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

> An attacker can exhaust server threads with keep-alive connections, denying service to legitimate users in Puma HTTP server.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 11 May 2021
- **Updated:** 3 Aug 2024
- **Also known as:** DEBIAN-CVE-2021-29509, GHSA-q28m-8xjw-8vr5, OESA-2021-1203, SUSE-SU-2021:2761-1, SUSE-SU-2021:2914-1

## What is CVE-2021-29509?

CVE-2021-29509 is a high-severity denial of service vulnerability in Debian Debian Linux, published on 11 May 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-667 (Improper Locking), CWE-400 (Uncontrolled Resource Consumption)

## Which versions are affected by CVE-2021-29509?

4 open source packages ship an affected version, including puma. It affects 2 products.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `puma` | RubyGems | All versions | 4.3.8 |
| `puma` | RubyGems | All versions | 4.3.8 |
| `puma` | RubyGems | From 5.0.0 | 5.3.1 |
| `puma` | RubyGems | From 5.0.0 | 5.3.1 |
| `rubygem-puma` | SUSE:OpenStack Cloud Crowbar 8 | All versions | 2.16.0-3.12.1 |
| `rubygem-puma` | SUSE:OpenStack Cloud Crowbar 9 | All versions | 2.16.0-4.12.1 |

Products: Debian Debian Linux, Puma Puma.

## How do I fix CVE-2021-29509?

Upgrade puma to a fixed version for your release line: 4.3.8 or 5.3.1.

## Is CVE-2021-29509 being exploited?

There is no known exploitation of CVE-2021-29509 in the wild and no public exploit code.

## Other vulnerabilities in Debian Debian Linux

- [CVE-2015-3306](https://insights.sbomify.com/CVE-2015-3306/): ProFTPD Improper Access Control (critical 10.0, exploited in the wild)
- [CVE-2025-49113](https://insights.sbomify.com/CVE-2025-49113/): RoundCube Webmail Deserialization of Untrusted Data (critical 9.9, exploited in the wild)
- [CVE-2026-24061](https://insights.sbomify.com/CVE-2026-24061/): GNU InetUtils Argument Injection (critical 9.8, exploited in the wild)
- [CVE-2025-39682](https://insights.sbomify.com/CVE-2025-39682/): Linux Kernel Improper Check for Unusual or Exceptional Conditions (critical 9.8, exploited in the wild)
- [CVE-2024-9680](https://insights.sbomify.com/CVE-2024-9680/): Mozilla Firefox Use-After-Free (critical 9.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-29509 and fix it if it is affected.

What it is: CVE-2021-29509 is a high-severity denial of service vulnerability in Debian Debian Linux, published on 11 May 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-29509 in the wild and no public exploit code.

Affected packages and fixed versions:
- puma (RubyGems): affected all versions; fixed in 4.3.8
- puma (RubyGems): affected all versions; fixed in 4.3.8
- puma (RubyGems): affected from 5.0.0; fixed in 5.3.1
- puma (RubyGems): affected from 5.0.0; fixed in 5.3.1

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-29509/ (Markdown: https://insights.sbomify.com/CVE-2021-29509/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-29509
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-29509
- OSV: https://osv.dev/vulnerability/CVE-2021-29509

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-29509/
Summaries are generated and may contain mistakes; the official description above is the record.