CVE-2021-29509: Debian Linux

High severity, CVSS 7.5. EPSS: 1.6% chance of exploitation in the next 30 days.

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.

Affected products

• Debian Debian Linux: version 10.0 only
• Puma Puma: before 4.3.8 (fixed in 4.3.8); from 5.0.0, before 5.3.1 (fixed in 5.3.1)

Published 2021-05-11. Last modified 2026-06-17.