{"id":"CVE-2021-29509","aliases":["GHSA-q28m-8xjw-8vr5"],"url":"https://o3.security/vulnerability/CVE-2021-29509","summary":"Puma's Keepalive Connections Causing Denial Of Service","details":"This vulnerability is related to [CVE-2019-16770](https://github.com/puma/puma/security/advisories/GHSA-7xx3-m584-x994).\n\n### Impact\n\nThe 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.\n\nA `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.\n\n### Patches\n\nThis problem has been fixed in `puma` 4.3.8 and 5.3.1.\n\n### Workarounds\n\nSetting `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](https://en.wikipedia.org/wiki/Slowloris_(computer_security))).\n\nThe fix is very small. [A git patch is available here](https://gist.github.com/nateberkopec/4b3ea5676c0d70cbb37c82d54be25837) for those using [unsupported versions](https://github.com/puma/puma/security/policy#supported-versions) of Puma.\n\n### For more information\n\nIf you have any questions or comments about this advisory:\n\n* Open an issue in [Puma](https://github.com/puma/puma).\n* To report problems with this fix or to report another vulnerability, see [our security policy.](https://github.com/puma/puma/security/policy)\n\n### Acknowledgements\n\nThank you to @MSP-Greg, @wjordan and @evanphx for their review on this issue. \n\nThank you to @ioquatix for providing a modified fork of `wrk` which made debugging this issue much easier.","published":"2021-05-11T17:15:07.627Z","modified":"2026-10-20T10:54:37.465952714Z","cvss":{"score":7.5,"severity":"HIGH","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"},"epss":null,"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"RubyGems","name":"puma","fixedVersion":"4.3.8"},{"ecosystem":"RubyGems","name":"puma","fixedVersion":"5.3.1"}],"fix":null,"references":[{"type":"ADVISORY","url":"https://github.com/puma/puma/security/policy"},{"type":"ADVISORY","url":"https://lists.debian.org/debian-lts-announce/2022/08/msg00015.html"},{"type":"ADVISORY","url":"https://rubygems.org/gems/puma"},{"type":"ADVISORY","url":"https://security.gentoo.org/glsa/202208-28"},{"type":"FIX","url":"https://gist.github.com/nateberkopec/4b3ea5676c0d70cbb37c82d54be25837"},{"type":"FIX","url":"https://github.com/puma/puma/security/advisories/GHSA-q28m-8xjw-8vr5"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2021-29509"},{"type":"PACKAGE","url":"https://github.com/puma/puma"},{"type":"WEB","url":"https://github.com/rubysec/ruby-advisory-db/blob/master/gems/puma/CVE-2021-29509.yml"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-20T10:54:37.465952714Z"}}