RubySec

Providing security resources for the Ruby community

CVE-2021-29509 (puma): Keepalive Connections Causing Denial Of Service in puma

ADVISORIES

GEM

puma

SEVERITY

CVSS v3: 7.5

PATCHED VERSIONS

  • ~> 4.3.8
  • >= 5.3.1

DESCRIPTION

Impact

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.

Patches

This problem has been fixed in puma 4.3.8 and 5.3.1.

Workarounds

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. A git patch is available here for those using unsupported versions of Puma.