In recent months, America Online and RealNames also have reported incursions on their systems.
Corporations and other entities large and small have often been targets of attacks. Sometimes hackers sneak their way into a company's Web site and deface the site with graffiti or more pointed messages. In September, for instance, a hacker smeared the Web sites of NASA and the Communications Workers of America, among others, with pro-Napster messages.
AOL in June confirmed that hackers had compromised member accounts by means of email attachments sent to AOL employees. At the time of the attack, AOL said it boosted the security of its email systems. An AOL spokesman declined to comment on the measures AOL employs to fight hackers.
Internet keyword service RealNames fell victim to an account hacker in February. The company, which substitutes complicated Web addresses with simple keywords, said the perpetrators may have accessed credit card numbers and passwords.
Microsoft itself has logged other breaches of security. In March and April, test copies of Whistler, a future Microsoft operating system for consumers, were leaked onto the Internet. The company said at the time that it had not yet come to any conclusions about how the internal test builds were being released to the Internet.
Industry experts say that while many companies have been beefing up security measures in an effort to safeguard valuable intellectual property and other data housed in internal systems, they will remain vulnerable to attacks.
Steve Englund, an intellectual property attorney and partner at Arnold & Porter, said corporate hacking incidents will continue to be a problem and will most likely increase no matter how well-armed companies are against breaches to their data security. Dealing with the aftermath is the real issue, he said.
"Once (a company's) information is compromised, it's hard to get that back," said Englund. "Having the FBI poking around may not even help the problem."
Like other companies that have experienced similar attacks, Microsoft this week enlisted the help of the FBI to probe the latest incident, in which a hacker succeeded in breaking into the company's network.
"What has happened to Microsoft may motivate other companies to wake up," said Englund. "If this can happen to Microsoft, maybe it can happen to you, too."