CVE-2021-23240

selinux is not the default MAC in Ubuntu, though users can
boot into it.

fs.protected_symlinks is '1' by default in Ubuntu,
and if not changed, can prevent this issue from being exploited.

There are backports available for version
1.8 but not straightforward for 1.8.16 (xenial baseline). Some of
the selinux specific code that needs to be patched was added later
in time and it seems to me that the backports for 1.8 are based on
1.8.32. So, in xenial, a fix would be very intrusive and likely
to introduce a regression.
The reproducer in sudo.ws reference does not reproduce
in xenial.
The fact that the reproducer does not reproduce, that
the kernel hardening fs.protected_symlinks is '1' by default (with
no clear reason to be turned off to '0'), that there is a very low
possibility to have Ubuntu running selinux without a working policy
and, mainly, that the risky of regression is high, we are marking
this CVE as ignored for xenial.