According to the Microsoft Security Response Center, Microsoft will issue two Security Bulletins addressing two vulnerabilities on Tuesday. It will also host a webcast to address customer questions about the bulletins the following day (May 12 at 11:00 am PST, if you're interested).
Both of the vulnerabilities are rated "Critical," and they earned their rating through a remote code execution impact, meaning a hacker could potentially gain control of an infected machine. At least one of the two patches will require a restart.