SSH provides remote, encrypted terminal access to hosts. Some SSH servers, when running on Microsoft Windows, set insecure permissions on the file storing the private SSH server hostkey. This could allow an authenticated user to obtain the SSH hostkey and use it to impersonate the server.
Some SSH servers create the hostkey with permissions that allow any user to read the file. As a result, any user logged into the system can read the private SSH hostkey.
The hostkey is used to authenticate the server to the client. This defends against redirection attacks, such as DNS hijacking that cause the client to connect to a malicious server. In such cases, clients that know the public hostkey can verify that the server has the private hostkey, thereby verifying the server is correct.
If an attacker copies the private hostkey of a server, they can configure a server with the same private key as the legitimate server. Such a server would appear valid to clients if another attack, such as DNS hijacking, was used to trick the client into connecting to the attacker's server.
SSH Communications Security
Thanks to SSH Communications Security for reporting this vulnerability.
This document was written by Hal Burch.
|Date First Published:||2005-07-18|
|Date Last Updated:||2005-09-09 18:48 UTC|