CVE-2021-41136: Puma is a HTTP 1.1 server for Ruby/Rack applications

Routine

LOW 3.7WATCHLISTUPGRADEMITIGATE

CVE-2021-41136 is a low-severity flaw (CVSS 3.7) affecting puma, debian linux. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Puma is a HTTP 1.1 server for Ruby/Rack applications. Prior to versions 5.5.1 and 4.3.9, using `puma` with a proxy which forwards HTTP header values which contain the LF character could allow HTTP request smugggling. A client could smuggle a request through a proxy, causing the proxy to send a response back to another unknown client. The only proxy which has this behavior, as far as the Puma team is aware of, is Apache Traffic Server. If the prox

Why this priority

On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	3.7 (Low)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	Low
User interaction	Required
EPSS exploit chance	1.2% (top 32%)
CISA exploited list	No
Weakness	CWE-444
Published	2021-10-12
Last modified	2026-06-17

Publisher (CNA): GitHub, Inc. · Vendor: debian · puma

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-10-12 16:15 UTCPublished CVE record published
2026-06-17 04:07 UTCUpdated Record last modified

puma puma UPGRADE

Affected: <= 4.3.8 · >= 5.0.0 and <= 5.5.0

Fixed in: newer than 5.5.0

Safe to downgrade to: a release before 5.0.0

1. Check installed version

apt-cache policy puma || rpm -q puma || puma --version

2. UPGRADE to newer than 5.5.0 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade puma

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade puma

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 5.0.0 (Debian / Ubuntu)

apt-cache madison puma sudo apt install puma=<VERSION_BEFORE_5.0.0> sudo apt-mark hold puma

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list puma sudo dnf downgrade puma-<VERSION_BEFORE_5.0.0> sudo dnf versionlock add puma

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

debian debian linux MITIGATE

Affected: 10.0 · 11.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 10.0

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-41136?

puma: upgrade to newer than 5.5.0; debian linux: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-41136 being exploited?

CVE-2021-41136 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-41136 affect?

CVE-2021-41136 affects puma, debian linux. Severity is low (CVSS 3.7). Puma is a HTTP 1.1 server for Ruby/Rack applications. Prior to versions 5.5.1 and 4.3.9, using `puma` with a proxy which forwards HTTP header values which contain the LF character could allow HTTP request smugggling. A client could smuggle

How urgent is CVE-2021-41136?

Priority 35/100. On your watchlist

Software pages

Debian vulnerabilities · Ruby vulnerabilities

Related CVEs

CVE-2024-45614 MEDIUM 5.4 UPGRADE Puma is a Ruby/Rack web server built for parallelism
CVE-2024-21647 MEDIUM 5.9 UPGRADE Puma is a web server for Ruby/Rack applications built for parallelism
CVE-2023-40175 HIGH 7.3 UPGRADE Puma is a Ruby/Rack web server built for parallelism
CVE-2022-24790 CRITICAL 9.1 UPGRADE MITIGATE Puma is a simple, fast, multi-threaded, parallel HTTP 1.1 server for Ruby/Rack applications
CVE-2022-23634 HIGH 8.0 UPGRADE UPGRADE Puma is a Ruby/Rack web server built for parallelism

References

← Back to latest CVEs