It's a common and sensible practice to install records of the form
"localhost. IN A 127.0.0.1" into nameserver configurations, bizarrely
however, administrators often mistakenly drop the trailing dot,
introducing an interesting variation of Cross-Site Scripting (XSS) I
call Same-Site Scripting. The missing dot indicates that the record is
not fully qualified, and thus queries of the form
"localhost.example.com" are resolved. While superficially this may
appear to be harmless, it does in fact allow an attacker to cheat the
RFC2109 (HTTP State Management Mechanism) same origin restrictions, and
therefore hijack state management data.
The result of this minor misconfiguration is that it is impossible to
access sites in affected domains securely from multi-user systems. The
attack is trivial, for example, from a shared UNIX system, an attacker
listens on an unprivileged port and then uses a typical XSS attack
vector (e.g. in an html email) to lure a victim into
requesting http://localhost.example.com:1024/example.gif, logging the
request. The request will include the RFC2109 Cookie header, which could
then be used to steal credentials or interact with the affected service
as if they were the victim.
Tavis recommends removing localhost entries from DNS servers that do not have the trailing period (i.e. "localhost" vs. "localhost."). The trailing period assures that somebody cannot setup camp on 127.0.0.1 and steal your web applications cookies or run any other malicious dynamic content in the same domain, exploiting DNS for same origin policy attacks.