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

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.6% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2021-05-11 · updated 2026-06-17
Weakness: CWE-400

Description

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 Linux: 10.0
puma: before 4.3.8; from 5.0.0 before 5.3.1 — fixed in 4.3.8, 5.3.1
Rubygems Puma: before 4.3.8; from 5.0.0 before 5.3.1; 0.8.0, 0.8.1, 0.8.2, 0.9.0, 0.9.1, 0.9.2 (+127) — fixed in 4.3.8, 5.3.1

How to fix

Upgrade to a fixed version: puma 4.3.8, 5.3.1; Rubygems Puma 4.3.8, 5.3.1. Check the vendor advisory for mitigations.

References

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