Microsoft on Thursday will issue a rare out-of-band patch for a critical flaw that affects all versions of Windows from 2000 forward, including Vista and Server 2008. It’s not known exactly what vulnerability the fix is for, but Microsoft said the flaw allows remote code execution. The company usually issues patches outside of its normal scheduled monthly cycle when there is a vulnerability that is being actively exploited. The patch, which will be available on Microsoft’s security bulletin site later today, is rated critical. The company also will have a special webcast at 4 p.m. EDT today to explain the vulnerability and the patch.