Online payment provider PayPal has patched a critical cross-site scripting vulnerability that a Finnish researcher disclosed late last week, the company said Monday.

Harry Sintonen, who goes by the alias "piru," revealed the cross-site scripting bug in an online chat on Friday, according to a report posted by U.K.-based anti-fraud vendor Netcraft Ltd. The cross-site scripting vulnerability's potential impact was even more serious than usual, Sintonen said, because the page was guarded by an Extended Validation (EV) SSL certificate

"PayPal says you can trust the URL if it begins with 'https://www.paypal.com,' which is not true in this case," Sintonen told Netcraft and others during an online chat session.

Sintonen claimed that he was able to inject his own code, which put a harmless message on-screen that read, "Is it safe?" a reference to a line in the 1976 movie "Marathon Man."

Cross-site scripting bugs, which are typically exploited by identity thieves and phishers, let attackers insert their own malicious code into legitimate pages, but have also been used for other purposes. Last month, for example, a cross-site scripting vulnerability in Sen. Barack Obama's (D-Ill.) presidential campaign Web site redirected some visitors to the URL of his rival, Sen. Hillary Clinton (D-N.Y.).

EVs are a step above standard SSL certificates -- their owners must go through more stringent background checks -- and were introduced to reassure users that an online site is legitimate, not a fake hosted by phishers. PayPal was one of the first commercial sites to use an EV.

To let users know they're at URL backed by an EV, browsers that support the certificates tint the address bar green.

Monday, the payment arm of eBay Inc. said it had plugged the cross-site scripting hole. "As soon as we learned of this exploit, we began working very quickly to shut it down and it is now closed," said PayPal spokesman Michael Olderburg in an e-mail. "To our knowledge, this exploit was not used in any phishing attacks."

EVs, cross-site scripting vulnerabilities and PayPal have some history. A month ago, PayPal's chief information security officer, Michael Barrett, seemed to say in a paper presented at the RSA Conference that the company would block browsers such as Apple Inc.'s Safari that didn't support EVs.

A few days later, however, PayPal said it had no plans to block Safari, or any current browser, from its site and service, but instead said it would bar only "obsolete browsers on outdated or unsupported operating systems." The example it gave then was Internet Explorer 4 running on Windows 98; both that browser and OS fell off Microsoft Corp.'s support list in mid-2006.