SSH, as implemented in OpenSSH before 4.0 and possibly other implementations, stores hostnames, IP addresses, and keys in plaintext in the known_hosts file, which makes it easier for an attacker that has compromised an SSH user's account to generate a list of additional targets that are more likely to have the same password or key.
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A10201
http://www.redhat.com/support/errata/RHSA-2007-0257.html
http://www.eweek.com/article2/0%2C1759%2C1815795%2C00.asp
http://secunia.com/advisories/25098