source: http://www.securityfocus.com/bid/5728/info

Lycos htmlGEAR guestGEAR does not sanitize HTML from CSS (Cascading Style-Sheets) elements in guestbook fields. An attacker could capitalize on this situation to include arbitrary HTML and script code in a guestbook entries, which would be rendered in the web client of users who view the malicious guestbook entry.

Code injected in this manner will be executed in the security context of the website hosting the guestbook. 

It has also been reported that it is possible, in some versions of the software, to inject HTML into image tags.

By specifying an e-mail address/web page URL like the following:

" STYLE="expression([javascript])

The JavaScript block will execute. Some less-paranoid versions of the
guestbook also allow a typical IMG attack:

<IMG SRC="javascript:[javascript]">