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

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.

Published: 2021-05-11 Last update: 2024-11-21 Assigner: [email protected] Source: [email protected]
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2021-29509 is rated Moderate Risk (58.1/100): CVSS High severity, with medium exploitation likelihood (EPSS 1.36%). Review affected assets and schedule remediation.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2021-29509

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2025-09-08 1.06% 1.36% +0.30%
2 2025-08-31 0.16% 1.06% +0.90%
3 2025-03-30 — 0.16% —

Full EPSS history (21 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2021-29509

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
7.5 3.1 HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 3.9 3.6 [email protected]
7.5 3.1 HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 3.9 3.6 [email protected]
5.0 2.0 MEDIUM
AV:N/AC:L/Au:N/C:N/I:N/A:P Click to expand
Access vector (AV:N)
Can be exploited remotely over network reachability.
Access complexity (AC:L)
Exploitation conditions are straightforward and predictable.
Authentication (AU:N)
No authentication is required.
Confidentiality impact (C:N)
No confidentiality impact.
Integrity impact (I:N)
No integrity impact.
Availability impact (A:P)
Partial availability impact.
 10.0 2.9 [email protected]

Weakness enumeration for CVE-2021-29509

GitHub Security Advisory for CVE-2021-29509

GHSA-q28m-8xjw-8vr5 · Severity: high · Ecosystem: rubygems — Puma's Keepalive Connections Causing Denial Of Service

OS Trackers for CVE-2021-29509

vendor priority summary link
debian not yet assigned CVE-2021-29509 not yet assigned priority: Debian including 1 source packages (puma), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 5. https://security-tracker.debian.org/tracker/CVE-2021-29509
gentoo low CVE-2021-29509: 1 GLSA(s) (202208-28), 1 atom(s) (www-servers/puma); latest impact low. https://bugs.gentoo.org/buglist.cgi?quicksearch=CVE-2021-29509
redhat medium — https://access.redhat.com/security/cve/CVE-2021-29509
suse high — https://www.suse.com/security/cve/CVE-2021-29509/
ubuntu medium CVE-2021-29509 medium priority: Ubuntu including 1 source packages (puma), 10 status rows across 10 suites (bionic, focal, groovy, hirsute, impish, jammy, kinetic, trusty, upstream, xenial): ignored 4, not-affected 3, DNE 2, released 1. https://ubuntu.com/security/CVE-2021-29509

Affected software / configurations for CVE-2021-29509

Vendor Product Version Raw CPE
puma puma < 4.3.8 cpe:2.3:a:puma:puma:*:*:*:*:*:ruby:*:*
puma puma >= 5.0.0, < 5.3.1 cpe:2.3:a:puma:puma:*:*:*:*:*:ruby:*:*
debian debian_linux 10.0 cpe:2.3:o:debian:debian_linux:10.0:*:*:*:*:*:*:*

References for CVE-2021-29509

cvelogic Threat Intelligence