Debian is an upstream of Ubuntu only in the development release. Ubuntu stable releases are effectively branches (like the stable releases of every other distro

Debian is an upstream of Ubuntu only in the development release. Ubuntu stable releases are effectively branches (like the stable releases of every other distro). Stable release maintenance of Debian and Ubuntu are thus independent in principle. In practice patches for one are likely to be similar to what is needed on the other, issues affecting one are likely to affect the other, there are Debian developers who are also Ubuntu developers, Canonical employees who are also Debian developers, everyone is working towards a common goal of stable maintenance, and so forth, so there is quite a bit of communication and cross-pollination between the two. For example: I'm a Canonical employee and Debian maintainer; today I've spent quite a bit of time working on a security fix for a package I maintain in Debian that I'm effectively being paid to do by Canonical.