I ran into this bug as well, as I tried to build a new Linux kernel as root. Downloading the sources as a regular user worked though.
apt-get source linux-image-extra-4.10.0-19-generic
W: Download is performed unsandboxed as root as file 'linux_4.10.0-21.23.dsc' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)
I ran into this bug as well, as I tried to build a new Linux kernel as root. Downloading the sources as a regular user worked though.
apt-get source linux-image- extra-4. 10.0-19- generic
W: Download is performed unsandboxed as root as file 'linux_ 4.10.0- 21.23.dsc' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)