A security flaw in the PayPal web site is being actively exploited by fraudsters to steal customers' credit card numbers and other personal information.

The issue was reported to security company Netcraft today via its anti-phishing toolbar. What is particularly dangerous and convincing about this scam is that it starts at a page hosted on the PayPal website and is using a valid security certificate.

Netcraft said users are initially tricked into accessing a URL hosted on the genuine PayPal web site.

The web address (URL) uses SSL to encrypt information transmitted to and from the site, and a valid 256-bit SSL certificate is presented to confirm that the site does indeed belong to PayPal.

However, some of the content on the page has been modified by the fraudsters via a cross-site scripting technique (XSS).

When the victim visits the page, they are presented with a message that has been 'injected' onto the genuine PayPal site that says: "Your account is currently disabled because we think it has been accessed by a third party. You will now be redirected to Resolution Center."

After a short pause, the victim is then redirected to an external server, which presents a fake PayPal Member log-in page.

At this crucial point, victims may be off guard, as the paypal.com domain name and SSL certificate previously seen are intended to suggest they have visited the genuine PayPal web site.

Victims therefore don't expect PayPal to redirect them to a fraudulent web site. If a victim logs in via the fake login page, their PayPal username and password is transmitted to the fraudsters.

Users are subsequently presented with another page which asks them to enter further details to remove limits on the access of their account.

Information requested includes social security number, credit card number, expiration date, card verification number and ATM PIN.

The server currently running the scam is hosted in Korea and is accessed via a hex-encoded IP address. The Netcraft Toolbar already protects PayPal users by blocking access to this site.

We have contacted PayPal for comment but have had no reply as yet.