Microsoft patched a hole in its .Net Passport identity management service last night after a security researcher disclosed a potentially serious flaw that could enable attackers to hijack Passport accounts.

The vulnerability was in the code for a "Secret Question" feature that helped users who had forgotten their Passport password, according to a message posted by Victor Manuel Alvarez Castro, who identified himself as a security consultant.

Some Passport accounts that were created before the Secret Question feature was implemented in August 1999 contained "bad data" in the Secret Question field, according to Jeff Jones, senior director of Trustworthy Computing Security at Microsoft.

That data enabled knowledgeable attackers to circumvent the Secret Question feature and reset the password for another Passport user's account, he said.

Attackers needed to know both the e-mail address and home country of the account owner. In the case of U.S.-based accounts, an attacker also needed the state and zip code of the account owner, Jones said.

Microsoft would not comment on how many Passport user accounts were affected, but Jones characterized the number as a "small fraction" of the subset of accounts that were created before August 1999 for which no secret question had been established.

After verifying the problem Monday, Microsoft temporarily suspended the password update feature that relied on the Secret Question for all its Passport users, Jones said.

Microsoft patched the errant code Monday evening so that the bad data could not be used to circumvent the Secret Question requirement, then updated the Passport service overnight, Jones said.

Microsoft does not have any evidence that the vulnerability was exploited, Jones said.

Customers whose Passport accounts were affected by the problem should still be able to log in to the service. Those individuals can overwrite the bad data by setting up a valid Secret Question, Jones said.

Customers can also use an e-mail-based update feature to update their password, he said.

The company did not receive any word from Castro prior to the public disclosure and first learned of the flaw when it was posted to software security newsgroups, he said.

Microsoft encourages security researchers to report problems to Microsoft before disclosing them to the public, Jones said.

The IDG News Service is a Network World affiliate.