How severe is CVE-2021-41136?

CVE-2021-41136 has a CVSS score of 3.7/10, rated LOW. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-41136?

CVE-2021-41136 affects the following packages: puma (RubyGems), puma (RubyGems). Ecosystems affected: RubyGems.

How do I fix CVE-2021-41136?

Update puma to 5.5.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-41136 is resolved across your whole dependency graph.

How do I detect CVE-2021-41136 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-2021-41136 if there is no patch (or I can't update yet)?

Constrain what reaches the vulnerable code: limit the size and shape of untrusted input, isolate the affected component in a sandboxed or least-privileged process, and enable the platform's memory-safety mitigations (ASLR, stack protector, hardened allocator) so an out-of-bounds access is more likely to fail closed than to be exploitable.

Is CVE-2021-41136 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-41136 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-2021-41136?

CVE-2021-41136 has an EPSS (Exploit Prediction Scoring System) score of 1.2%, placing it in the 69th 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-2021-41136?

CVE-2021-41136 is classified as CWE-444 (CWE-444). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-41136 published, and has it been updated?

CVE-2021-41136 was published on October 12, 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

LOW severity

CVE-2021-41136 — puma

LOWFix: puma/puma@436c718

CVE-2021-41136 is a low-severity (CVSS 3.7) CWE-444 vulnerability in puma. A fix is available for puma — see the affected versions and patch details below.

Puma with proxy which forwards LF characters as line endings could allow HTTP request smuggling

Also known asGHSA-48w2-rm65-62xx

Published

Oct 12, 2021

Updated

Oct 20, 2026

Affected

2 pkgs

Patched

2 / 2

Exploits

None indexed

Exploitation data as of Nov 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

1.2%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs69th percentile — riskier than 69% 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

Impact

Prior to puma version 5.5.0, using puma with a proxy which forwards LF characters as line endings could allow HTTP request smuggling. A client could smuggle a request through a proxy, causing the proxy to send a response back to another unknown client.

This behavior (forwarding LF characters as line endings) is very uncommon amongst proxy servers, so we have graded the impact here as "low". Puma is only aware of a single proxy server which has this behavior.

If the proxy uses persistent connections and the client adds another request in via HTTP pipelining, the proxy may mistake it as the first request's body. Puma, however, would see it as two requests, and when processing the second request, send back a response that the proxy does not expect. If the proxy has reused the persistent connection to Puma to send another request for a different client, the second response from the first client will be sent to the second client.

Patches

This vulnerability was patched in Puma 5.5.1 and 4.3.9.

Workarounds

This vulnerability only affects Puma installations without any proxy in front.

Use a proxy which does not forward LF characters as line endings.

Proxies which do not forward LF characters as line endings:

Nginx
Apache (>2.4.25)
Haproxy
Caddy
Traefik

Possible Breakage

If you are dealing with legacy clients that want to send LF as a line ending in an HTTP header, this will cause those clients to receive a 400 error.

References

HTTP Request Smuggling

For more information

If you have any questions or comments about this advisory:

Open an issue in Puma
See our security policy

Affected Packages

2 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
💎RubyGems	`puma`	≥ 5.0.0&&< 5.5.1	5.5.1`bundle update puma --conservative`
💎RubyGems	`puma`	all versions	4.3.9`bundle update puma --conservative`

Affected Products

2 products · 4 configurations

debian linuxdebian

2 versions

10.011.0

Application

pumapuma

≥ 5.0.0 && ≤ 5.5.0

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 5.5.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-41136 is resolved across your whole dependency graph.
Workarounds
Constrain what reaches the vulnerable code: limit the size and shape of untrusted input, isolate the affected component in a sandboxed or least-privileged process, and enable the platform's memory-safety mitigations (ASLR, stack protector, hardened allocator) so an out-of-bounds access is more likely to fail closed than to be exploitable.

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 HatLow

Red Hat CloudForms 5.0 (CFME 5.11) is in the maintenance phase and Medium and Low impact security bugs will not be fixed. See https://access.redhat.com/support/policy/updates/cloudforms for additional information.

Product	Fixed in	Advisory
Red Hat Satellite 6.11 for RHEL 7	`tfm-rubygem-puma-0:5.6.2-1.el7sat`	RHSA-2022:5498

Frequently Asked Questions

### Impact Prior to `puma` version 5.5.0, using `puma` with a proxy which forwards LF characters as line endings could allow HTTP request smuggling. A client could smuggle a request through a proxy, causing the proxy to send a response back to another unknown client. This behavior (forwarding LF characters as line endings) is very uncommon amongst proxy servers, so we have graded the impact here as "low". Puma is only aware of a single proxy server which has this behavior. If the proxy uses persistent connections and the client adds another request in via HTTP pipelining, the proxy may mist