If you use the Internet Explorer, Google Chrome or Apple Safari browsers to conduct PayPal transactions, now would be a good time to switch over to the decidedly more secure Firefox alternative.

That's because a hacker on Monday published a counterfeit secure sockets layer certificate that exploits a gaping hole in a Microsoft library used by all three of those browsers. Although the certificate is fraudulent, it appears to all three to be a completely legitimate credential vouching for the online payment service. The bug was disclosed more than nine weeks ago, but Microsoft has yet to fix it.

Monday's release of the so-called null-prefix certificate for PayPal is a serious blow to online security because it makes it trivial for cybercrooks to defeat one of the web's oldest and most relied upon defenses against man-in-the-middle attacks. PayPal and thousands of other financial websites use the certificates to generate a digital signature that mathematically proves login pages aren't forgeries that were set up by con artists who are sitting in between the user and the website he's trying to view.

The certificate exploits a security hole in a Microsoft application programming interface known as the CryptoAPI, which is used by the IE, Google Chrome and Apple Safari for Windows browsers to parse a website's SSL certificates. Even though the certificate is demonstrably forged, it can be used with a previously available hacking tool called SSLSniff to cause all three browsers to display a spoofed page with no warnings, even when its address begins with "https."

"Use this with SSLSniff and it's game over," Moxie Marlinspike, a hacker who demonstrated the SSL weakness at the Black Hat security conference in Las Vegas, Nevada, said of the bogus PayPal cert. "It's true that posting this doesn't exactly seem prudent and is personally frustrating for me. Technically, though, it might be more fair to say that Windows users are at risk because of a vulnerability that remains unpatched by Microsoft."

A PayPal spokeswoman said the company's information security team is aware of the fraudulent certificate. "We're working to see if there are any technical workarounds on the PayPal side which can be put into place," she said.

The certificate is the latest to target a weakness that causes browsers, email clients, and other SSL-enabled apps to ignore all text following the \ and 0 characters, which are used to denote the end of a string of characters in C-based languages. Attackers can exploit that weakness by registering a normal SSL certificate for a site under their control and then inserting the domain name and the null character immediately following the name of the site they want to impersonate.

The name on the certificate looked something like the following:

www.paypal.com\0ssl.secureconnection.cc

While the vulnerability was disclosed in July, Microsoft has yet to acknowledge or fix it in the crypto library, which is used by a wide variety of applications. A spokesman for the software giant said last week that members of its security team are "investigating a possible vulnerability in Windows presented during Black Hat" and "will take appropriate actions to protect customers" once it's completed.

The take-away from all of this is that if you use IE, Chrome or Safari for Windows to browse SSL-protected parts of PayPal, there's no way to know if they are genuine - at least until Microsoft gets around to fixing the bug. And because it's entirely possible null-prefix certificates for other sites have been issued more quietly, there's no way to rely on SSL at all for those browsers.

The obvious answer now is for the certificate authority that issued the fraudulent credential to revoke it. But even then, there's no way to guarantee the rogue certificate is taken out of circulation because of vulnerabilities in the Online Certificate Status Protocol, which Marlinspike also discussed at Black Hat.

Fortunately, Mozilla developers patched the hole a few days after Marlinspike's demo and Apple followed suit a few weeks later with Safari for OS X. That means if you're on Windows, the only way to protect yourself against this critical vulnerability is to use versions 3.5 or 3.0.13 or later of Firefox. At least until Microsoft fixes the CryptoAPI, whenever that may be. ®