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.