/usr/lib/mono/xbuild-frameworks/ is in mono-xbuild 3.2.8+dfsg-4ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/mono/xbuild-frameworks/.NETFramework/
/usr/lib/mono/xbuild-frameworks/.NETFramework/v2.0/
/usr/lib/mono/xbuild-frameworks/.NETFramework/v2.0/RedistList/
/usr/lib/mono/xbuild-frameworks/.NETFramework/v2.0/RedistList/FrameworkList.xml
/usr/lib/mono/xbuild-frameworks/.NETFramework/v3.0/
/usr/lib/mono/xbuild-frameworks/.NETFramework/v3.0/RedistList/
/usr/lib/mono/xbuild-frameworks/.NETFramework/v3.0/RedistList/FrameworkList.xml
/usr/lib/mono/xbuild-frameworks/.NETFramework/v3.5/
/usr/lib/mono/xbuild-frameworks/.NETFramework/v3.5/RedistList/
/usr/lib/mono/xbuild-frameworks/.NETFramework/v3.5/RedistList/FrameworkList.xml
/usr/lib/mono/xbuild-frameworks/.NETFramework/v4.0/
/usr/lib/mono/xbuild-frameworks/.NETFramework/v4.0/Profile/
/usr/lib/mono/xbuild-frameworks/.NETFramework/v4.0/Profile/Client/
/usr/lib/mono/xbuild-frameworks/.NETFramework/v4.0/Profile/Client/RedistList/
/usr/lib/mono/xbuild-frameworks/.NETFramework/v4.0/Profile/Client/RedistList/FrameworkList.xml
/usr/lib/mono/xbuild-frameworks/.NETFramework/v4.0/RedistList/
/usr/lib/mono/xbuild-frameworks/.NETFramework/v4.0/RedistList/FrameworkList.xml
/usr/lib/mono/xbuild-frameworks/.NETFramework/v4.5/
/usr/lib/mono/xbuild-frameworks/.NETFramework/v4.5/RedistList/
/usr/lib/mono/xbuild-frameworks/.NETFramework/v4.5/RedistList/FrameworkList.xml