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

NIEDRIG 3.7CVE-2021-41136 · GHSA-48w2-rm65-62xx · Published 12/10/2021 · updated 17/06/2026

Kurz gesagt

CVE-2021-41136 ist eine Schwachstelle (NIEDRIG (CVSS 3.7)) in puma, puma (puma).
Wahrscheinlichkeit einer Ausnutzung in den nächsten 30 Tagen (EPSS): 1.2%, höher als bei 68.3% aller Schwachstellen.
Abhilfe: auf 5.5.1, 4.3.9 oder neuer aktualisieren.

Package	Affected versions	Patched versions
puma RubyGems	`≥ 5.0.0, < 5.5.1< 4.3.9`	`5.5.14.3.9`
puma puma	`≥ 5.0.0, ≤ 5.5.0≤ 4.3.8`	No fix yet

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
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NVD description

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`.