Original posting date: December 29, 2010
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The following is a RightScale Services Security Announcement. All v4 and v5 Ubuntu RightImages, and some customer built Ubuntu-based images, share the same SSH host key fingerprint.
Note: SSH host keys serve as a fingerprint for server identification. These are not related to the root user keys that the RightScale Dashboard generates to provide login authentication to servers. No SSH root user keys are impacted by this advisory and they should not be regenerated.
Ubuntu generates host keys when the openssh-server package is installed. This package is installed during the RightImage creation process and not at boot time. Therefore, all Ubuntu RightImages share the same SSH host key, making Ubuntu-based RightImages vulnerable to an active middleperson attack.
Note: CentOS based images are not impacted by this issue as SSH for CentOS generates keys at boot time.
This vulnerability would allow an attacker to hijack an SSH session by interfering with the SSH key exchange protocol, provided:
(a) they knew the exact RightImage from which an instance was booted
(b) they could actively hijack the TCP connection between an authorized SSH client and the instance.
Both of these criteria must be satisfied in order for the attack to succeed; a passive middleperson cannot mount an attack, nor can knowledge of a host's private key alone be used to gain unauthorized access to an instance.
The severity of this vulnerability is rated Medium, but because knowledge of server launch parameters and an active middleperson attack are both required to leverage this vulnerability.
RightScale rates the likelihood of this attack as Low in most environments, except in environments where the likelihood of an insider attack is increased.
There are three ways to remediate the problem. In order of preference, they are:
The three remediation methods are detailed below.
Important! Should you need to implement remediation steps, you only need to perform one of the three below.
We have published a RightScript that will regenerate SSH keys on a running server.
To patch running instances using the RightScript:
Patching a running instance has significant drawbacks and should be used with caution. If your deployment uses SSH as a mechanism for inter-server communication and enables strict host checking, you may find that some operational scripts or automation tasks no longer function correctly. This patch does not affect client SSH keys (you will still be able to connect with your current client keys), but if you SSH into an instance for which you have regenerated the host key, you may receive errors from your SSH client like the following:
If this happens, you can remove the offending line from the known_hosts file to get rid of the error.
Relaunching your servers will automatically apply a hot patch which fixes the SSH host key issue. A hotpatch is available for the following Ubuntu RightImage/RightLink versions:
Relaunching is appropriate in the following circumstances
To relaunch your servers, simply click "Relaunch" on the affected server.
Note: v5.6 RightImages are scheduled to be published the week of Jan 3, 2011.
Updating your ServerTemplates and Servers to RightImage v5.6 is the recommended course of action because the image has numerous stability fixes in addition to fixing the SSH host key issue. Update to v5.6 is appropriate in the following circumstances:
To update ServerTemplates, perform the following:
If you have any additional questions regarding this known issue, please contact firstname.lastname@example.org or click on 'Support' inside the Dashboard to file a ticket.
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