A vulnerability could enable attackers to reset the password and hijack older Microsoft .net Passport accounts, according to a message posted to an online software vulnerability discussion mailing list.
.Net Passport is Microsoft's online identity management service which enables customers to use a single e-mail address and account password to sign on to a variety of affiliated services and websites, including Microsoft's Hotmail free e-mail service.
The vulnerability is in code used to help users who have forgotten their account password.
A "secret question" feature used to validate the identity of a user who needs to reset his account password can be manipulated by attackers on .net Passport accounts set up before Microsoft implemented the secret question feature, according to a message posted by Victor Manuel Alvarez Castro, who identified himself as a security consultant.
Microsoft did not immediately respond to requests for comment.
Accounts created since the secret question feature was implemented require the account owner to establish a secret question to retrieve their password, so not all .net Passport users are affected by the flaw.
It has been "a couple of years" since the secret question feature was implemented, Castro said.
The vulnerability requires that attackers know both the e-mail address and home country of the account owner. In the case of US based accounts, an attacker would also need the owner's state and zip code.
Those conditions make it more difficult to exploit the vulnerability, according to Rafael Núñez, a senior research scientist at Scientech de Venezuela in Caracas, Venezuela, who is known online as "[RaFa]."
However, given the estimated 200 million .net Passport accounts and the length of time that services like Hotmail have been online, there may be a large number of accounts affected by the vulnerability, Núñez said.
The attack would be especially useful for targeted attacks by those familiar with the victim.
Once in control of the victim's .net Passport account, an attacker could pose as that person online, using the victim's e-mail account or other services such as Microsoft's MSN instant messenger to pose as the victim online and perform "social engineering" attacks to collect other sensitive information, Núñez said.
This is the second vulnerability affecting .net Passport in as many months. In May, Muhammad Faisal Rauf Danka, a security researcher in Pakistan, reported a flaw in a function that enabled Passport users who had forgotten their password to change it using an e-mail message sent to an address associated with their Passport account.
The flaw enabled an attacker to have the password update e-mail sent to an e-mail address of their choice, and required little more than knowledge of their victim's e-mail address.
In that instance, repeated e-mail messages from Danka to Hotmail support went unanswered, prompting him to disclose the problem publicly.
Similar confusion about the correct procedure for reporting vulnerabilities may be at play in the latest revelation, which was not first disclosed to Microsoft, according to Núñez, who learned of the vulnerability from a Spanish language vulnerability discussion list on Friday.
Núñez worked with Castro to direct him to the proper security group at Microsoft, but the researcher released the information on the internet instead, he said.
Paul Roberts writes for IDG News Service