Dave Nielsen and Patrick Breitenbach PayPal Web Services (aka PHP Toolkit) 0.50 and possibly earlier has (1) world-readable permissions for ipn/logs/ipn_success.txt, which allows local users to view sensitive information (payment data), and (2) world-writable permissions for ipn/logs, which allows local users to delete or replace payment data.
http://www.vupen.com/english/advisories/2006/0183
http://www.uinc.ru/articles/vuln/ptpaypal050.shtml
http://www.securityfocus.com/bid/16218