TLDR: If you're using the group assignment feature of Hypnotoad or the 
prefork daemon, you should upgrade to Mojolicious 5.53.

Yesterday we've been informed by Klaus Madsen of a security flaw in our use 
of POSIX::setgid(), which only changes the real group id and the effective 
group id, but does not make any changes to supplementary group ids. So if 
you were starting Hypnotoad as root user, to be able to bind to a 
privileged port like 80, and then used the user/group assignment feature to 
drop privileges of spawned worker processes, those worker processes would 
retain all permissions from supplementary group ids of the root user. These 
permissions tend to be very broad on common operating systems.

This flaw has been fixed in Mojolicious 5.53, which is already available on 
CPAN.

  
  https://github.com/kraih/mojo/commit/37cf15fb23139bc76731dc5b59de570a99981ec9

P.S.: If you're using 3rd party Perl web servers with a similar group 
assignment feature, we strongly recommend that you verify that they don't 
contain the same flaw.

--
sebastian

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