Microsoft Corp. has scrambled to shut down a flaw in its Passport service that could potentially reveal users’ critical personal information, a company spokesperson confirmed Thursday.

The flaw, which was reported to the company late Wednesday, was located in the service’s password recovery system and would allow attackers to change an account password if they knew the user name.

Adam Sohn, a product manager with the Passport team, said Thursday that the flaw has been shut down and that the company is working to quickly fix the matter.

While Sohn said a preliminary investigation suggested that the vulnerability was not seriously exploited, it could potentially pose a large security threat to Passport users who store critical personal information such as credit card information with the service to access various online sites and services without having to re-type information.

The vulnerability was in the function that allowed users to request a forgotten Passport password via e-mail. By tricking the system into initiating an e-mail password reset process, a malicious attacker could then request that the password be sent to a different e-mail address, Sohn said.

Microsoft has turned off this feature while it fixes the problem, and users requesting a forgotten password were instructed to use other means, such as going through the customer service support page.

Sohn said that the problem should be fixed “within hours” and that the company is actively investigating the matter.