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If a Mac app you’ve already trusted asks for keychain access
If an app is changed or infected by a virus after being granted access to your keychain, the app is no longer trusted, and you must grant access again.
A dialogue asks if you want to change the keychain to allow access. This dialogue appears if you recently updated your system software or the app, or if the app has been modified.
If the dialogue appears and you haven’t recently updated software: The app may be damaged or infected with a virus. Click Don’t Change, then consider checking the software for malware.
If you recently updated your system software: You must sometimes reauthorise updated apps.
To reauthorise servers, in the Finder 
on your Mac, choose Go > Connect to Server, and then log in to the server.
See alsoWhat is Keychain Access on Mac?