Avian’s Blog

By Tomaž

21.08.2017 16:52

Piping Curl to Bash is dangerous. Hopefully, this is well known at this point and many projects no longer advertise this installation method. Several popular posts by security researchers demonstrated all kinds of covert ways such instructions could be subverted by an attacker who managed to gain access to the server hosting the install script. Passing HTTP responses uninspected to other sensitive system commands is no safer, however.

This is how Docker documentation currently advertises adding their GPG key to the trusted keyring for Debian's APT:

Screenshot of instructions for adding Docker GPG key.

The APT keyring is used to authenticate packages that are installed by Debian's package manager. If an attacker can put their public key into this keyring they can for example provide compromised package updates to your system. Obviously you want to be extra sure no keys from questionable origins get added.

Docker documentation correctly prompts you to verify that the added key matches the correct fingerprint (in contrast to some other projects). However, the suggested method is flawed since the fingerprint is only checked after an arbitrary number of keys has already been imported. Even more, the command shown specifically checks only the fingerprint of the Docker signing key, not of the key (or keys) that were imported.

Consider the case where download.docker.com starts serving an evil key file that contains both the official Docker signing key and the attacker's own key. In this case, the instructions above will not reveal anything suspicious. The displayed fingerprint matches the one shown in the documentation:

$ curl -fsSL https://www.tablix.org/~avian/docker/evil_gpg | sudo apt-key add -
OK
$ sudo apt-key fingerprint 0EBFCD88
pub 4096R/0EBFCD88 2017-02-22 Key fingerprint = 9DC8 5822 9FC7 DD38 854A E2D8 8D81 803C 0EBF CD88
uid Docker Release (CE deb) <docker@docker.com>
sub 4096R/F273FCD8 2017-02-22

However, importing our key file also silently added another key to the keyring. Considering that apt-key list outputs several screen-fulls of text on a typical system, this is not easy to spot after the fact without specifically looking for it:

$ sudo apt-key list|grep -C1 evil
pub 1024R/6537017F 2017-08-21
uid Dr. Evil <evil@example.com>
sub 1024R/F2F8E843 2017-08-21

The solution is obviously not to pipe the key file directly into apt-key without inspecting it first. Interestingly, this is not straightforward. pgpdump tool is recommended by the first answer that comes up when asking Google about inspecting PGP key files. The version in Debian Jessie fails to find anything suspicious about our evil file:

$ pgpdump -v
pgpdump version 0.28, Copyright (C) 1998-2013 Kazu Yamamoto
$ curl -so evil_gpg https://www.tablix.org/~avian/docker/evil_gpg
$ pgpdump evil_gpg|grep -i evil

Debian developers suggest importing the key into a personal GnuPG keyring, inspecting the integrity and then exporting it into apt-key. That is more of a hassle, and not that useful for Docker's key that doesn't use the web of trust. In our case, inspecting the file with GnuPG directly is enough to show that it in fact contains two keys with different fingerprints:

$ gpg --version|head -n1
gpg (GnuPG) 1.4.18
$ gpg --with-fingerprint evil_gpg
pub 4096R/0EBFCD88 2017-02-22 Docker Release (CE deb) <docker@docker.com> Key fingerprint = 9DC8 5822 9FC7 DD38 854A E2D8 8D81 803C 0EBF CD88
sub 4096R/F273FCD8 2017-02-22
pub 1024R/6537017F 2017-08-21 Dr. Evil <evil@example.com> Key fingerprint = 3097 2749 79E6 C35F A009 E25E 6CD6 1308 6537 017F
sub 1024R/F2F8E843 2017-08-21

The key file used in the example above was created by simply concatenating the two ASCII-armored public key blocks. It looks pretty suspicious in a text editor because it contains two ASCII headers (which is probably why pgpdump stops processing it before the end). However, two public key blocks could easily have also been exported into a single ASCII-armor block.