The flaw, in Passport's password recovery mechanism, could have allowed an attacker to change the password on any account to which the username is known. The flaw was disclosed late Wednesday night on the security mailing list Full Disclosure.
The simplicity of the attack method and the high value of the data frequently stored in Passport accounts combined to make the vulnerability critical.
"It is hardly an exploit or even vulnerability; it's just a flaw, in their Web-application logic," the person who posted the vulnerability said in an e-mail to CNET News.com. "The flaw has been there since a long time. I just discovered it recently," wrote the individual who identified himself as Muhammad Faisal Rauf Danka. He claimed to be a Pakistani security consultant and M.B.A. candidate.
Microsoft has touted. Passport accounts are central repositories for a person's online data and can include personal information such as birthdays and credit card numbers as well as acting as the single key for the customer's online accounts.
Microsoft moved quickly to prevent online vandals from exploiting the issue, and posted its advisory just before 8 p.m. PDT. By 11:30 p.m., the software giant had essentially turned off the vulnerable feature. "We have shut down all ability to reset passwords," said Sean Sundwall, a spokesman for the company.
The flaw allowed a single Web address--or URL--to be used to request a password reset from the Passport servers. The URL contains the e-mail address of the account to be changed and the address where the attacker would like to have the reset message sent. By entering the single line into a Web browser an attacker can cause the Passport servers to return a link that allows an account's password to be reset. By following the link returned in the message, the attacker can change the password for the victim's account.
Danka claims to have found the issue after a friend's account had been hacked.
"Later, my friend gave the 'attacker' my passport address as a challenge, and mine was compromised as well," he wrote in the e-mail. Not long after, he figured out how the attacker had compromised the accounts.
The security consultant also said that he had repeatedly sent e-mail warnings to Microsoft's abuse and security addresses at Hotmail.com to no avail. However, he didn't send an e-mail to Microsoft's standard security contact point, firstname.lastname@example.org.
It wasn't clear Wednesday night whether the flaw affected all Passport accounts, or a smaller subset of accounts. Several security experts confirmed that the flaw could be exploited in the manner described by Danka.
"I tried it on my own account and I tried it on my friends' accounts, with full permission; it worked on all occasions," said Wayne Chang, a student at the University of Massachusetts at Amherst. "This is definitely a big security flaw."
The issue couldn't be confirmed by everyone. In some cases, security experts didn't get an e-mail back from the server.
"I just tried again, and have not yet received an e-mail with the change password link in it," Marc Slemko, a Seattle-area software engineer, wrote to CNET News.com in an e-mail. "That either means it is much slower now or has been disabled."
The engineer believed Microsoft would rally the security teams to handle the vulnerability, as the issue had enormous implications for customers.