Microsoft has reported attacks against a previously unannounced flaw within its Excel spreadsheet program. According to Microsoft, "In order for this attack to be carried out, a user must first open a malicious Excel document that is sent as an e-mail attachment or otherwise provided to them by an attacker. (Note that opening it out of e-mail will prompt you to be careful about opening the attachment.) So remember to be very careful opening unsolicited attachments from both known and unknown sources." Microsoft says its Office team is working on a workaround and that antivirus companies have been notified to create signatures to block these attacks.