/usr/lib/mono/xbuild/Microsoft/ is in mono-xbuild 3.2.8+dfsg-4ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/mono/xbuild/Microsoft/Portable/
/usr/lib/mono/xbuild/Microsoft/Portable/Microsoft.Portable.Core.props
/usr/lib/mono/xbuild/Microsoft/Portable/Microsoft.Portable.Core.targets
/usr/lib/mono/xbuild/Microsoft/Portable/v4.0/
/usr/lib/mono/xbuild/Microsoft/Portable/v4.0/Microsoft.Portable.CSharp.targets
/usr/lib/mono/xbuild/Microsoft/Portable/v4.5/
/usr/lib/mono/xbuild/Microsoft/Portable/v4.5/Microsoft.Portable.CSharp.targets
/usr/lib/mono/xbuild/Microsoft/VisualStudio/
/usr/lib/mono/xbuild/Microsoft/VisualStudio/v10.0/
/usr/lib/mono/xbuild/Microsoft/VisualStudio/v10.0/WebApplications/
/usr/lib/mono/xbuild/Microsoft/VisualStudio/v10.0/WebApplications/Microsoft.WebApplication.targets
/usr/lib/mono/xbuild/Microsoft/VisualStudio/v11.0/
/usr/lib/mono/xbuild/Microsoft/VisualStudio/v11.0/WebApplications/
/usr/lib/mono/xbuild/Microsoft/VisualStudio/v11.0/WebApplications/Microsoft.WebApplication.targets
/usr/lib/mono/xbuild/Microsoft/VisualStudio/v9.0/
/usr/lib/mono/xbuild/Microsoft/VisualStudio/v9.0/WebApplications/
/usr/lib/mono/xbuild/Microsoft/VisualStudio/v9.0/WebApplications/Microsoft.WebApplication.targets