Adobe Systems today released an update to plug a dangerous security hole that hackers first began exploiting in January. The update, available here, is for Adobe Reader and Acrobat programs on both Windows and Mac systems. Adobe said it expects updates for Adobe Reader 7 and 8, and Acrobat 7 and 8, to be available by March 18. If you’ve chosen to read PDF documents using the popular alternative to Adobe — Foxit Reader — you also need to update. On Monday, Foxit shipped an update that fixes at least three serious vulnerabilities in its Reader products. That update, which brings Foxit Reader to version 3.0, is available from this link.