.dat · application/octet-stream
What is a .dat file?
Registry Hive / Generic Data
About the format
A .dat extension promises nothing — it's the generic 'data' suffix — but on Windows the most famous .dat files are registry hives: NTUSER.DAT in every user profile holds that user's entire HKEY_CURRENT_USER branch, and UsrClass.dat holds per-user file associations. Hives use the binary 'regf' format and are locked while the user is logged in, which is why copying NTUSER.DAT of the active user fails. Other .dat files are simply whatever some program decided to store: game saves, caches, application databases. Identification therefore starts with the bytes, not the name: regf at offset zero means registry hive; otherwise check the first bytes against known signatures. Registry hives can be loaded read-only into Registry Editor on another machine (File → Load Hive) — a standard technique for repairing a profile or inspecting a dead system's settings. Random application .dat files, by contrast, are only meaningful to the app that wrote them.
How to open a .dat file
Windows
- Registry Editor (File → Load Hive, for regf hives)
- the application that created it (for app data)
macOS
- hivex tools (for hives)
Linux
- hivexsh / hivexregedit (for hives)
Is a .dat file dangerous?
Not directly executable. A hive from an unknown source is just data; the risk with .dat files is misidentifying what they are.
Related file types
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