Almost anyone can snoop the secure data traffic of unpatched iPhones and iPads using a recently-revised tool, a researcher said today as he urged owners to apply Apple's latest iOS fix.

The nine-year-old bug was quashed Monday when Apple issued a patch for the iPhone 4, iPhone 3GS and third- and fourth-generation iPod Touch.

If those devices aren't patched, attackers can easily intercept and decrypt secure traffic -- the kind guarded by SSL, which is used by banks, e-tailers and other sites -- at a public Wi-Fi hotspot, said Chet Wisniewski, a security researcher with U.K.-based Sophos.

"This is a nine-year-old bug that Moxie Marlinspike disclosed in 2002," said Wisniewski in an interview today.

On Monday, Marlinspike released a revision of his long-available "sslsniff" traffic sniffing tool that allows a user to intercept SSL traffic from vulnerable iOS devices. "My mother could actually use this," said Wisniewski, alluding to the tool's simplicity.

The bug Apple patched was in the parsing of SSL certificates on iOS, according to Wisniewski and the researchers Apple credited with reporting the flaw.

"iOS's SSL certificate parsing contains a flaw where it fails to check the basicConstraints parameter of certificates in the chain," said Trustwave, a Chicago security firm, in a Monday advisory. "By signing a new certificate using a legitimate end entity certificate, an attacker can obtain a 'valid' certificate for any domain."

Apple credited Trustwave's Paul Kehrer and Recurity Lab's Gregor Kopf with reporting the vulnerability.

Wisniewski confirmed the bug by using a legitimate certificate for his own website to create a valid certificate for PayPal. If he had proceeded, he could have intercepted others' iOS-generated traffic to the real PayPal site and steal their usernames and passwords.

"Anybody can sign anything," said Wisniewski.

Marlinspike disclosed the underlying vulnerability in 2002, and created sslsniff that same year as a proof-of-concept demonstration of a "man-in-the-middle" attack using rogue certificates. Microsoft patched the bug in Windows' cryptographic component in 2002, but Apple had overlooked it in iOS.

"It's probably been in [iOS] since day one," said Wisniewski, who speculated that even attackers hadn't known of the flaw. "Someone would likely would noticed if it had been used, because every Windows user would have been getting browser warnings [of an invalid certificate] on a public Wi-Fi network even as iPhone users were seeing no such warning."