CVE-2021-29509

Puma is a concurrent HTTP 1.1 server for Ruby/Rack applications. The fix
for CVE-2019-16770 was incomplete. The original fix only protected existing
connections that had already been accepted from having their requests
starved by greedy persistent-connections saturating all threads in the same
process. However, new connections may still be starved by greedy
persistent-connections saturating all threads in all processes in the
cluster. A puma server which received more concurrent keep-alive
connections than the server had threads in its threadpool would service
only a subset of connections, denying service to the unserved connections.
This problem has been fixed in puma 4.3.8 and 5.3.1. Setting
queue_requests false also fixes the issue. This is not advised when using
puma without a reverse proxy, such as nginx or apache, because you
will open yourself to slow client attacks (e.g. slowloris). The fix is very
small and a git patch is available for those using unsupported versions of
Puma.