What is the CVSS score of CVE-2021-41136?

CVE-2021-41136 has a CVSS 3.1 base score of 3.7 (Low). CVSS vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:L/I:L/A:N. EPSS exploitation probability: 1.1%.

Is there a patch available for CVE-2021-41136?

Yes, a patch is available for CVE-2021-41136. Update the affected software to the latest version immediately.

Puma CVE-2021-41136

LOW

HTTP Request/Response Smuggling (CWE-444)

2021-10-12 security-advisories@github.com

Apache Information Disclosure Debian Request Smuggling Debian Linux Puma

3.7

CVSS 3.1 · NVD

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Vulnerability Assessment

Weakness	CWE CWE-444 OWASP Top 10:2025 A06:2025 - Insecure Design ASVS 5.0 V4 - API and Web Service HTTP request consistency
Impact	CVSS score of 3.7 (LOW). Attack characteristics: network attack vector, high complexity, low privileges required, user interaction required.
Exploit ScenarioDerived	A remote attacker could manipulate HTTP request interpretation between frontend and backend servers by sending crafted requests to the affected application.

Monitor Track and reassess. Why: LOW severity (CVSS 0.1-3.9).

Patch available 1.1% exploitation probability (EPSS)

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AnalysisAI

Puma is a HTTP 1.1 server for Ruby/Rack applications. Rated low severity (CVSS 3.7), this vulnerability is remotely exploitable. This HTTP Request/Response Smuggling vulnerability could allow attackers to manipulate HTTP request interpretation between frontend and backend servers.

RemediationAI

A vendor patch is available. Apply the latest security update as soon as possible. Enforce strict HTTP parsing, normalize requests at proxy layer, use HTTP/2 end-to-end, reject ambiguous headers.

Affected Packages

rubygems puma fix: 4.3.9, 5.5.1

Source: OSV (Open Source Vulnerabilities)

Affects your stack

Affects 1 package in rubygems

Checking this CVE against your own stacks and servers is part of the Basic plan and up.

Technical ContextAI · show details

This vulnerability is classified as HTTP Request/Response Smuggling (CWE-444), which allows attackers to manipulate HTTP request interpretation between frontend and backend servers. 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 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. This vulnerability was patched in Puma 5.5.1 and 4.3.9. As a workaround, do not use Apache Traffic Server with puma. Affected products include: Puma, Debian Debian Linux.

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Exploit and PoC references, threat intelligence from 22 sources

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Severity by source

NVD PRIMARY 3.7 LOW

AV:N/AC:H/PR:L/UI:R/S:U/C:L/I:L/A:N

Primary rating from NVD · only source for this CVE.

CVSS VectorNVD

Attack Vector

Network

Attack Complexity

High

Privileges Required

Low

User Interaction

Required

Scope

Unchanged

Confidentiality

Low

Integrity

Low

Availability

None

DescriptionNVD

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 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. This vulnerability was patched in Puma 5.5.1 and 4.3.9. As a workaround, do not use Apache Traffic Server with puma.

Threat Intelligence Overview

EPSS Score

1.1%

Top 38% exploitable

Data Quality

3 sources

Confidence: Low

NVD

CVE.org

EPSS

GitHub

How this decision was reached

The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.

SignalValue

Severity LOW

CISA KEV No

Public POC No

EPSS 1.1%

Patch available Yes

Active threat No

Deciding rule

LOW severity (CVSS 0.1-3.9)

What every action level means

Emergency Immediate action required. Criteria: CRITICAL + in CISA KEV (actively exploited) without patch, OR CRITICAL + EPSS > 30% without patch, OR CRITICAL + active threat signals without patch.

Act Now Prioritize within 24-48h. Criteria: in KEV with patch available, OR CRITICAL + public POC, OR HIGH + EPSS > 10%, OR CRITICAL + active threat, OR any CRITICAL severity.

This Week Schedule remediation this week. Criteria: HIGH severity (CVSS 7.0-8.9). Patch within normal change window.

This Month Plan within 30 days. Criteria: MEDIUM severity (CVSS 4.0-6.9), lower exploitation probability. Include in regular patch cycle.

Monitor Track and reassess. Criteria: LOW severity (CVSS 0.1-3.9), minimal exploitation risk. No immediate action needed.

SSVC AssessmentENISA

SSVC Decision· derived

Defer

Exploitation· derived

None

System Exposure· derived

Open

Automatable· derived

Human Impact· derived

Low

Priority Score

Low Medium High Critical

KEV: 0

EPSS: +1

CVSS: +18

PoC: 0

Weaponized: 0

Threat intel: 0

Gov advisory: 0

Attack Technique

HTTP Request Smuggling HIGH

HTTP request smuggling exploits inconsistencies in how front-end and back-end servers parse HTTP message boundaries.

Information Disclosure MEDIUM

Information disclosure occurs when an application unintentionally exposes sensitive data that aids attackers in reconnaissance or directly compromises security.