S9Y Serendipity is prone to an HTML-injection vulnerability and multiple cross-site scripting vulnerabilities because it fails to properly sanitize user-supplied input.
An attacker may leverage these issues to execute arbitrary script code in the browser of an unsuspecting user in the context of the affected site. This may allow the attacker to steal cookie-based authentication credentials, control how the site is rendered to the user, or launch other attacks.
S9Y Serendipity 1.3 is vulnerable; other versions may also be affected.
The following proof of concept is available for the referrer issue:
wget --referer='http://<hr onMouseOver="alert(7)">' http://someblog.com/