How severe is CVE-2021-29509?

CVE-2021-29509 has a CVSS score of 7.5/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-29509?

CVE-2021-29509 affects the following packages: puma (RubyGems), puma (RubyGems). Ecosystems affected: RubyGems.

How do I fix CVE-2021-29509?

Update puma to 4.3.8 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-29509 is resolved across your whole dependency graph.

How do I detect CVE-2021-29509 in my RubyGems dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for puma, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-29509 if there is no patch (or I can't update yet)?

Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Is CVE-2021-29509 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-29509 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-29509?

CVE-2021-29509 has an EPSS (Exploit Prediction Scoring System) score of 1.6%, placing it in the 76th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-29509?

CVE-2021-29509 is classified as Uncontrolled Resource Consumption (CWE-400), CWE-667 (CWE-667). These weakness types describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-29509 published, and has it been updated?

CVE-2021-29509 was published on May 11, 2021 and was last updated on October 20, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

💎

💎 RubyGems

Not in CISA KEV

HIGH severity

CVE-2021-29509 — puma

HIGH

CVE-2021-29509 is a high-severity (CVSS 7.5) Uncontrolled Resource Consumption vulnerability in puma. A fix is available for puma — see the affected versions and patch details below.

Puma's Keepalive Connections Causing Denial Of Service

Also known asGHSA-q28m-8xjw-8vr5

Published

May 11, 2021

Updated

Oct 20, 2026

Affected

2 pkgs

Patched

2 / 2

Exploits

None indexed

Exploitation data as of Nov 5, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

1.6%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs76th percentile — riskier than 76% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

2 pkgs affected

💎puma💎puma

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects RubyGems packages — download data is not available via public APIs for these ecosystems.

Description

This vulnerability is related to CVE-2019-16770.

Impact

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.

Patches

This problem has been fixed in puma 4.3.8 and 5.3.1.

Workarounds

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. A git patch is available here for those using unsupported versions of Puma.

For more information

If you have any questions or comments about this advisory:

Open an issue in Puma.
To report problems with this fix or to report another vulnerability, see our security policy.

Acknowledgements

Thank you to @MSP-Greg, @wjordan and @evanphx for their review on this issue.

Thank you to @ioquatix for providing a modified fork of wrk which made debugging this issue much easier.

Affected Packages

2 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
💎RubyGems	`puma`	all versions	4.3.8`bundle update puma --conservative`
💎RubyGems	`puma`	≥ 5.0.0&&< 5.3.1	5.3.1`bundle update puma --conservative`

Affected Products

2 products · 3 configurations

debian linuxdebian

1 version

10.0

Application

pumapuma

≥ 5.0.0 && < 5.3.1

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for puma, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update puma to 4.3.8 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-29509 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatModerate

Red Hat CloudForms 5.0 (CFME 5.11) is in the maintenance support phase and we are no longer fixing Medium/Low severity security bugs. Reference: https://access.redhat.com/support/policy/updates/cloudforms

Product	Fixed in	Advisory
Red Hat Satellite 6.10 for RHEL 7	`tfm-rubygem-puma-0:5.3.2-1.el7sat`	RHSA-2021:4702

Frequently Asked Questions

This vulnerability is related to [CVE-2019-16770](https://github.com/puma/puma/security/advisories/GHSA-7xx3-m584-x994). ### Impact 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