Masking sensitive fields

This is not something M-Files supports natively: you can define property permissions as you mentioned, but if the user has the right to see the property they will see the full value as it is recorded.

It might be possible to create a custom extension that would display the data in a separate dashboard in the preferred masked format. Or, you could hide the actual property with the full value with permissions and store the masked version to another property in an event handler script so that this property would be visible for most users. Perhaps someone else can think of other ways to achieve the same.

For redacting sensitive information from document contents, there are some partner solutions available. Here's one for example: catalog.m-files.com/.../

You can use Extension Kit for this case. Module Extended property operations, besides many other features, gives you the possibility to use Regular expression (regex). For this particular case, we created a rule in 5 minutes which is setting a new property named "Sensitive Data" to read only the last 4 characters from the property "Data". You can then set the visibility of a regular property and a "masked" one according to permissions.

Which extension kit is it?

Extension Kit by Unitfly.
https://catalog.m-files.com/shop/extension-kit-for-m-files/
The module from Extension Kit I used here is Extended property operations. You can find more about it here: unitfly.com/.../

Thanks