"Several thousand Windows Live Hotmail customers' credentials were exposed on a third-party site," as a result of the scam, Microsoft officials conceded in a blog post Monday.
"As a result of our investigation we are taking measures to block access to all of the accounts that were exposed and have resources in place to help those users reclaim their accounts," the officials said.
Meanwhile, it's been reported that the hackers may also have targeted Google and Yahoo e-mail accounts.
Microsoft said the criminals who breached users' Hotmail accounts did so without penetrating the company's servers.
"We determined that this was not a breach of internal Microsoft data and initiated our standard process of working to help customers regain control of their accounts," Microsoft officials said.
Phishing is a fraudulent scheme carried out by cyber-criminals.
It involves tricking Internet users into providing passwords and other sensitive data to Web sites that mimic in almost every detail the legitimate, commercial sites operated by banks, online retailers, and other businesses that offer e-commerce services.
Often, Web surfers are directed to the bogus sites through links contained directly in an e-mail or in an e-mail attachment.
Microsoft is advising customers who believe they've been victimized by a phishing scheme to immediately update their accounts by changing their passwords and answers to security questions.
The company also recommends that Web users change their passwords every 90 days even if they haven't fallen victim to hackers.
Microsoft said it's cooperating with law enforcement authorities investigating the attacks.
bMighty has published a report on the secrets of e-mail management. Download the report here (registration required). Also, visit bMighty's IT Management How-To Center here.