1 The following instructions relate to CVE-2008-0166. They were prepared by
2 Matt Zimmerman, assisted by Colin Watson.
6 A weakness has been discovered in the random number generator used by OpenSSL
7 on Debian and Ubuntu systems. As a result of this weakness, certain encryption
8 keys are much more common than they should be, such that an attacker could
9 guess the key through a brute-force attack given minimal knowledge of the
10 system. This particularly affects the use of encryption keys in OpenSSH,
11 OpenVPN and SSL certificates.
13 This vulnerability only affects operating systems which (like Ubuntu) are based
14 on Debian. However, other systems can be indirectly affected if weak keys are
17 We consider this an extremely serious vulnerability, and urge all users to act
18 immediately to secure their systems.
22 Systems which are running any of the following releases:
25 * Ubuntu 7.04 (Feisty)
27 * Ubuntu 8.04 LTS (Hardy)
28 * Ubuntu "Intrepid Ibex" (development): libssl <= 0.9.8g-8
30 and have openssh-server installed or have been used to create an OpenSSH key or
31 X.509 (SSL) certificate.
33 All OpenSSH and X.509 keys generated on such systems must be considered
34 untrustworthy, regardless of the system on which they are used, even after the
35 update has been applied.
37 This includes the automatically generated host keys used by OpenSSH, which are
38 the basis for its server spoofing and man-in-the-middle protection.
40 The specific package versions affected are:
42 * Debian 4.0: libssl <= 0.9.8c-4etch3
43 * Ubuntu 7.04: libssl <= 0.9.8c-4ubuntu0.2
44 * Ubuntu 7.10: libssl <= 0.9.8e-5ubuntu3.1
45 * Ubuntu 8.04: libssl <= 0.9.8g-4ubuntu3
47 == What to do if you are affected ==
51 1. Install the security updates
53 Once the update is applied, weak user keys will be automatically rejected
54 where possible (though they cannot be detected in all cases). If you are
55 using such keys for user authentication, they will immediately stop working
56 and will need to be replaced (see step 3).
58 OpenSSH host keys can be automatically regenerated when the OpenSSH security
59 update is applied. The update will prompt for confirmation before taking
62 2. Update OpenSSH known_hosts files
64 The regeneration of host keys will cause a warning to be displayed when
65 connecting to the system using SSH until the host key is updated in the
66 known_hosts file. The warning will look like this:
68 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
69 @ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @
70 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
71 IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
72 Someone could be eavesdropping on you right now (man-in-the-middle attack)!
73 It is also possible that the RSA host key has just been changed.
75 In this case, the host key has simply been changed, and you should update
76 the relevant known_hosts file as indicated in the error message.
78 3. Check all OpenSSH user keys
80 The safest course of action is to regenerate all OpenSSH user keys,
81 except where it can be established to a high degree of certainty that the
82 key was generated on an unaffected system.
84 Check whether your key is affected by running the ssh-vulnkey tool, included
85 in the security update. By default, ssh-vulnkey will check the standard
86 location for user keys (~/.ssh/id_rsa, ~/.ssh/id_dsa and ~/.ssh/identity),
87 your authorized_keys file (~/.ssh/authorized_keys and
88 ~/.ssh/authorized_keys2), and the system's host keys
89 (/etc/ssh/ssh_host_dsa_key and /etc/ssh/ssh_host_rsa_key).
91 To check all your own keys, assuming they are in the standard
92 locations (~/.ssh/id_rsa, ~/.ssh/id_dsa, or ~/.ssh/identity):
96 To check all keys on your system:
100 To check a key in a non-standard location:
102 ssh-vulnkey /path/to/key
104 If ssh-vulnkey says "Unknown (no blacklist information)", then it has no
105 information about whether that key is affected. If in doubt, destroy the
106 key and generate a new one.
108 4. Regenerate any affected user keys
110 OpenSSH keys used for user authentication must be manually regenerated,
111 including those which may have since been transferred to a different system
112 after being generated.
114 New keys can be generated using ssh-keygen, e.g.:
117 Generating public/private rsa key pair.
118 Enter file in which to save the key (/home/user/.ssh/id_rsa):
119 Enter passphrase (empty for no passphrase):
120 Enter same passphrase again:
121 Your identification has been saved in /home/user/.ssh/id_rsa.
122 Your public key has been saved in /home/user/.ssh/id_rsa.pub.
123 The key fingerprint is:
124 00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00 user@host
126 5. Update authorized_keys files (if necessary)
128 Once the user keys have been regenerated, the relevant public keys must
129 be propagated to any authorized_keys files on remote systems. Be sure to
130 delete the affected key.
134 1. Install the security update
136 2. Create new certificates to replace any server or client certificates in use
139 3. If certificates have been generated for use on other systems, they must be
140 found and replaced as well.
142 == Removing openssh-blacklist ==
144 For the moment, the openssh-server package depends on openssh-blacklist, in
145 order that the blacklist is deployed to the maximum possible number of
146 systems to reduce the potential spread of worms exploiting this
147 vulnerability. We acknowledge that this may be inconvenient for some small
148 systems, but nevertheless feel that this was the best course of action.
150 If you absolutely need to remove the blacklist from your system, then you
151 can run the following commands to substitute a fake package for
154 sudo apt-get install equivs
155 equivs-control openssh-blacklist.ctl
156 sed -i 's/^Package:.*/Package: openssh-blacklist/' openssh-blacklist.ctl
157 sed -i 's/^# Version:.*/Version: 9:1.0/' openssh-blacklist.ctl
158 equivs-build openssh-blacklist.ctl
159 sudo dpkg -i openssh-blacklist_1.0_all.deb
161 Be warned: this circumvents a security measure for the sake of disk space.
162 You should only do this if you have no other option, and if you are certain
163 that no compromised keys will ever be generated on or copied onto this
166 Once a sufficient amount of time and number of releases have passed, the
167 openssh-blacklist package will be phased out.