By Aidan Malley

Published: 06:20 PM EST

As part of a multi-tiered approach to guarding against online fraud on its site, PayPal says it will block the use of any web browser that doesn't provided added validation measures, potentially restricting the current version of Safari from the e-commerce site.

The money transfer service's Chief Information Security Officer, Michael Barrett, makes the new policy clear in a white paper (PDF) posted this week, which highlights the browser as a key means of putting an end to phishing (false website) scams alongside such steps as blocking fraudulent e-mail messages and criminal charges.

When addressing web access, Barrett argues that any user visiting a financial site such as PayPal should know not only that their browser will block fake sites meant to steal information, but also that the browser can properly indicate a legitimate site. Without either precaution, visitors may not only be victims of scams but may lose all trust in an otherwise safe business. This doubly harmful outcome is likened to a car crash without protection.

"In our view, letting users view the PayPal site on one of these browsers is equal to a car manufacturer allowing drivers to buy one of their vehicles without seatbelts," the expert says.

To that end, PayPal is said to be implementing steps that will first provide warnings against, and eventually block, any browser that doesn't meet these criteria.

Most modern web browsers, including Firefox and newer versions of Microsoft's Internet Explorer, are able to support at least basic blocking of phishing sites. The newest, such as Internet Explorer 7 or the upcoming Firefox 3, also support a new feature known as an Extended Validation Secure Socket Layer (EV SSL) certificate. The measure of authenticity turns the address bar green and identifies the company running the site, letting the user know any secure transactions are genuine.

Safari, however, lacks either of these features and so could fall prey to the blocks and warning messages. Barrett doesn't mention the browser by name but notes that any "very old and vulnerable" software would ultimately be blacklisted from the future update to PayPal's service, placing Safari in the same category of dangerous clients as Microsoft's ten-year-old Internet Explorer 4.

Apple's approach to browser security has so far been tentative. The Mac maker has briefly incorporated Google's database of fraudulent sites into a beta builds of Mac OS X Leopard this past fall, only to pull the feature in later test versions. Release builds of the stand-alone browser for both Macs and Windows PCs have also gone without the anti-phishing warnings, but notably leave code traces inside the software that raise the possiblity of improvements through a later update.

Apple hasn't responded to the white paper but is likely to face pressure as PayPal and similar institutions ask for an all-encompassing approach to fighting scams that involves EV SSL and other software techniques. Internet Explorer 7's debut has already had a demonstrated effect on customers, who are more likely to finish signing up for PayPal knowing that the web browser has authenticated the registration page.

"We couldn’t eradicate this problem on our own – to make a dent in phishing, it would take collaboration with the Internet industry, law enforcement, and government around the world," Barrett explains.