CVE-2021-41136

What it is

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`.

At a glance

Exploitability

Can be exploited over the network with a low-privilege account, but needs a user to act. Limited direct impact.

Exploit odds

1.2% chance of exploitation in the next 30 days (EPSS).

Fix

A patch, advisory or fixed version is referenced. Upgrade or apply it.

Timeline

1. 2021-10-12 16:15 UTC PublishedDisclosed publicly on CVE.org and NVD
2. 2026-06-17 04:07 UTC Last updatedMost recent change to the record

Why it ranks here

• Exploited in the wild: no, CISA does not list it.
• Exploit odds: 1.2% in the next 30 days (EPSS).
• Severity: 3.7 LOW (CVSS).
• Fix: a patch or advisory is linked.

Details

Published

2021-10-12 16:15 UTC

Last modified

2026-06-17 04:07 UTC

Exploit probability

1.2% · EPSS, next 30 days

Weakness

CWE-444

Record status

Modified

Affected

• Puma ≤ 4.3.8
• Puma ≤ 5.5.0
• Debian Linux 10.0
• Debian Linux 11.0

Attack profile CVSS 3.1 · 3.7

Attack vector

Network

Complexity

High

Privileges needed

Low

User interaction

Required

Scope

Unchanged

Confidentiality

Low

Integrity

Low

Availability

None

Is it in your code?

Hand this to your coding agent. It searches your project for the affected products and checks versions. Nothing leaves your machine.

Read https://cve-list.com/agent/cve/CVE-2021-41136.md and follow it. Check whether this project is affected by CVE-2021-41136. Read only. Don't change anything and don't upload my code. Tell me yes, no or not sure, with the file as proof.

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