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Security advisory

Type: advisorySources: OSVCVE: 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.

Products
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Affected versions
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Fixed versions
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CVEs
CVE-2021-29509
Confirmed fixed CVEs
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Updated in this feed
31 Oct 2026, 11:44 UTC · Revision 1

Source evidence

CVE-2021-29509

core-osv · Original source

Published 11 May 2021, 17:15 UTC

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.

osv:CVE-2021-29509
Complete source record
{
  "id": "27f5c37da131cc2dfec0cdba2a1bd6a235686bc155bf10f51136d5aee1c1cba5",
  "url": "https://osv.dev/vulnerability/CVE-2021-29509",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "CVE-2021-29509",
    "summary": "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.",
    "evidence": "osv:CVE-2021-29509",
    "products": [],
    "severity": "high",
    "structured": {
      "id": "CVE-2021-29509",
      "aliases": [
        "CVE-2021-29509",
        "GHSA-q28m-8xjw-8vr5"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/puma/puma",
              "type": "GIT",
              "events": [
                {
                  "introduced": "0"
                },
                {
                  "fixed": "b911c13f8797aacaa8decf8532d6d7d45fda334f"
                },
                {
                  "introduced": "13e18e8078c800adfc52af687acc1d8de5f3988d"
                },
                {
                  "fixed": "1c91a4f1af23328118dbfe5b615f812af5e817ef"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "cpe": "cpe:2.3:a:puma:puma:*:*:*:*:*:ruby:*:*",
                    "range": [
                      {
                        "introduced": "0"
                      },
                      {
                        "fixed": "4.3.8"
                      }
                    ],
                    "source": "CPE_RANGE"
                  },
                  {
                    "cpe": "cpe:2.3:a:puma:puma:*:*:*:*:*:ruby:*:*",
                    "range": [
                      {
                        "introduced": "5.0.0"
                      },
                      {
                        "fixed": "5.3.1"
                      }
                    ],
                    "source": "CPE_RANGE"
                  }
                ]
              }
            }
          ],
          "versions": [
            "v4.3.7",
            "v5.3.0",
            "v5.2.2",
            "v5.2.1",
            "v5.2.0",
            "v5.1.0",
            "v4.3.6",
            "v5.0.3",
            "v5.0.2",
            "v5.0.1",
            "v5.0.0",
            "v4.3.5",
            "v4.3.4",
            "v4.3.3",
            "v4.3.2",
            "v4.3.1",
            "v4.3.0",
            "v4.2.1",
            "v4.2.0",
            "v4.1.0",
            "v4.0.1",
            "v4.0.0",
            "v3.12.0",
            "v3.11.4",
            "v3.11.3",
            "v3.11.2",
            "v3.11.1",
            "v3.11.0",
            "v3.10.0",
            "v3.9.1",
            "v3.9.0",
            "v3.8.0",
            "v3.7.1",
            "v3.6.0",
            "v3.5.2",
            "v3.5.1",
            "v3.5.0",
            "v3.4.0",
            "v3.3.0",
            "v3.2.0",
            "v3.1.1",
            "v3.1.0",
            "v3.0.2",
            "v3.0.1",
            "v3.0.0",
            "v3.0.0.rc1",
            "v2.16.0",
            "v2.15.3",
            "v2.15.2",
            "v2.15.1",
            "v2.15.0",
            "v2.14.0",
            "v2.13.4",
            "v2.13.3",
            "v2.13.2",
            "v2.13.1",
            "v2.13.0",
            "v2.12.3",
            "v2.12.2",
            "v2.12.1",
            "v2.12.0",
            "v2.11.3",
            "v2.11.2",
            "v2.11.0",
            "v2.10.2",
            "v2.10.1",
            "v2.10.0",
            "v2.9.2",
            "v2.9.1",
            "v2.9.0",
            "v2.8.2",
            "v2.8.1",
            "v2.7.1",
            "v2.7.0",
            "v2.6.0",
            "v2.5.1",
            "v2.5.0",
            "v2.4.1",
            "v2.4.0",
            "v2.3.2",
            "v2.3.1",
            "v2.3.0",
            "v2.2.2",
            "v2.2.1",
            "v2.2.0",
            "v2.1.1",
            "v2.1.0",
            "v2.0.1",
            "v2.0.0",
            "v2.0.0.b7",
            "v2.0.0.b6",
            "v2.0.0.b5",
            "v2.0.0.b4",
            "v2.0.0.b3",
            "v2.0.0.b2",
            "v1.5.0",
            "v1.4.0",
            "v1.3.1",
            "v1.3.0",
            "v1.2.2",
            "v1.2.1",
            "v1.2.0",
            "v1.1.1",
            "v1.1.0"
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2021-29509.json"
          }
        }
      ],
      "modified": "2026-10-20T10:54:37.465952714Z",
      "provider": "osv",
      "references": [
        "https://github.com/puma/puma/security/policy",
        "https://lists.debian.org/debian-lts-announce/2022/08/msg00015.html",
        "https://rubygems.org/gems/puma",
        "https://security.gentoo.org/glsa/202208-28",
        "https://gist.github.com/nateberkopec/4b3ea5676c0d70cbb37c82d54be25837",
        "https://github.com/puma/puma/security/advisories/GHSA-q28m-8xjw-8vr5"
      ],
      "provider_metadata": {
        "unresolved_ranges": [
          {
            "cpes": [
              "cpe:2.3:o:debian:debian_linux:10.0:*:*:*:*:*:*:*"
            ],
            "vendor_product": "debian:debian_linux",
            "extracted_events": [
              {
                "range": [
                  {
                    "introduced": "10.0"
                  },
                  {
                    "last_affected": "10.0"
                  }
                ],
                "source": "CPE_STRING"
              }
            ]
          }
        ]
      }
    },
    "related_cves": [
      {
        "value": "CVE-2021-29509",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 385774,
  "published_at": "2021-05-11T17:15:07.627+00:00"
}