If you’ve ever seen this annoying error in your various build server logs, or even when running msbuild locally from the command line, you’re probably getting annoyed.
There are a few solutions (such as including the user credentials in plain text in a config file – eep!) but this is one which I’ve used when I really get stuck.
Ensure you’re logged in as the user which will be running the builds (if not yourself), and update the nuget source reference (which will be in a user-specific appdata config file) with the password:
nuget sources update -Name <whatever you called it> -source http://your.nuget.repo/authed/feed/ -User <your username> -pass <your pwd>
This will save an encrypted password in a user-specific config file on that computer, and should mean you don’t get prompted for that source anymore.
Several more options are detailed over here: http://www.xavierdecoster.com/nuget-package-restore-from-a-secured-feed