Modern ransomware campaigns specifically target older formats because security tools often scan new .docx files rigorously but ignore a .xls file from 2003. If you are in IT support, you know the ticket. A senior executive tries to open a 15-year-old budget file. They see: "Microsoft Excel cannot open or save any more documents because there is not enough available memory or disk space." (This error is a lie. The problem isn't memory; it is the File Block Settings.)
They allow you to say: "I will never touch a Word 6.0 document again. Please treat it as a potential bomb." file block settings in the trust center
After 90 days of Phase 2, change the policy to "Hard Block Open" . Any remaining legacy files become inaccessible. You will get three angry emails, but the migration will be over. Common Misconceptions Myth 1: "File Block Settings protect against all zero-day exploits." Reality: No. They protect against exploits in specific parsing libraries for specific old formats . A zero-day in .docx will bypass them completely. They see: "Microsoft Excel cannot open or save
Between 1997 and 2007, Microsoft Office used the OLE Compound File format ( .doc , .xls , .ppt ). These were not simple text files; they were virtual file systems inside a single file. They contained streams, storages, and binary blobs. Malware authors loved them because it was easy to hide shellcode in unused sectors. Any remaining legacy files become inaccessible