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 c...

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.

Metrics

CVSS 3.1

7.5/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS Probability

1.60%

72.7th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

Affected Software

Vendor	Product	Versions
Puma	Puma	< 4.3.8
Puma	Puma	>= 5.0.0, < 5.3.1
Debian	Debian Linux	10.0

References

https://gist.github.com/nateberkopec/4b3ea5676c0d70cbb37c82d54be25837Patch, Third Party Advisory
https://github.com/puma/puma/security/advisories/GHSA-q28m-8xjw-8vr5Patch, Third Party Advisory
https://github.com/puma/puma/security/policyThird Party Advisory
https://lists.debian.org/debian-lts-announce/2022/08/msg00015.htmlMailing List, Third Party Advisory
https://rubygems.org/gems/pumaProduct, Third Party Advisory
https://security.gentoo.org/glsa/202208-28Third Party Advisory
https://gist.github.com/nateberkopec/4b3ea5676c0d70cbb37c82d54be25837Patch, Third Party Advisory
https://github.com/puma/puma/security/advisories/GHSA-q28m-8xjw-8vr5Patch, Third Party Advisory
https://github.com/puma/puma/security/policyThird Party Advisory
https://lists.debian.org/debian-lts-announce/2022/08/msg00015.htmlMailing List, Third Party Advisory
https://rubygems.org/gems/pumaProduct, Third Party Advisory
https://security.gentoo.org/glsa/202208-28Third Party Advisory

Timeline

Published: May 11, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is 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.

How severe is CVE-2021-29509?

CVE-2021-29509 has a CVSS score of 7.5/10 (HIGH severity). The EPSS model estimates a 1.60% probability of exploitation in the next 30 days.

How do I fix CVE-2021-29509?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.