 (f) <f () f>
sig! 3F272F5B 2012-06-13 f...31 (f) <f () f>
sub 1024R/2376C859 2012-06-13
sig! 76A4410F 2012-06-13 f...31 (f) <f () f>
sub 2180R/3F272F5B 2012-06-13
sig! 76A4410F 2012-06-13 f...31 (f) <f () f>

/usr/share/keyrings/ubuntu-master-keyring.gpg
---------------------------------------------
pub 4096R/3F272F5B 2007-11-09
uid Ubuntu Archive Master Signing Key <ftpmaster () ubuntu com>
sig!3 3F272F5B 2007-11-09 f...31 (f) <f () f> #wrong

1 signature not checked due to a missing key

master first:

$gpg --ignore-time-conflict --no-options --no-default-keyring --secret-keyring /etc/apt/secring.gpg --keyring
/usr/share/keyrings/ubuntu-master-keyring.gpg --keyring /tmp/sec1 --check-sigs
/usr/share/keyrings/ubuntu-master-keyring.gpg
---------------------------------------------
pub 4096R/3F272F5B 2007-11-09
uid Ubuntu Archive Master Signing Key <ftpmaster () ubuntu com>
sig!3 3F272F5B 2007-11-09 Ubuntu Archive Master Signing Key <ftpmaster () ubuntu com>

/tmp/sec1
---------
pub 1024R/76A4410F 2012-06-13
uid f...31 (f) <f () f>
sig!3 76A4410F 2012-06-13 [User ID not found]
sig- 3F272F5B 2012-06-13 Ubuntu Archive Master Signing Key <ftpmaster () ubuntu com> # wrong, signer is a
subkey of f () f
sig- 3F272F5B 2012-06-13 Ubuntu Archive Master Signing Key <ftpmaster () ubuntu com> # wrong, signer is a
subkey of f () f
sub 1024R/2376C859 2012-06-13
sig! 76A4410F 2012-06-13 [User ID not found]
sub 2180R/3F272F5B 2012-06-13
sig! 76A4410F 2012-06-13 [User ID not found]

2 bad signatures
1 signature not checked due to a missing key"

[Jamie Strandboge (jdstrand)]

Ok, Michael, Marc and I looked at this a bit. If copy sec1.bin to /var/www/ubuntu-archive-keyring.gpg then update ARCHIVE_KEYRING_URI to have ARCHIVE_KEYRING_URI=http://localhost/ubuntu-archive-keyring.gpg, the key does not get updated on lucid/amd64. If I change the order as the report suggests, it is imported. Ie, I use this:
if $GPG_CMD --keyring $TMP_KEYRING --keyring $MASTER --check-sigs --with-colons $add_key | grep '^sig:!:' | cut -d: -f5 | grep -q $master_key; then

instead of:
if $GPG_CMD --keyring $MASTER --keyring $TMP_KEYRING --check-sigs --with-colons $add_key | grep '^sig:!:' | cut -d: -f5 | grep -q $master_key; then

At this point it needs to be verified that gpg will never internally reverse the order on all architectures. If it ever does, then this is a problem. If not, this attack would require an additional step (to update apt-key's code to reverse the order).

[Jamie Strandboge (jdstrand)]

I've uploaded these to the security ppa. Michael, do you want to coordinate this with Debian or shall I?

[Launchpad Janitor (janitor)]

This bug was fixed in the package apt - 0.9.6ubuntu2

---------------
apt (0.9.6ubuntu2) quantal; urgency=low

  * adjust apt-key to ensure no collisions on subkeys too. Patch thanks to
    Marc Deslauriers. (LP: #1013128)
 -- Jamie Strandboge <email address hidden> Thu, 14 Jun 2012 11:29:48 -0500

[Launchpad Janitor (janitor)]

This bug was fixed in the package apt - 0.8.16~exp12ubuntu10.1

---------------
apt (0.8.16~exp12ubuntu10.1) precise-security; urgency=low

  * adjust apt-key to ensure no collisions on subkeys too. Patch thanks to
    Marc Deslauriers. (LP: #1013128)
 -- Jamie Strandboge <email address hidden> Thu, 14 Jun 2012 11:30:50 -0500

[Launchpad Janitor (janitor)]

This bug was fixed in the package apt - 0.8.16~exp5ubuntu13.4

---------------
apt (0.8.16~exp5ubuntu13.4) oneiric-security; urgency=low

  * adjust apt-key to ensure no collisions on subkeys too. Patch thanks to
    Marc Deslauriers. (LP: #1013128)
 -- Jamie Strandboge <email address hidden> Thu, 14 Jun 2012 11:31:28 -0500

[Launchpad Janitor (janitor)]

This bug was fixed in the package apt - 0.8.13.2ubuntu4.5

---------------
apt (0.8.13.2ubuntu4.5) natty-security; urgency=low

  * adjust apt-key to ensure no collisions on subkeys too. Patch thanks to
    Marc Deslauriers. (LP: #1013128)
 -- Jamie Strandboge <email address hidden> Thu, 14 Jun 2012 11:32:34 -0500

[Launchpad Janitor (janitor)]

This bug was fixed in the package apt - 0.7.25.3ubuntu9.12

---------------
apt (0.7.25.3ubuntu9.12) lucid-security; urgency=low

  * adjust apt-key to ensure no collisions on subkeys too. Patch thanks to
    Marc Deslauriers. (LP: #1013128)
 -- Jamie Strandboge <email address hidden> Thu, 14 Jun 2012 10:57:16 -0500

[Launchpad Janitor (janitor)]

This bug was fixed in the package apt - 0.7.9ubuntu17.5

---------------
apt (0.7.9ubuntu17.5) hardy-security; urgency=low

  * adjust apt-key to ensure no collisions on subkeys too. Patch thanks to
    Marc Deslauriers. (LP: #1013128)
 -- Jamie Strandboge <email address hidden> Thu, 14 Jun 2012 11:00:24 -0500

[georgi (guninski)]

Created better keyring that works with master first.

The keyring is at:

http://lists.grok.org.uk/pipermail/full-disclosure/attachments/20120614/a24a7974/attachment.obj
(name sec3)

master first:

gpg --ignore-time-conflict --no-options --no-default-keyring --secret-keyring /etc/apt/secring.gpg --keyring /usr/share/keyrings/ubuntu-master-keyring.gpg --keyring /tmp/sec3 --check-sigs

/usr/share/keyrings/ubuntu-master-keyring.gpg
---------------------------------------------
pub 4096R/3F272F5B 2007-11-09
uid Ubuntu Archive Master Signing Key <email address hidden>
sig!3 3F272F5B 2007-11-09 Ubuntu Archive Master Signing Key <email address hidden>

/tmp/sec3
---------
pub 1024R/B1C08810 2012-06-14
uid kkkkkkk5 <k@k>
sig!3 B1C08810 2012-06-14 [User ID not found]
sig! 3F272F5B 2012-06-14 Ubuntu Archive Master Signing Key <email address hidden>
sig! 3F272F5B 2012-06-14 Ubuntu Archive Master Signing Key <email address hidden>
sub 1024R/0354AE88 2012-06-14
sig! B1C08810 2012-06-14 [User ID not found]
sub 2179R/3F272F5B 2012-06-14
sig! B1C08810 2012-06-14 [User ID not found]

Can't import it yet...

[Jamie Strandboge (jdstrand)]

This is too brittle and we are going to disable net-update again (bug #1013639) in the short term and try to fix this properly. Bug #1013681 will track the actual fix.