Simple Streams (simplestreams) does not properly verify the GPG signatures of disk image files, which allows remote mirror servers to spoof disk images and have unspecified other impact via a 403 (aka Forbidden) response.
https://euvd.enisa.europa.eu/vulnerability/EUVD-2015-1477
https://bugs.launchpad.net/ubuntu/%2Bsource/simplestreams/%2Bbug/1487004