{"cve":"CVE-2021-29509","nvd":{"published":"2021-05-11T17:15:07.627","modified":"2026-06-17T03:47:48.050","status":"Modified","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.","cvss_v4":null,"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","score":7.5},"cwes":["CWE-667","CWE-400"],"references":["https://gist.github.com/nateberkopec/4b3ea5676c0d70cbb37c82d54be25837","https://github.com/puma/puma/security/advisories/GHSA-q28m-8xjw-8vr5","https://github.com/puma/puma/security/policy","https://lists.debian.org/debian-lts-announce/2022/08/msg00015.html","https://rubygems.org/gems/puma"],"products":["debian:debian_linux","puma:puma"],"source":"nvd","synced_at":"2026-10-19T21:18:09.669877188+00:00"},"epss":{"cve":"CVE-2021-29509","score":0.01599,"percentile":0.75776,"date":"2026-10-31"},"kev":null,"osv":[{"id":"GHSA-q28m-8xjw-8vr5","url":"https://osv.dev/vulnerability/GHSA-q28m-8xjw-8vr5","summary":"Puma's Keepalive Connections Causing Denial Of Service","modified":"2026-07-08T06:00:01.447117147Z","packages":[{"ecosystem":"RubyGems","name":"puma","purl":"pkg:gem/puma","ranges":["<4.3.8"],"fixed":["4.3.8"]},{"ecosystem":"RubyGems","name":"puma","purl":"pkg:gem/puma","ranges":[">=5.0.0 <5.3.1"],"fixed":["5.3.1"]}]}],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}